
****
NAME
****


parallel - build and execute shell command lines from standard input in parallel


********
SYNOPSIS
********


\ **parallel**\  [options] [\ *command*\  [arguments]] < list_of_arguments

\ **parallel**\  [options] [\ *command*\  [arguments]] ( \ **:::**\  arguments | \ **:::+**\  arguments | \ **::::**\  argfile(s) | \ **::::+**\  argfile(s) ) ...

\ **parallel**\  --semaphore [options] \ *command*\ 

\ **#!/usr/bin/parallel**\  --shebang [options] [\ *command*\  [arguments]]

\ **#!/usr/bin/parallel**\  --shebang-wrap [options] [\ *command*\  [arguments]]


***********
DESCRIPTION
***********


STOP!

Read the \ **Reader's guide**\  below if you are new to GNU \ **parallel**\ .

GNU \ **parallel**\  is a shell tool for executing jobs in parallel using one or more computers. A job can be a single command or a small script that has to be run for each of the lines in the input. The typical input is a list of files, a list of hosts, a list of users, a list of URLs, or a list of tables. A job can also be a command that reads from a pipe. GNU \ **parallel**\  can then split the input into blocks and pipe a block into each command in parallel.

If you use xargs and tee today you will find GNU \ **parallel**\  very easy to use as GNU \ **parallel**\  is written to have the same options as xargs. If you write loops in shell, you will find GNU \ **parallel**\  may be able to replace most of the loops and make them run faster by running several jobs in parallel.

GNU \ **parallel**\  makes sure output from the commands is the same output as you would get had you run the commands sequentially. This makes it possible to use output from GNU \ **parallel**\  as input for other programs.

For each line of input GNU \ **parallel**\  will execute \ *command*\  with the line as arguments. If no \ *command*\  is given, the line of input is executed. Several lines will be run in parallel. GNU \ **parallel**\  can often be used as a substitute for \ **xargs**\  or \ **cat | bash**\ .

Reader's guide
==============


GNU \ **parallel**\  includes the 4 types of documentation: Tutorial, how-to, reference and explanation.

Tutorial
--------


If you prefer reading a book buy \ **GNU Parallel 2018**\  at https://www.lulu.com/shop/ole-tange/gnu-parallel-2018/paperback/product-23558902.html or download it at: https://doi.org/10.5281/zenodo.1146014 Read at least chapter 1+2. It should take you less than 20 minutes.

Otherwise start by watching the intro videos for a quick introduction: https://youtube.com/playlist?list=PL284C9FF2488BC6D1

If you want to dive deeper: spend a couple of hours walking through the tutorial (\ **man parallel_tutorial**\ ). Your command line will love you for it.


How-to
------


You can find a lot of \ **EXAMPLE**\ s of use after the list of \ **OPTIONS**\  in \ **man parallel**\  (Use \ **LESS=+/EXAMPLE: man parallel**\ ). That will give you an idea of what GNU \ **parallel**\  is capable of, and you may find a solution you can simply adapt to your situation.


Reference
---------


If you need a one page printable cheat sheet you can find it on: https://www.gnu.org/software/parallel/parallel_cheat.pdf

The man page is the reference for all options.


Design discussion
-----------------


If you want to know the design decisions behind GNU \ **parallel**\ , try: \ **man parallel_design**\ . This is also a good intro if you intend to change GNU \ **parallel**\ .




*******
OPTIONS
*******



- \ *command*\ 
 
 Command to execute.
 
 If \ *command*\  or the following arguments contain replacement strings (such as \ **{}**\ ) every instance will be substituted with the input.
 
 If \ *command*\  is given, GNU \ **parallel**\  solve the same tasks as \ **xargs**\ . If \ *command*\  is not given GNU \ **parallel**\  will behave similar to \ **cat | sh**\ .
 
 The \ *command*\  must be an executable, a script, a composed command, an alias, or a function.
 
 \ **Bash functions**\ : \ **export -f**\  the function first or use \ **env_parallel**\ .
 
 \ **Bash, Csh, or Tcsh aliases**\ : Use \ **env_parallel**\ .
 
 \ **Zsh, Fish, Ksh, and Pdksh functions and aliases**\ : Use \ **env_parallel**\ .
 


- \ **{}**\ 
 
 Input line.
 
 This replacement string will be replaced by a full line read from the input source. The input source is normally stdin (standard input), but can also be given with \ **--arg-file**\ , \ **:::**\ , or \ **::::**\ .
 
 The replacement string \ **{}**\  can be changed with \ **-I**\ .
 
 If the command line contains no replacement strings then \ **{}**\  will be appended to the command line.
 
 Replacement strings are normally quoted, so special characters are not parsed by the shell. The exception is if the command starts with a replacement string; then the string is not quoted.
 
 See also: \ **--plus**\  \ **{.}**\  \ **{/}**\  \ **{//}**\  \ **{/.}**\  \ **{#}**\  \ **{%}**\  \ **{**\ \ *n*\ \ **}**\  \ **{=**\ \ *perl expression*\ \ **=}**\ 
 


- \ **{.}**\ 
 
 Input line without extension.
 
 This replacement string will be replaced by the input with the extension removed. If the input line contains \ **.**\  after the last \ **/**\ , the last \ **.**\  until the end of the string will be removed and \ **{.}**\  will be replaced with the remaining. E.g. \ *foo.jpg*\  becomes \ *foo*\ , \ *subdir/foo.jpg*\  becomes \ *subdir/foo*\ , \ *sub.dir/foo.jpg*\  becomes \ *sub.dir/foo*\ , \ *sub.dir/bar*\  remains \ *sub.dir/bar*\ . If the input line does not contain \ **.**\  it will remain unchanged.
 
 The replacement string \ **{.}**\  can be changed with \ **--extensionreplace**\ 
 
 See also: \ **{}**\  \ **--extensionreplace**\ 
 


- \ **{/}**\ 
 
 Basename of input line.
 
 This replacement string will be replaced by the input with the directory part removed.
 
 See also: \ **{}**\  \ **--basenamereplace**\ 
 


- \ **{//}**\ 
 
 Dirname of input line.
 
 This replacement string will be replaced by the dir of the input line. See \ **dirname**\ (1).
 
 See also: \ **{}**\  \ **--dirnamereplace**\ 
 


- \ **{/.}**\ 
 
 Basename of input line without extension.
 
 This replacement string will be replaced by the input with the directory and extension part removed.  \ **{/.}**\  is a combination of \ **{/}**\  and \ **{.}**\ .
 
 See also: \ **{}**\  \ **--basenameextensionreplace**\ 
 


- \ **{#}**\ 
 
 Sequence number of the job to run.
 
 This replacement string will be replaced by the sequence number of the job being run. It contains the same number as $PARALLEL_SEQ.
 
 See also: \ **{}**\  \ **--seqreplace**\ 
 


- \ **{%}**\ 
 
 Job slot number.
 
 This replacement string will be replaced by the job's slot number between 1 and number of jobs to run in parallel. There will never be 2 jobs running at the same time with the same job slot number.
 
 If the job needs to be retried (e.g using \ **--retries**\  or \ **--retry-failed**\ ) the job slot is not automatically updated. You should then instead use \ **$PARALLEL_JOBSLOT**\ :
 
 
 .. code-block:: perl
 
    $ do_test() {
        id="$3 {%}=$1 PARALLEL_JOBSLOT=$2"
        echo run "$id";
        sleep 1
        # fail if {%} is odd
        return `echo $1%2 | bc`
      }
    $ export -f do_test
    $ parallel -j3 --jl mylog do_test {%} \$PARALLEL_JOBSLOT {} ::: A B C D
    run A {%}=1 PARALLEL_JOBSLOT=1
    run B {%}=2 PARALLEL_JOBSLOT=2
    run C {%}=3 PARALLEL_JOBSLOT=3
    run D {%}=1 PARALLEL_JOBSLOT=1
    $ parallel --retry-failed -j3 --jl mylog do_test {%} \$PARALLEL_JOBSLOT {} ::: A B C D
    run A {%}=1 PARALLEL_JOBSLOT=1
    run C {%}=3 PARALLEL_JOBSLOT=2
    run D {%}=1 PARALLEL_JOBSLOT=3
 
 
 Notice how {%} and $PARALLEL_JOBSLOT differ in the retry run of C and D.
 
 See also: \ **{}**\  \ **--jobs**\  \ **--slotreplace**\ 
 


- \ **{**\ \ *n*\ \ **}**\ 
 
 Argument from input source \ *n*\  or the \ *n*\ 'th argument.
 
 This positional replacement string will be replaced by the input from input source \ *n*\  (when used with \ **--arg-file**\  or \ **::::**\ ) or with the \ *n*\ 'th argument (when used with \ **-N**\ ). If \ *n*\  is negative it refers to the \ *n*\ 'th last argument.
 
 See also: \ **{}**\  \ **{**\ \ *n*\ .\ **}**\  \ **{**\ \ *n*\ /\ **}**\  \ **{**\ \ *n*\ //\ **}**\  \ **{**\ \ *n*\ /.\ **}**\ 
 


- \ **{**\ \ *n*\ .\ **}**\ 
 
 Argument from input source \ *n*\  or the \ *n*\ 'th argument without extension.
 
 \ **{**\ \ *n*\ .\ **}**\  is a combination of \ **{**\ \ *n*\ \ **}**\  and \ **{.}**\ .
 
 This positional replacement string will be replaced by the input from input source \ *n*\  (when used with \ **--arg-file**\  or \ **::::**\ ) or with the \ *n*\ 'th argument (when used with \ **-N**\ ). The input will have the extension removed.
 
 See also: \ **{**\ \ *n*\ \ **}**\  \ **{.}**\ 
 


- \ **{**\ \ *n*\ /\ **}**\ 
 
 Basename of argument from input source \ *n*\  or the \ *n*\ 'th argument.
 
 \ **{**\ \ *n*\ /\ **}**\  is a combination of \ **{**\ \ *n*\ \ **}**\  and \ **{/}**\ .
 
 This positional replacement string will be replaced by the input from input source \ *n*\  (when used with \ **--arg-file**\  or \ **::::**\ ) or with the \ *n*\ 'th argument (when used with \ **-N**\ ). The input will have the directory (if any) removed.
 
 See also: \ **{**\ \ *n*\ \ **}**\  \ **{/}**\ 
 


- \ **{**\ \ *n*\ //\ **}**\ 
 
 Dirname of argument from input source \ *n*\  or the \ *n*\ 'th argument.
 
 \ **{**\ \ *n*\ //\ **}**\  is a combination of \ **{**\ \ *n*\ \ **}**\  and \ **{//}**\ .
 
 This positional replacement string will be replaced by the dir of the input from input source \ *n*\  (when used with \ **--arg-file**\  or \ **::::**\ ) or with the \ *n*\ 'th argument (when used with \ **-N**\ ). See \ **dirname**\ (1).
 
 See also: \ **{**\ \ *n*\ \ **}**\  \ **{//}**\ 
 


- \ **{**\ \ *n*\ /.\ **}**\ 
 
 Basename of argument from input source \ *n*\  or the \ *n*\ 'th argument without extension.
 
 \ **{**\ \ *n*\ /.\ **}**\  is a combination of \ **{**\ \ *n*\ \ **}**\ , \ **{/}**\ , and \ **{.}**\ .
 
 This positional replacement string will be replaced by the input from input source \ *n*\  (when used with \ **--arg-file**\  or \ **::::**\ ) or with the \ *n*\ 'th argument (when used with \ **-N**\ ). The input will have the directory (if any) and extension removed.
 
 See also: \ **{**\ \ *n*\ \ **}**\  \ **{/.}**\ 
 


- \ **{=**\ \ *perl expression*\ \ **=}**\ 
 
 Replace with calculated \ *perl expression*\ .
 
 \ **$_**\  will contain the same as \ **{}**\ . After evaluating \ *perl expression*\  \ **$_**\  will be used as the value. It is recommended to only change $_ but you have full access to all of GNU \ **parallel**\ 's internal functions and data structures.
 
 The expression must give the same result if evaluated twice - otherwise the behaviour is undefined. E.g. this will not work as expected:
 
 
 .. code-block:: perl
 
      parallel echo '{= $_= ++$wrong_counter =}' ::: a b c
 
 
 A few convenience functions and data structures have been made:
 
 
 - \ **Q(**\ \ *string*\ \ **)**\ 
  
  shell quote a string
  
 
 
 - \ **pQ(**\ \ *string*\ \ **)**\ 
  
  perl quote a string
  
 
 
 - \ **uq()**\  (or \ **uq**\ )
  
  do not quote current replacement string
  
 
 
 - \ **hash(val)**\ 
  
  compute B::hash(val)
  
 
 
 - \ **total_jobs()**\ 
  
  number of jobs in total
  
 
 
 - \ **slot()**\ 
  
  slot number of job
  
 
 
 - \ **seq()**\ 
  
  sequence number of job
  
 
 
 - \ **@arg**\ 
  
  the arguments
  
 
 
 - \ **skip()**\ 
  
  skip this job (see also \ **--filter**\ )
  
 
 
 - \ **yyyy_mm_dd_hh_mm_ss()**\ 
 
 
 
 - \ **yyyy_mm_dd_hh_mm()**\ 
 
 
 
 - \ **yyyy_mm_dd()**\ 
 
 
 
 - \ **hh_mm_ss()**\ 
 
 
 
 - \ **hh_mm()**\ 
 
 
 
 - \ **yyyymmddhhmmss()**\ 
 
 
 
 - \ **yyyymmddhhmm()**\ 
 
 
 
 - \ **yyyymmdd()**\ 
 
 
 
 - \ **hhmmss()**\ 
 
 
 
 - \ **hhmm()**\ 
  
  time functions
  
 
 
 Example:
 
 
 .. code-block:: perl
 
    seq 10 | parallel echo {} + 1 is {= '$_++' =}
    parallel csh -c {= '$_="mkdir ".Q($_)' =} ::: '12" dir'
    seq 50 | parallel echo job {#} of {= '$_=total_jobs()' =}
 
 
 See also: \ **--rpl**\  \ **--parens**\  \ **{}**\  \ **{=**\ \ *n*\  \ *perl expression*\ \ **=}**\ 
 


- \ **{=**\ \ *n*\  \ *perl expression*\ \ **=}**\ 
 
 Positional equivalent to \ **{=**\ \ *perl expression*\ \ **=}**\ .
 
 To understand positional replacement strings see \ **{**\ \ *n*\ \ **}**\ .
 
 See also: \ **{=**\ \ *perl expression*\ \ **=}**\  \ **{**\ \ *n*\ \ **}**\ 
 


- \ **:::**\  \ *arguments*\ 
 
 Use arguments on the command line as input source.
 
 Unlike other options for GNU \ **parallel**\  \ **:::**\  is placed after the \ *command*\  and before the arguments.
 
 The following are equivalent:
 
 
 .. code-block:: perl
 
    (echo file1; echo file2) | parallel gzip
    parallel gzip ::: file1 file2
    parallel gzip {} ::: file1 file2
    parallel --arg-sep ,, gzip {} ,, file1 file2
    parallel --arg-sep ,, gzip ,, file1 file2
    parallel ::: "gzip file1" "gzip file2"
 
 
 To avoid treating \ **:::**\  as special use \ **--arg-sep**\  to set the argument separator to something else.
 
 If multiple \ **:::**\  are given, each group will be treated as an input source, and all combinations of input sources will be generated. E.g. ::: 1 2 ::: a b c will result in the combinations (1,a) (1,b) (1,c) (2,a) (2,b) (2,c). This is useful for replacing nested for-loops.
 
 \ **:::**\ , \ **::::**\ , and \ **--arg-file**\  can be mixed. So these are equivalent:
 
 
 .. code-block:: perl
 
    parallel echo {1} {2} {3} ::: 6 7 ::: 4 5 ::: 1 2 3
    parallel echo {1} {2} {3} :::: <(seq 6 7) <(seq 4 5) \
      :::: <(seq 1 3)
    parallel -a <(seq 6 7) echo {1} {2} {3} :::: <(seq 4 5) \
      :::: <(seq 1 3)
    parallel -a <(seq 6 7) -a <(seq 4 5) echo {1} {2} {3} \
      ::: 1 2 3
    seq 6 7 | parallel -a - -a <(seq 4 5) echo {1} {2} {3} \
      ::: 1 2 3
    seq 4 5 | parallel echo {1} {2} {3} :::: <(seq 6 7) - \
      ::: 1 2 3
 
 
 See also: \ **--arg-sep**\  \ **--arg-file**\  \ **::::**\  \ **:::+**\  \ **::::+**\  \ **--link**\ 
 


- \ **:::+**\  \ *arguments*\ 
 
 Like \ **:::**\  but linked like \ **--link**\  to the previous input source.
 
 Contrary to \ **--link**\ , values do not wrap: The shortest input source determines the length.
 
 Example:
 
 
 .. code-block:: perl
 
    parallel echo ::: a b c :::+ 1 2 3 ::: X Y :::+ 11 22
 
 
 See also: \ **::::+**\  \ **--link**\ 
 


- \ **::::**\  \ *argfiles*\ 
 
 Another way to write \ **--arg-file**\  \ *argfile1*\  \ **--arg-file**\  \ *argfile2*\  ...
 
 \ **:::**\  and \ **::::**\  can be mixed.
 
 See also: \ **--arg-file**\  \ **:::**\  \ **::::+**\  \ **--link**\ 
 


- \ **::::+**\  \ *argfiles*\ 
 
 Like \ **::::**\  but linked like \ **--link**\  to the previous input source.
 
 Contrary to \ **--link**\ , values do not wrap: The shortest input source determines the length.
 
 See also: \ **--arg-file**\  \ **:::+**\  \ **--link**\ 
 


- \ **--null**\ 



- \ **-0**\ 
 
 Use NUL as delimiter.
 
 Normally input lines will end in \n (newline). If they end in \0 (NUL), then use this option. It is useful for processing arguments that may contain \n (newline).
 
 Shorthand for \ **--delimiter '\0'**\ .
 
 See also: \ **--delimiter**\ 
 


- \ **--arg-file**\  \ *input-file*\ 



- \ **-a**\  \ *input-file*\ 
 
 Use \ *input-file*\  as input source.
 
 If you use this option, stdin (standard input) is given to the first process run.  Otherwise, stdin (standard input) is redirected from /dev/null.
 
 If multiple \ **--arg-file**\  are given, each \ *input-file*\  will be treated as an input source, and all combinations of input sources will be generated. E.g. The file \ **foo**\  contains \ **1 2**\ , the file \ **bar**\  contains \ **a b c**\ .  \ **-a foo**\  \ **-a bar**\  will result in the combinations (1,a) (1,b) (1,c) (2,a) (2,b) (2,c). This is useful for replacing nested for-loops.
 
 See also: \ **--link**\  \ **{**\ \ *n*\ \ **}**\  \ **::::**\  \ **::::+**\  \ **:::**\ 
 


- \ **--arg-file-sep**\  \ *sep-str*\ 
 
 Use \ *sep-str*\  instead of \ **::::**\  as separator string between command and argument files.
 
 Useful if \ **::::**\  is used for something else by the command.
 
 See also: \ **::::**\ 
 


- \ **--arg-sep**\  \ *sep-str*\ 
 
 Use \ *sep-str*\  instead of \ **:::**\  as separator string.
 
 Useful if \ **:::**\  is used for something else by the command.
 
 Also useful if you command uses \ **:::**\  but you still want to read arguments from stdin (standard input): Simply change \ **--arg-sep**\  to a string that is not in the command line.
 
 See also: \ **:::**\ 
 


- \ **--bar**\ 
 
 Show progress as a progress bar.
 
 In the bar is shown: % of jobs completed, estimated seconds left, and number of jobs started.
 
 It is compatible with \ **zenity**\ :
 
 
 .. code-block:: perl
 
    seq 1000 | parallel -j30 --bar '(echo {};sleep 0.1)' \
      2> >(perl -pe 'BEGIN{$/="\r";$|=1};s/\r/\n/g' |
           zenity --progress --auto-kill) | wc
 
 


- \ **--basefile**\  \ *file*\ 



- \ **--bf**\  \ *file*\ 
 
 \ *file*\  will be transferred to each sshlogin before first job is started.
 
 It will be removed if \ **--cleanup**\  is active. The file may be a script to run or some common base data needed for the job.  Multiple \ **--bf**\  can be specified to transfer more basefiles. The \ *file*\  will be transferred the same way as \ **--transferfile**\ .
 
 See also: \ **--sshlogin**\  \ **--transfer**\  \ **--return**\  \ **--cleanup**\  \ **--workdir**\ 
 


- \ **--basenamereplace**\  \ *replace-str*\ 



- \ **--bnr**\  \ *replace-str*\ 
 
 Use the replacement string \ *replace-str*\  instead of \ **{/}**\  for basename of input line.
 
 See also: \ **{/}**\ 
 


- \ **--basenameextensionreplace**\  \ *replace-str*\ 



- \ **--bner**\  \ *replace-str*\ 
 
 Use the replacement string \ *replace-str*\  instead of \ **{/.}**\  for basename of input line without extension.
 
 See also: \ **{/.}**\ 
 


- \ **--bin**\  \ *binexpr*\ 
 
 Use \ *binexpr*\  as binning key and bin input to the jobs.
 
 \ *binexpr*\  is [column number|column name] [perlexpression] e.g.:
 
 
 .. code-block:: perl
 
    3
    Address
    3 $_%=100
    Address s/\D//g
 
 
 Each input line is split using \ **--colsep**\ . The value of the column is put into $_, the perl expression is executed, the resulting value is is the job slot that will be given the line. If the value is bigger than the number of jobslots the value will be modulo number of jobslots.
 
 This is similar to \ **--shard**\  but the hashing algorithm is a simple modulo, which makes it predictible which jobslot will receive which value.
 
 The performance is in the order of 100K rows per second. Faster if the \ *bincol*\  is small (<10), slower if it is big (>100).
 
 \ **--bin**\  requires \ **--pipe**\  and a fixed numeric value for \ **--jobs**\ .
 
 See also: SPREADING BLOCKS OF DATA \ **--group-by**\  \ **--round-robin**\  \ **--shard**\ 
 


- \ **--bg**\ 
 
 Run command in background.
 
 GNU \ **parallel**\  will normally wait for the completion of a job. With \ **--bg**\  GNU \ **parallel**\  will not wait for completion of the command before exiting.
 
 This is the default if \ **--semaphore**\  is set.
 
 Implies \ **--semaphore**\ .
 
 See also: \ **--fg**\  \ **man sem**\ 
 


- \ **--bibtex**\ 



- \ **--citation**\ 
 
 Print the citation notice and BibTeX entry for GNU \ **parallel**\ , silence citation notice for all future runs, and exit. It will not run any commands.
 
 If it is impossible for you to run \ **--citation**\  you can instead use \ **--will-cite**\ , which will run commands, but which will only silence the citation notice for this single run.
 
 If you use \ **--will-cite**\  in scripts to be run by others you are making it harder for others to see the citation notice.  The development of GNU \ **parallel**\  is indirectly financed through citations, so if your users do not know they should cite then you are making it harder to finance development. However, if you pay 10000 EUR, you have done your part to finance future development and should feel free to use \ **--will-cite**\  in scripts.
 
 If you do not want to help financing future development by letting other users see the citation notice or by paying, then please consider using another tool instead of GNU \ **parallel**\ . You can find some of the alternatives in \ **man parallel_alternatives**\ .
 


- \ **--block**\  \ *size*\ 



- \ **--block-size**\  \ *size*\ 
 
 Size of block in bytes to read at a time.
 
 The \ *size*\  can be postfixed with K, M, G, T, P, k, m, g, t, or p.
 
 GNU \ **parallel**\  tries to meet the block size but can be off by the length of one record. For performance reasons \ *size*\  should be bigger than a two records. GNU \ **parallel**\  will warn you and automatically increase the size if you choose a \ *size*\  that is too small.
 
 If you use \ **-N**\ , \ **--block**\  should be bigger than N+1 records.
 
 \ *size*\  defaults to 1M.
 
 When using \ **--pipe-part**\  a negative block size is not interpreted as a blocksize but as the number of blocks each jobslot should have. So this will run 10\*5 = 50 jobs in total:
 
 
 .. code-block:: perl
 
    parallel --pipe-part -a myfile --block -10 -j5 wc
 
 
 This is an efficient alternative to \ **--round-robin**\  because data is never read by GNU \ **parallel**\ , but you can still have very few jobslots process large amounts of data.
 
 See also: UNIT PREFIX \ **-N**\  \ **--pipe**\  \ **--pipe-part**\  \ **--round-robin**\  \ **--block-timeout**\ 
 


- \ **--block-timeout**\  \ *duration*\ 



- \ **--bt**\  \ *duration*\ 
 
 Timeout for reading block when using \ **--pipe**\ .
 
 If it takes longer than \ *duration*\  to read a full block, use the partial block read so far.
 
 \ *duration*\  is in seconds, but can be postfixed with s, m, h, or d.
 
 See also: TIME POSTFIXES \ **--pipe**\  \ **--block**\ 
 


- \ **--cat**\ 
 
 Create a temporary file with content.
 
 Normally \ **--pipe**\ /\ **--pipe-part**\  will give data to the program on stdin (standard input). With \ **--cat**\  GNU \ **parallel**\  will create a temporary file with the name in \ **{}**\ , so you can do: \ **parallel --pipe --cat wc {}**\ .
 
 Implies \ **--pipe**\  unless \ **--pipe-part**\  is used.
 
 See also: \ **--pipe**\  \ **--pipe-part**\  \ **--fifo**\ 
 


- \ **--cleanup**\ 
 
 Remove transferred files.
 
 \ **--cleanup**\  will remove the transferred files on the remote computer after processing is done.
 
 
 .. code-block:: perl
 
    find log -name '*gz' | parallel \
      --sshlogin server.example.com --transferfile {} \
      --return {.}.bz2 --cleanup "zcat {} | bzip -9 >{.}.bz2"
 
 
 With \ **--transferfile {}**\  the file transferred to the remote computer will be removed on the remote computer. Directories on the remote computer containing the file will be removed if they are empty.
 
 With \ **--return**\  the file transferred from the remote computer will be removed on the remote computer. Directories on the remote computer containing the file will be removed if they are empty.
 
 \ **--cleanup**\  is ignored when not used with \ **--basefile**\ , \ **--transfer**\ , \ **--transferfile**\  or \ **--return**\ .
 
 See also: \ **--basefile**\  \ **--transfer**\  \ **--transferfile**\  \ **--sshlogin**\  \ **--return**\ 
 


- \ **--color**\  (alpha testing)
 
 Colour output.
 
 Colour the output. Each job gets its own colour combination (background+foreground).
 
 \ **--color**\  is ignored when using \ **-u**\ .
 
 See also: \ **--color-failed**\ 
 


- \ **--color-failed**\  (alpha testing)



- \ **--cf**\  (alpha testing)
 
 Colour the output from failing jobs white on red.
 
 Useful if you have a lot of jobs and want to focus on the failing jobs.
 
 \ **--color-failed**\  is ignored when using \ **-u**\ , \ **--line-buffer**\  and unreliable when using \ **--latest-line**\ .
 
 See also: \ **--color**\ 
 


- \ **--colsep**\  \ *regexp*\ 



- \ **-C**\  \ *regexp*\ 
 
 Column separator.
 
 The input will be treated as a table with \ *regexp*\  separating the columns. The n'th column can be accessed using \ **{**\ \ *n*\ \ **}**\  or \ **{**\ \ *n*\ .\ **}**\ . E.g. \ **{3}**\  is the 3rd column.
 
 If there are more input sources, each input source will be separated, but the columns from each input source will be linked.
 
 
 .. code-block:: perl
 
    parallel --colsep '-' echo {4} {3} {2} {1} \
      ::: A-B C-D ::: e-f g-h
 
 
 \ **--colsep**\  implies \ **--trim rl**\ , which can be overridden with \ **--trim n**\ .
 
 \ *regexp*\  is a Perl Regular Expression: https://perldoc.perl.org/perlre.html
 
 See also: \ **--csv**\  \ **{**\ \ *n*\ \ **}**\  \ **--trim**\  \ **--link**\ 
 


- \ **--compress**\ 
 
 Compress temporary files.
 
 If the output is big and very compressible this will take up less disk space in $TMPDIR and possibly be faster due to less disk I/O.
 
 GNU \ **parallel**\  will try \ **pzstd**\ , \ **lbzip2**\ , \ **pbzip2**\ , \ **zstd**\ , \ **pigz**\ , \ **lz4**\ , \ **lzop**\ , \ **plzip**\ , \ **lzip**\ , \ **lrz**\ , \ **gzip**\ , \ **pxz**\ , \ **lzma**\ , \ **bzip2**\ , \ **xz**\ , \ **clzip**\ , in that order, and use the first available.
 
 See also: \ **--compress-program**\ 
 


- \ **--compress-program**\  \ *prg*\ 



- \ **--decompress-program**\  \ *prg*\ 
 
 Use \ *prg*\  for (de)compressing temporary files.
 
 It is assumed that \ *prg -dc*\  will decompress stdin (standard input) to stdout (standard output) unless \ **--decompress-program**\  is given.
 
 See also: \ **--compress**\ 
 


- \ **--csv**\ 
 
 Treat input as CSV-format.
 
 \ **--colsep**\  sets the field delimiter. It works very much like \ **--colsep**\  except it deals correctly with quoting. Compare:
 
 
 .. code-block:: perl
 
     echo '"1 big, 2 small","2""x4"" plank",12.34' |
       parallel --csv echo {1} of {2} at {3}
  
     echo '"1 big, 2 small","2""x4"" plank",12.34' |
       parallel --colsep ',' echo {1} of {2} at {3}
 
 
 Even quoted newlines are parsed correctly:
 
 
 .. code-block:: perl
 
     (echo '"Start of field 1 with newline'
      echo 'Line 2 in field 1";value 2') |
       parallel --csv --colsep ';' echo Field 1: {1} Field 2: {2}
 
 
 When used with \ **--pipe**\  only pass full CSV-records.
 
 See also: \ **--pipe**\  \ **--link**\  \ **{**\ \ *n*\ \ **}**\  \ **--colsep**\  \ **--header**\ 
 


- \ **--ctag**\  (obsolete: use \ **--color**\  \ **--tag**\ )
 
 Color tag.
 
 If the values look very similar looking at the output it can be hard to tell when a new value is used. \ **--ctag**\  gives each value a random color.
 
 See also: \ **--color**\  \ **--tag**\ 
 


- \ **--ctagstring**\  \ *str*\  (obsolete: use \ **--color**\  \ **--tagstring**\ )
 
 Color tagstring.
 
 See also: \ **--color**\  \ **--ctag**\  \ **--tagstring**\ 
 


- \ **--delay**\  \ *duration*\ 
 
 Delay starting next job by \ *duration*\ .
 
 GNU \ **parallel**\  will not start another job for the next \ *duration*\ .
 
 \ *duration*\  is in seconds, but can be postfixed with s, m, h, or d.
 
 If you append 'auto' to \ *duration*\  (e.g. 13m3sauto) GNU \ **parallel**\  will automatically try to find the optimal value: If a job fails, \ *duration*\  is increased by 30%. If a job succeeds, \ *duration*\  is decreased by 10%.
 
 See also: TIME POSTFIXES \ **--retries**\  \ **--ssh-delay**\ 
 


- \ **--delimiter**\  \ *delim*\ 



- \ **-d**\  \ *delim*\ 
 
 Input items are terminated by \ *delim*\ .
 
 The specified delimiter may be characters, C-style character escapes such as \n, or octal or hexadecimal escape codes.  Octal and hexadecimal escape codes are understood as for the printf command.
 
 See also: \ **--colsep**\ 
 


- \ **--dirnamereplace**\  \ *replace-str*\ 



- \ **--dnr**\  \ *replace-str*\ 
 
 Use the replacement string \ *replace-str*\  instead of \ **{//}**\  for dirname of input line.
 
 See also: \ **{//}**\ 
 


- \ **--dry-run**\ 
 
 Print the job to run on stdout (standard output), but do not run the job.
 
 Use \ **-v -v**\  to include the wrapping that GNU \ **parallel**\  generates (for remote jobs, \ **--tmux**\ , \ **--nice**\ , \ **--pipe**\ , \ **--pipe-part**\ , \ **--fifo**\  and \ **--cat**\ ). Do not count on this literally, though, as the job may be scheduled on another computer or the local computer if : is in the list.
 
 See also: \ **-v**\ 
 


- \ **-E**\  \ *eof-str*\ 
 
 Set the end of file string to \ *eof-str*\ .
 
 If the end of file string occurs as a line of input, the rest of the input is not read.  If neither \ **-E**\  nor \ **-e**\  is used, no end of file string is used.
 


- \ **--eof**\ [=\ *eof-str*\ ]



- \ **-e**\ [\ *eof-str*\ ]
 
 This option is a synonym for the \ **-E**\  option.
 
 Use \ **-E**\  instead, because it is POSIX compliant for \ **xargs**\  while this option is not.  If \ *eof-str*\  is omitted, there is no end of file string.  If neither \ **-E**\  nor \ **-e**\  is used, no end of file string is used.
 


- \ **--embed**\ 
 
 Embed GNU \ **parallel**\  in a shell script.
 
 If you need to distribute your script to someone who does not want to install GNU \ **parallel**\  you can embed GNU \ **parallel**\  in your own shell script:
 
 
 .. code-block:: perl
 
    parallel --embed > new_script
 
 
 After which you add your code at the end of \ **new_script**\ . This is tested on \ **ash**\ , \ **bash**\ , \ **dash**\ , \ **ksh**\ , \ **sh**\ , and \ **zsh**\ .
 


- \ **--env**\  \ *var*\ 
 
 Copy exported environment variable \ *var*\ .
 
 This will copy \ *var*\  to the environment that the command is run in. This is especially useful for remote execution.
 
 In Bash \ *var*\  can also be a Bash function - just remember to \ **export -f**\  the function.
 
 The variable '_' is special. It will copy all exported environment variables except for the ones mentioned in ~/.parallel/ignored_vars.
 
 To copy the full environment (both exported and not exported variables, arrays, and functions) use \ **env_parallel**\ .
 
 See also: \ **--record-env**\  \ **--session**\  \ **--sshlogin**\  \ *command*\  \ **env_parallel**\ 
 


- \ **--eta**\ 
 
 Show the estimated number of seconds before finishing.
 
 This forces GNU \ **parallel**\  to read all jobs before starting to find the number of jobs. GNU \ **parallel**\  normally only reads the next job to run.
 
 The estimate is based on the runtime of finished jobs, so the first estimate will only be shown when the first job has finished.
 
 Implies \ **--progress**\ .
 
 See also: \ **--bar**\  \ **--progress**\ 
 


- \ **--fg**\ 
 
 Run command in foreground.
 
 With \ **--tmux**\  and \ **--tmuxpane**\  GNU \ **parallel**\  will start \ **tmux**\  in the foreground.
 
 With \ **--semaphore**\  GNU \ **parallel**\  will run the command in the foreground (opposite \ **--bg**\ ), and wait for completion of the command before exiting. Exit code will be that of the command.
 
 See also: \ **--bg**\  \ **man sem**\ 
 


- \ **--fifo**\ 
 
 Create a temporary fifo with content.
 
 Normally \ **--pipe**\  and \ **--pipe-part**\  will give data to the program on stdin (standard input). With \ **--fifo**\  GNU \ **parallel**\  will create a temporary fifo with the name in \ **{}**\ , so you can do:
 
 
 .. code-block:: perl
 
    parallel --pipe --fifo wc {}
 
 
 Beware: If the fifo is never opened for reading, the job will block forever:
 
 
 .. code-block:: perl
 
    seq 1000000 | parallel --fifo echo This will block
    seq 1000000 | parallel --fifo 'echo This will not block < {}'
 
 
 Implies \ **--pipe**\  unless \ **--pipe-part**\  is used.
 
 See also: \ **--cat**\  \ **--pipe**\  \ **--pipe-part**\ 
 


- \ **--filter**\  \ *filter*\ 
 
 Only run jobs where \ *filter*\  is true.
 
 \ *filter*\  can contain replacement strings and Perl code. Example:
 
 
 .. code-block:: perl
 
     parallel --filter '{1} < {2}+1' echo ::: {1..3} ::: {1..3}
 
 
 Outputs: 1,1 1,2 1,3 2,2 2,3 3,3
 
 See also: \ **skip()**\  \ **--no-run-if-empty**\ 
 


- \ **--filter-hosts**\ 
 
 Remove down hosts.
 
 For each remote host: check that login through ssh works. If not: do not use this host.
 
 For performance reasons, this check is performed only at the start and every time \ **--sshloginfile**\  is changed. If an host goes down after the first check, it will go undetected until \ **--sshloginfile**\  is changed; \ **--retries**\  can be used to mitigate this.
 
 Currently you can \ *not*\  put \ **--filter-hosts**\  in a profile, $PARALLEL, /etc/parallel/config or similar. This is because GNU \ **parallel**\  uses GNU \ **parallel**\  to compute this, so you will get an infinite loop. This will likely be fixed in a later release.
 
 See also: \ **--sshloginfile**\  \ **--sshlogin**\  \ **--retries**\ 
 


- \ **--gnu**\ 
 
 Behave like GNU \ **parallel**\ .
 
 This option historically took precedence over \ **--tollef**\ . The \ **--tollef**\  option is now retired, and therefore may not be used. \ **--gnu**\  is kept for compatibility.
 


- \ **--group**\ 
 
 Group output.
 
 Output from each job is grouped together and is only printed when the command is finished. Stdout (standard output) first followed by stderr (standard error).
 
 This takes in the order of 0.5ms CPU time per job and depends on the speed of your disk for larger output. It can be disabled with \ **-u**\ , but this means output from different commands can get mixed.
 
 \ **--group**\  is the default. Can be reversed with \ **-u**\ .
 
 See also: \ **--line-buffer**\  \ **--ungroup**\  \ **--tag**\ 
 


- \ **--group-by**\  \ *val*\ 
 
 Group input by value.
 
 Combined with \ **--pipe**\ /\ **--pipe-part**\  \ **--group-by**\  groups lines with the same value into a record.
 
 The value can be computed from the full line or from a single column.
 
 \ *val*\  can be:
 
 
 - column number
  
  Use the value in the column numbered.
  
 
 
 - column name
  
  Treat the first line as a header and use the value in the column named.
  
  (Not supported with \ **--pipe-part**\ ).
  
 
 
 - perl expression
  
  Run the perl expression and use $_ as the value.
  
 
 
 - column number perl expression
  
  Put the value of the column put in $_, run the perl expression, and use $_ as the value.
  
 
 
 - column name perl expression
  
  Put the value of the column put in $_, run the perl expression, and use $_ as the value.
  
  (Not supported with \ **--pipe-part**\ ).
  
 
 
 Example:
 
 
 .. code-block:: perl
 
    UserID, Consumption
    123,    1
    123,    2
    12-3,   1
    221,    3
    221,    1
    2/21,   5
 
 
 If you want to group 123, 12-3, 221, and 2/21 into 4 records and pass one record at a time to \ **wc**\ :
 
 
 .. code-block:: perl
 
    tail -n +2 table.csv | \
      parallel --pipe --colsep , --group-by 1 -kN1 wc
 
 
 Make GNU \ **parallel**\  treat the first line as a header:
 
 
 .. code-block:: perl
 
    cat table.csv | \
      parallel --pipe --colsep , --header : --group-by 1 -kN1 wc
 
 
 Address column by column name:
 
 
 .. code-block:: perl
 
    cat table.csv | \
      parallel --pipe --colsep , --header : --group-by UserID -kN1 wc
 
 
 If 12-3 and 123 are really the same UserID, remove non-digits in UserID when grouping:
 
 
 .. code-block:: perl
 
    cat table.csv | parallel --pipe --colsep , --header : \
      --group-by 'UserID s/\D//g' -kN1 wc
 
 
 See also: SPREADING BLOCKS OF DATA \ **--pipe**\  \ **--pipe-part**\  \ **--bin**\  \ **--shard**\  \ **--round-robin**\ 
 


- \ **--help**\ 



- \ **-h**\ 
 
 Print a summary of the options to GNU \ **parallel**\  and exit.
 


- \ **--halt-on-error**\  \ *val*\ 



- \ **--halt**\  \ *val*\ 
 
 When should GNU \ **parallel**\  terminate?
 
 In some situations it makes no sense to run all jobs. GNU \ **parallel**\  should simply stop as soon as a condition is met.
 
 \ *val*\  defaults to \ **never**\ , which runs all jobs no matter what.
 
 \ *val*\  can also take on the form of \ *when*\ ,\ *why*\ .
 
 \ *when*\  can be 'now' which means kill all running jobs and halt immediately, or it can be 'soon' which means wait for all running jobs to complete, but start no new jobs.
 
 \ *why*\  can be 'fail=X', 'fail=Y%', 'success=X', 'success=Y%', 'done=X', or 'done=Y%' where X is the number of jobs that has to fail, succeed, or be done before halting, and Y is the percentage of jobs that has to fail, succeed, or be done before halting.
 
 Example:
 
 
 - --halt now,fail=1
  
  exit when the first job fails. Kill running jobs.
  
 
 
 - --halt soon,fail=3
  
  exit when 3 jobs fail, but wait for running jobs to complete.
  
 
 
 - --halt soon,fail=3%
  
  exit when 3% of the jobs have failed, but wait for running jobs to complete.
  
 
 
 - --halt now,success=1
  
  exit when a job succeeds. Kill running jobs.
  
 
 
 - --halt soon,success=3
  
  exit when 3 jobs succeeds, but wait for running jobs to complete.
  
 
 
 - --halt now,success=3%
  
  exit when 3% of the jobs have succeeded. Kill running jobs.
  
 
 
 - --halt now,done=1
  
  exit when one of the jobs finishes. Kill running jobs.
  
 
 
 - --halt soon,done=3
  
  exit when 3 jobs finishes, but wait for running jobs to complete.
  
 
 
 - --halt now,done=3%
  
  exit when 3% of the jobs have finished. Kill running jobs.
  
 
 
 For backwards compatibility these also work:
 
 
 - 0
  
  never
  
 
 
 - 1
  
  soon,fail=1
  
 
 
 - 2
  
  now,fail=1
  
 
 
 - -1
  
  soon,success=1
  
 
 
 - -2
  
  now,success=1
  
 
 
 - 1-99%
  
  soon,fail=1-99%
  
 
 


- \ **--header**\  \ *regexp*\ 
 
 Use regexp as header.
 
 For normal usage the matched header (typically the first line: \ **--header '.\*\n'**\ ) will be split using \ **--colsep**\  (which will default to '\t') and column names can be used as replacement variables: \ **{column name}**\ , \ **{column name/}**\ , \ **{column name//}**\ , \ **{column name/.}**\ , \ **{column name.}**\ , \ **{=column name perl expression =}**\ , ..
 
 For \ **--pipe**\  the matched header will be prepended to each output.
 
 \ **--header :**\  is an alias for \ **--header '.\*\n'**\ .
 
 If \ *regexp*\  is a number, it is a fixed number of lines.
 
 See also: \ **--colsep**\  \ **--pipe**\  \ **--pipe-part**\ 
 


- \ **--hostgroups**\ 



- \ **--hgrp**\ 
 
 Enable hostgroups on arguments.
 
 If an argument contains '@' the string after '@' will be removed and treated as a list of hostgroups on which this job is allowed to run. If there is no \ **--sshlogin**\  with a corresponding group, the job will run on any hostgroup.
 
 Example:
 
 
 .. code-block:: perl
 
    parallel --hostgroups \
      --sshlogin @grp1/myserver1 -S @grp1+grp2/myserver2 \
      --sshlogin @grp3/myserver3 \
      echo ::: my_grp1_arg@grp1 arg_for_grp2@grp2 third@grp1+grp3
 
 
 \ **my_grp1_arg**\  may be run on either \ **myserver1**\  or \ **myserver2**\ , \ **third**\  may be run on either \ **myserver1**\  or \ **myserver3**\ , but \ **arg_for_grp2**\  will only be run on \ **myserver2**\ .
 
 See also: \ **--sshlogin**\  \ **$PARALLEL_HOSTGROUPS**\  \ **$PARALLEL_ARGHOSTGROUPS**\ 
 


- \ **-I**\  \ *replace-str*\ 
 
 Use the replacement string \ *replace-str*\  instead of \ **{}**\ .
 
 See also: \ **{}**\ 
 


- \ **--replace**\  [\ *replace-str*\ ]



- \ **-i**\  [\ *replace-str*\ ]
 
 This option is deprecated; use \ **-I**\  instead.
 
 This option is a synonym for \ **-I**\ \ *replace-str*\  if \ *replace-str*\  is specified, and for \ **-I {}**\  otherwise.
 
 See also: \ **{}**\ 
 


- \ **--joblog**\  \ *logfile*\ 
 
 Logfile for executed jobs.
 
 Save a list of the executed jobs to \ *logfile*\  in the following TAB separated format: sequence number, sshlogin, start time as seconds since epoch, run time in seconds, bytes in files transferred, bytes in files returned, exit status, signal, and command run.
 
 For \ **--pipe**\  bytes transferred and bytes returned are number of input and output of bytes.
 
 If \ **logfile**\  is prepended with '+' log lines will be appended to the logfile.
 
 To convert the times into ISO-8601 strict do:
 
 
 .. code-block:: perl
 
    cat logfile | perl -a -F"\t" -ne \
      'chomp($F[2]=`date -d \@$F[2] +%FT%T`); print join("\t",@F)'
 
 
 If the host is long, you can use \ **column -t**\  to pretty print it:
 
 
 .. code-block:: perl
 
    cat joblog | column -t
 
 
 See also: \ **--resume**\  \ **--resume-failed**\ 
 


- \ **--jobs**\  \ *N*\ 



- \ **-j**\  \ *N*\ 



- \ **--max-procs**\  \ *N*\ 



- \ **-P**\  \ *N*\ 
 
 Number of jobslots on each machine.
 
 Run up to N jobs in parallel.  0 means as many as possible (this can take a while to determine). Default is 100% which will run one job per CPU thread on each machine.
 
 Due to a bug \ **-j 0**\  will also evaluate replacement strings twice up to the number of joblots:
 
 
 .. code-block:: perl
 
    # This will not count from 1 but from number-of-jobslots
    seq 10000 | parallel -j0   echo '{= $_ = $foo++; =}' | head
    # This will count from 1
    seq 10000 | parallel -j100 echo '{= $_ = $foo++; =}' | head
 
 
 If \ **--semaphore**\  is set, the default is 1 thus making a mutex.
 
 See also: \ **--use-cores-instead-of-threads**\  \ **--use-sockets-instead-of-threads**\ 
 


- \ **--jobs**\  \ *+N*\ 



- \ **-j**\  \ *+N*\ 



- \ **--max-procs**\  \ *+N*\ 



- \ **-P**\  \ *+N*\ 
 
 Add N to the number of CPU threads.
 
 Run this many jobs in parallel.
 
 See also: \ **--number-of-threads**\  \ **--number-of-cores**\  \ **--number-of-sockets**\ 
 


- \ **--jobs**\  \ *-N*\ 



- \ **-j**\  \ *-N*\ 



- \ **--max-procs**\  \ *-N*\ 



- \ **-P**\  \ *-N*\ 
 
 Subtract N from the number of CPU threads.
 
 Run this many jobs in parallel.  If the evaluated number is less than 1 then 1 will be used.
 
 See also: \ **--number-of-threads**\  \ **--number-of-cores**\  \ **--number-of-sockets**\ 
 


- \ **--jobs**\  \ *N*\ %



- \ **-j**\  \ *N*\ %



- \ **--max-procs**\  \ *N*\ %



- \ **-P**\  \ *N*\ %
 
 Multiply N% with the number of CPU threads.
 
 Run this many jobs in parallel.
 
 See also: \ **--number-of-threads**\  \ **--number-of-cores**\  \ **--number-of-sockets**\ 
 


- \ **--jobs**\  \ *procfile*\ 



- \ **-j**\  \ *procfile*\ 



- \ **--max-procs**\  \ *procfile*\ 



- \ **-P**\  \ *procfile*\ 
 
 Read parameter from file.
 
 Use the content of \ *procfile*\  as parameter for \ *-j*\ . E.g. \ *procfile*\  could contain the string 100% or +2 or 10. If \ *procfile*\  is changed when a job completes, \ *procfile*\  is read again and the new number of jobs is computed. If the number is lower than before, running jobs will be allowed to finish but new jobs will not be started until the wanted number of jobs has been reached.  This makes it possible to change the number of simultaneous running jobs while GNU \ **parallel**\  is running.
 


- \ **--keep-order**\ 



- \ **-k**\ 
 
 Keep sequence of output same as the order of input.
 
 Normally the output of a job will be printed as soon as the job completes. Try this to see the difference:
 
 
 .. code-block:: perl
 
    parallel -j4 sleep {}\; echo {} ::: 2 1 4 3
    parallel -j4 -k sleep {}\; echo {} ::: 2 1 4 3
 
 
 If used with \ **--onall**\  or \ **--nonall**\  the output will grouped by sshlogin in sorted order.
 
 \ **--keep-order**\  cannot keep the output order when used with \ **--pipe --round-robin**\ . Here it instead means, that the jobslots will get the same blocks as input in the same order in every run if the input is kept the same. Run each of these twice and compare:
 
 
 .. code-block:: perl
 
    seq 10000000 | parallel --pipe --round-robin 'sleep 0.$RANDOM; wc'
    seq 10000000 | parallel --pipe -k --round-robin 'sleep 0.$RANDOM; wc'
 
 
 \ **-k**\  only affects the order in which the output is printed - not the order in which jobs are run.
 
 See also: \ **--group**\  \ **--line-buffer**\ 
 


- \ **-L**\  \ *recsize*\ 
 
 When used with \ **--pipe**\ : Read records of \ *recsize*\ .
 
 When used otherwise: Use at most \ *recsize*\  nonblank input lines per command line.  Trailing blanks cause an input line to be logically continued on the next input line.
 
 \ **-L 0**\  means read one line, but insert 0 arguments on the command line.
 
 \ *recsize*\  can be postfixed with K, M, G, T, P, k, m, g, t, or p.
 
 Implies \ **-X**\  unless \ **-m**\ , \ **--xargs**\ , or \ **--pipe**\  is set.
 
 See also: UNIT PREFIX \ **-N**\  \ **--max-lines**\  \ **--block**\  \ **-X**\  \ **-m**\  \ **--xargs**\  \ **--pipe**\ 
 


- \ **--max-lines**\  [\ *recsize*\ ]



- \ **-l**\ [\ *recsize*\ ]
 
 When used with \ **--pipe**\ : Read records of \ *recsize*\  lines.
 
 When used otherwise: Synonym for the \ **-L**\  option.  Unlike \ **-L**\ , the \ *recsize*\  argument is optional.  If \ *recsize*\  is not specified, it defaults to one.  The \ **-l**\  option is deprecated since the POSIX standard specifies \ **-L**\  instead.
 
 \ **-l 0**\  is an alias for \ **-l 1**\ .
 
 Implies \ **-X**\  unless \ **-m**\ , \ **--xargs**\ , or \ **--pipe**\  is set.
 
 See also: UNIT PREFIX \ **-N**\  \ **--block**\  \ **-X**\  \ **-m**\  \ **--xargs**\  \ **--pipe**\ 
 


- \ **--limit**\  "\ *command*\  \ *args*\ "
 
 Dynamic job limit.
 
 Before starting a new job run \ *command*\  with \ *args*\ . The exit value of \ *command*\  determines what GNU \ **parallel**\  will do:
 
 
 - 0
  
  Below limit. Start another job.
  
 
 
 - 1
  
  Over limit. Start no jobs.
  
 
 
 - 2
  
  Way over limit. Kill the youngest job.
  
 
 
 You can use any shell command. There are 3 predefined commands:
 
 
 - "io \ *n*\ "
  
  Limit for I/O. The amount of disk I/O will be computed as a value 0-100, where 0 is no I/O and 100 is at least one disk is 100% saturated.
  
 
 
 - "load \ *n*\ "
  
  Similar to \ **--load**\ .
  
 
 
 - "mem \ *n*\ "
  
  Similar to \ **--memfree**\ .
  
 
 
 See also: \ **--memfree**\  \ **--load**\ 
 


- \ **--latest-line**\  (alpha testing)



- \ **--ll**\  (alpha testing)
 
 Print the lastest line of each running job.
 
 This only works if the currently running jobs fit on the screen.
 
 Example:
 
 
 .. code-block:: perl
 
    slow_seq() {
      seq "$@" |
        perl -ne '$|=1; for(split//){ print; select($a,$a,$a,0.03);}'
    }
    export -f slow_seq
    parallel --shuf -j10 --ll --tag --bar --color slow_seq {} ::: {1..100}
 
 
 See also: \ **--line-buffer**\ 
 


- \ **--line-buffer**\  (beta testing)



- \ **--lb**\  (beta testing)
 
 Buffer output on line basis.
 
 \ **--group**\  will keep the output together for a whole job. \ **--ungroup**\  allows output to mixup with half a line coming from one job and half a line coming from another job. \ **--line-buffer**\  fits between these two: GNU \ **parallel**\  will print a full line, but will allow for mixing lines of different jobs.
 
 \ **--line-buffer**\  takes more CPU power than both \ **--group**\  and \ **--ungroup**\ , but can be much faster than \ **--group**\  if the CPU is not the limiting factor.
 
 Normally \ **--line-buffer**\  does not buffer on disk, and can thus process an infinite amount of data, but it will buffer on disk when combined with: \ **--keep-order**\ , \ **--results**\ , \ **--compress**\ , and \ **--files**\ . This will make it as slow as \ **--group**\  and will limit output to the available disk space.
 
 With \ **--keep-order**\  \ **--line-buffer**\  will output lines from the first job continuously while it is running, then lines from the second job while that is running. It will buffer full lines, but jobs will not mix. Compare:
 
 
 .. code-block:: perl
 
    parallel -j0 'echo {};sleep {};echo {}' ::: 1 3 2 4
    parallel -j0 --lb 'echo {};sleep {};echo {}' ::: 1 3 2 4
    parallel -j0 -k --lb 'echo {};sleep {};echo {}' ::: 1 3 2 4
 
 
 See also: \ **--group**\  \ **--ungroup**\  \ **--keep-order**\  \ **--tag**\ 
 


- \ **--link**\ 



- \ **--xapply**\ 
 
 Link input sources.
 
 Read multiple input sources like the command \ **xapply**\ . If multiple input sources are given, one argument will be read from each of the input sources. The arguments can be accessed in the command as \ **{1}**\  .. \ **{**\ \ *n*\ \ **}**\ , so \ **{1}**\  will be a line from the first input source, and \ **{6}**\  will refer to the line with the same line number from the 6th input source.
 
 Compare these two:
 
 
 .. code-block:: perl
 
    parallel echo {1} {2} ::: 1 2 3 ::: a b c
    parallel --link echo {1} {2} ::: 1 2 3 ::: a b c
 
 
 Arguments will be recycled if one input source has more arguments than the others:
 
 
 .. code-block:: perl
 
    parallel --link echo {1} {2} {3} \
      ::: 1 2 ::: I II III ::: a b c d e f g
 
 
 See also: \ **--header**\  \ **:::+**\  \ **::::+**\ 
 


- \ **--load**\  \ *max-load*\ 
 
 Only start jobs if load is less than max-load.
 
 Do not start new jobs on a given computer unless the number of running processes on the computer is less than \ *max-load*\ . \ *max-load*\  uses the same syntax as \ **--jobs**\ , so \ *100%*\  for one per CPU is a valid setting. Only difference is 0 which is interpreted as 0.01.
 
 See also: \ **--limit**\  \ **--jobs**\ 
 


- \ **--controlmaster**\ 



- \ **-M**\ 
 
 Use ssh's ControlMaster to make ssh connections faster.
 
 Useful if jobs run remote and are very fast to run. This is disabled for sshlogins that specify their own ssh command.
 
 See also: \ **--ssh**\  \ **--sshlogin**\ 
 


- \ **-m**\ 
 
 Multiple arguments.
 
 Insert as many arguments as the command line length permits. If multiple jobs are being run in parallel: distribute the arguments evenly among the jobs. Use \ **-j1**\  or \ **--xargs**\  to avoid this.
 
 If \ **{}**\  is not used the arguments will be appended to the line.  If \ **{}**\  is used multiple times each \ **{}**\  will be replaced with all the arguments.
 
 Support for \ **-m**\  with \ **--sshlogin**\  is limited and may fail.
 
 If in doubt use \ **-X**\  as that will most likely do what is needed.
 
 See also: \ **-X**\  \ **--xargs**\ 
 


- \ **--memfree**\  \ *size*\ 
 
 Minimum memory free when starting another job.
 
 The \ *size*\  can be postfixed with K, M, G, T, P, k, m, g, t, or p.
 
 If the jobs take up very different amount of RAM, GNU \ **parallel**\  will only start as many as there is memory for. If less than \ *size*\  bytes are free, no more jobs will be started. If less than 50% \ *size*\  bytes are free, the youngest job will be killed (as per \ **--term-seq**\ ), and put back on the queue to be run later.
 
 \ **--retries**\  must be set to determine how many times GNU \ **parallel**\  should retry a given job.
 
 See also: UNIT PREFIX \ **--term-seq**\  \ **--retries**\  \ **--memsuspend**\ 
 


- \ **--memsuspend**\  \ *size*\ 
 
 Suspend jobs when there is less memory available.
 
 If the available memory falls below 2 \* \ *size*\ , GNU \ **parallel**\  will suspend some of the running jobs. If the available memory falls below \ *size*\ , only one job will be running.
 
 If a single job takes up at most \ *size*\  RAM, all jobs will complete without running out of memory. If you have swap available, you can usually lower \ *size*\  to around half the size of a single job - with the slight risk of swapping a little.
 
 Jobs will be resumed when more RAM is available - typically when the oldest job completes.
 
 \ **--memsuspend**\  only works on local jobs because there is no obvious way to suspend remote jobs.
 
 \ *size*\  can be postfixed with K, M, G, T, P, k, m, g, t, or p.
 
 See also: UNIT PREFIX \ **--memfree**\ 
 


- \ **--minversion**\  \ *version*\ 
 
 Print the version GNU \ **parallel**\  and exit.
 
 If the current version of GNU \ **parallel**\  is less than \ *version*\  the exit code is 255. Otherwise it is 0.
 
 This is useful for scripts that depend on features only available from a certain version of GNU \ **parallel**\ :
 
 
 .. code-block:: perl
 
     parallel --minversion 20170422 &&
       echo halt done=50% supported from version 20170422 &&
       parallel --halt now,done=50% echo ::: {1..100}
 
 
 See also: \ **--version**\ 
 


- \ **--max-args**\  \ *max-args*\ 



- \ **-n**\  \ *max-args*\ 
 
 Use at most \ *max-args*\  arguments per command line.
 
 Fewer than \ *max-args*\  arguments will be used if the size (see the \ **-s**\  option) is exceeded, unless the \ **-x**\  option is given, in which case GNU \ **parallel**\  will exit.
 
 \ **-n 0**\  means read one argument, but insert 0 arguments on the command line.
 
 \ *max-args*\  can be postfixed with K, M, G, T, P, k, m, g, t, or p (see UNIT PREFIX).
 
 Implies \ **-X**\  unless \ **-m**\  is set.
 
 See also: \ **-X**\  \ **-m**\  \ **--xargs**\  \ **--max-replace-args**\ 
 


- \ **--max-replace-args**\  \ *max-args*\ 



- \ **-N**\  \ *max-args*\ 
 
 Use at most \ *max-args*\  arguments per command line.
 
 Like \ **-n**\  but also makes replacement strings \ **{1}**\  .. \ **{**\ \ *max-args*\ \ **}**\  that represents argument 1 .. \ *max-args*\ . If too few args the \ **{**\ \ *n*\ \ **}**\  will be empty.
 
 \ **-N 0**\  means read one argument, but insert 0 arguments on the command line.
 
 This will set the owner of the homedir to the user:
 
 
 .. code-block:: perl
 
    tr ':' '\n' < /etc/passwd | parallel -N7 chown {1} {6}
 
 
 Implies \ **-X**\  unless \ **-m**\  or \ **--pipe**\  is set.
 
 \ *max-args*\  can be postfixed with K, M, G, T, P, k, m, g, t, or p.
 
 When used with \ **--pipe**\  \ **-N**\  is the number of records to read. This is somewhat slower than \ **--block**\ .
 
 See also: UNIT PREFIX \ **--pipe**\  \ **--block**\  \ **-m**\  \ **-X**\  \ **--max-args**\ 
 


- \ **--nonall**\ 
 
 \ **--onall**\  with no arguments.
 
 Run the command on all computers given with \ **--sshlogin**\  but take no arguments. GNU \ **parallel**\  will log into \ **--jobs**\  number of computers in parallel and run the job on the computer. \ **-j**\  adjusts how many computers to log into in parallel.
 
 This is useful for running the same command (e.g. uptime) on a list of servers.
 
 See also: \ **--onall**\  \ **--sshlogin**\ 
 


- \ **--onall**\ 
 
 Run all the jobs on all computers given with \ **--sshlogin**\ .
 
 GNU \ **parallel**\  will log into \ **--jobs**\  number of computers in parallel and run one job at a time on the computer. The order of the jobs will not be changed, but some computers may finish before others.
 
 When using \ **--group**\  the output will be grouped by each server, so all the output from one server will be grouped together.
 
 \ **--joblog**\  will contain an entry for each job on each server, so there will be several job sequence 1.
 
 See also: \ **--nonall**\  \ **--sshlogin**\ 
 


- \ **--open-tty**\  (beta testing)



- \ **-o**\  (beta testing)
 
 Open terminal tty.
 
 Similar to \ **--tty**\  but does not set \ **--jobs**\  or \ **--ungroup**\ .
 
 See also: \ **--tty**\ 
 


- \ **--output-as-files**\ 



- \ **--outputasfiles**\ 



- \ **--files**\ 
 
 Save output to files.
 
 Instead of printing the output to stdout (standard output) the output of each job is saved in a file and the filename is then printed.
 
 See also: \ **--results**\ 
 


- \ **--pipe**\ 



- \ **--spreadstdin**\ 
 
 Spread input to jobs on stdin (standard input).
 
 Read a block of data from stdin (standard input) and give one block of data as input to one job.
 
 The block size is determined by \ **--block**\  (default: 1M). The strings \ **--recstart**\  and \ **--recend**\  tell GNU \ **parallel**\  how a record starts and/or ends. The block read will have the final partial record removed before the block is passed on to the job. The partial record will be prepended to next block.
 
 You can limit the number of records to be passed with \ **-N**\ , and set the record size with \ **-L**\ .
 
 \ **--pipe**\  maxes out at around 1 GB/s input, and 100 MB/s output. If performance is important use \ **--pipe-part**\ .
 
 \ **--fifo**\  and \ **--cat**\  will give stdin (standard input) on a fifo or a temporary file.
 
 If data is arriving slowly, you can use \ **--block-timeout**\  to finish reading a block early.
 
 The data can be spread between the jobs in specific ways using \ **--round-robin**\ , \ **--bin**\ , \ **--shard**\ , \ **--group-by**\ . See the section: SPREADING BLOCKS OF DATA
 
 See also: \ **--block**\  \ **--block-timeout**\  \ **--recstart**\  \ **--recend**\  \ **--fifo**\  \ **--cat**\  \ **--pipe-part**\  \ **-N**\  \ **-L**\  \ **--round-robin**\ 
 


- \ **--pipe-part**\ 
 
 Pipe parts of a physical file.
 
 \ **--pipe-part**\  works similar to \ **--pipe**\ , but is much faster.
 
 \ **--pipe-part**\  has a few limitations:
 
 
 - 
  
  The file must be a normal file or a block device (technically it must be seekable) and must be given using \ **--arg-file**\  or \ **::::**\ . The file cannot be a pipe, a fifo, or a stream as they are not seekable.
  
  If using a block device with lot of NUL bytes, remember to set \ **--recend ''**\ .
  
 
 
 - 
  
  Record counting (\ **-N**\ ) and line counting (\ **-L**\ /\ **-l**\ ) do not work. Instead use \ **--recstart**\  and \ **--recend**\  to determine where records end.
  
 
 
 See also: \ **--pipe**\  \ **--recstart**\  \ **--recend**\  \ **--arg-file**\  \ **::::**\ 
 


- \ **--plain**\ 
 
 Ignore \ **--profile**\ , $PARALLEL, and ~/.parallel/config.
 
 Ignore any \ **--profile**\ , $PARALLEL, and ~/.parallel/config to get full control on the command line (used by GNU \ **parallel**\  internally when called with \ **--sshlogin**\ ).
 
 See also: \ **--profile**\ 
 


- \ **--plus**\  (alpha testing)
 
 Add more replacement strings.
 
 Activate additional replacement strings: {+/} {+.} {+..} {+...} {..} {...} {/..} {/...} {##}. The idea being that '{+foo}' matches the opposite of '{foo}' and {} = {+/}/{/} = {.}.{+.} = {+/}/{/.}.{+.} = {..}.{+..} = {+/}/{/..}.{+..} = {...}.{+...} = {+/}/{/...}.{+...}
 
 \ **{##}**\  is the total number of jobs to be run. It is incompatible with \ **-X**\ /\ **-m**\ /\ **--xargs**\ .
 
 \ **{0%}**\  zero-padded jobslot.
 
 \ **{0#}**\  zero-padded sequence number.
 
 \ **{choose_k}**\  is inspired by n choose k: Given a list of n elements, choose k. k is the number of input sources and n is the number of arguments in an input source.  The content of the input sources must be the same and the arguments must be unique.
 
 \ **{uniq}**\  skips jobs where values from two input sources are the same.
 
 Shorthands for variables:
 
 
 .. code-block:: perl
 
    {slot}         $PARALLEL_JOBSLOT (see {%})
    {sshlogin}     $PARALLEL_SSHLOGIN
    {host}         $PARALLEL_SSHHOST
    {agrp}         $PARALLEL_ARGHOSTGROUPS
    {hgrp}         $PARALLEL_HOSTGROUPS
 
 
 The following dynamic replacement strings are also activated. They are inspired by bash's parameter expansion:
 
 
 .. code-block:: perl
 
    {:-str}        str if the value is empty
    {:num}         remove the first num characters
    {:pos:len}     substring from position pos length len
    {#regexp}      remove prefix regexp (non-greedy)
    {##regexp}     remove prefix regexp (greedy)
    {%regexp}      remove postfix regexp (non-greedy)
    {%%regexp}     remove postfix regexp (greedy)
    {/regexp/str}  replace one regexp with str
    {//regexp/str} replace every regexp with str
    {^str}         uppercase str if found at the start
    {^^str}        uppercase str
    {,str}         lowercase str if found at the start
    {,,str}        lowercase str
 
 
 See also: \ **--rpl**\  \ **{}**\ 
 


- \ **--process-slot-var**\  \ *varname*\  (beta testing)
 
 Set the environment variable \ *varname*\  to the jobslot number-1.
 
 
 .. code-block:: perl
 
    seq 10 | parallel --process-slot-var=name echo '$name' {}
 
 


- \ **--progress**\ 
 
 Show progress of computations.
 
 List the computers involved in the task with number of CPUs detected and the max number of jobs to run. After that show progress for each computer: number of running jobs, number of completed jobs, and percentage of all jobs done by this computer. The percentage will only be available after all jobs have been scheduled as GNU \ **parallel**\  only read the next job when ready to schedule it - this is to avoid wasting time and memory by reading everything at startup.
 
 By sending GNU \ **parallel**\  SIGUSR2 you can toggle turning on/off \ **--progress**\  on a running GNU \ **parallel**\  process.
 
 See also: \ **--eta**\  \ **--bar**\ 
 


- \ **--max-line-length-allowed**\ 
 
 Print maximal command line length.
 
 Print the maximal number of characters allowed on the command line and exit (used by GNU \ **parallel**\  itself to determine the line length on remote computers).
 
 See also: \ **--show-limits**\ 
 


- \ **--number-of-cpus**\  (obsolete)
 
 Print the number of physical CPU cores and exit.
 


- \ **--number-of-cores**\ 
 
 Print the number of physical CPU cores and exit (used by GNU \ **parallel**\  itself to determine the number of physical CPU cores on remote computers).
 
 See also: \ **--number-of-sockets**\  \ **--number-of-threads**\  \ **--use-cores-instead-of-threads**\  \ **--jobs**\ 
 


- \ **--number-of-sockets**\ 
 
 Print the number of filled CPU sockets and exit (used by GNU \ **parallel**\  itself to determine the number of filled CPU sockets on remote computers).
 
 See also: \ **--number-of-cores**\  \ **--number-of-threads**\  \ **--use-sockets-instead-of-threads**\  \ **--jobs**\ 
 


- \ **--number-of-threads**\ 
 
 Print the number of hyperthreaded CPU cores and exit (used by GNU \ **parallel**\  itself to determine the number of hyperthreaded CPU cores on remote computers).
 
 See also: \ **--number-of-cores**\  \ **--number-of-sockets**\  \ **--jobs**\ 
 


- \ **--no-keep-order**\ 
 
 Overrides an earlier \ **--keep-order**\  (e.g. if set in \ **~/.parallel/config**\ ).
 


- \ **--nice**\  \ *niceness*\ 
 
 Run the command at this niceness.
 
 By default GNU \ **parallel**\  will run jobs at the same nice level as GNU \ **parallel**\  is started - both on the local machine and remote servers, so you are unlikely to ever use this option.
 
 Setting \ **--nice**\  will override this nice level. If the nice level is smaller than the current nice level, it will only affect remote jobs (e.g. if current level is 10 then \ **--nice 5**\  will cause local jobs to be run at level 10, but remote jobs run at nice level 5).
 


- \ **--interactive**\ 



- \ **-p**\ 
 
 Ask user before running a job.
 
 Prompt the user about whether to run each command line and read a line from the terminal.  Only run the command line if the response starts with 'y' or 'Y'.  Implies \ **-t**\ .
 


- \ **--_parset**\  \ *type*\ ,\ *varname*\ 
 
 Used internally by \ **parset**\ .
 
 Generate shell code to be eval'ed which will set the variable(s) \ *varname*\ . \ *type*\  can be 'assoc' for associative array or 'var' for normal variables.
 
 The only supported use is as part of \ **parset**\ .
 


- \ **--parens**\  \ *parensstring*\ 
 
 Use \ *parensstring*\  instead of \ **{==}**\ .
 
 Define start and end parenthesis for \ **{=perl expression=}**\ . The left and the right parenthesis can be multiple characters and are assumed to be the same length. The default is \ **{==}**\  giving \ **{=**\  as the start parenthesis and \ **=}**\  as the end parenthesis.
 
 Another useful setting is \ **,,,,**\  which would make both parenthesis \ **,,**\ :
 
 
 .. code-block:: perl
 
    parallel --parens ,,,, echo foo is ,,s/I/O/g,, ::: FII
 
 
 See also: \ **--rpl**\  \ **{=**\ \ *perl expression*\ \ **=}**\ 
 


- \ **--profile**\  \ *profilename*\ 



- \ **-J**\  \ *profilename*\ 
 
 Use profile \ *profilename*\  for options.
 
 This is useful if you want to have multiple profiles. You could have one profile for running jobs in parallel on the local computer and a different profile for running jobs on remote computers.
 
 \ *profilename*\  corresponds to the file ~/.parallel/\ *profilename*\ .
 
 You can give multiple profiles by repeating \ **--profile**\ . If parts of the profiles conflict, the later ones will be used.
 
 Default: ~/.parallel/config
 
 See also: PROFILE FILES
 


- \ **--quote**\ 



- \ **-q**\ 
 
 Quote \ *command*\ .
 
 If your command contains special characters that should not be interpreted by the shell (e.g. ; \ | \*), use \ **--quote**\  to escape these. The command must be a simple command (see \ **man bash**\ ) without redirections and without variable assignments.
 
 Most people will not need this. Quoting is disabled by default.
 
 See also: QUOTING \ *command*\  \ **--shell-quote**\  \ **uq()**\  \ **Q()**\ 
 


- \ **--no-run-if-empty**\ 



- \ **-r**\ 
 
 Do not run empty input.
 
 If the stdin (standard input) only contains whitespace, do not run the command.
 
 If used with \ **--pipe**\  this is slow.
 
 See also: \ *command*\  \ **--pipe**\  \ **--interactive**\ 
 


- \ **--noswap**\ 
 
 Do not start job is computer is swapping.
 
 Do not start new jobs on a given computer if there is both swap-in and swap-out activity.
 
 The swap activity is only sampled every 10 seconds as the sampling takes 1 second to do.
 
 Swap activity is computed as (swap-in)\*(swap-out) which in practice is a good value: swapping out is not a problem, swapping in is not a problem, but both swapping in and out usually indicates a problem.
 
 \ **--memfree**\  and \ **--memsuspend**\  may give better results, so try using those first.
 
 See also: \ **--memfree**\  \ **--memsuspend**\ 
 


- \ **--record-env**\ 
 
 Record exported environment.
 
 Record current exported environment variables in \ **~/.parallel/ignored_vars**\ .  This will ignore variables currently set when using \ **--env _**\ . So you should set the variables/fuctions, you want to use \ *after*\  running \ **--record-env**\ .
 
 See also: \ **--env**\  \ **--session**\  \ **env_parallel**\ 
 


- \ **--recstart**\  \ *startstring*\ 



- \ **--recend**\  \ *endstring*\ 
 
 Split record between \ *endstring*\  and \ *startstring*\ .
 
 If \ **--recstart**\  is given \ *startstring*\  will be used to split at record start.
 
 If \ **--recend**\  is given \ *endstring*\  will be used to split at record end.
 
 If both \ **--recstart**\  and \ **--recend**\  are given the combined string \ *endstring*\ \ *startstring*\  will have to match to find a split position. This is useful if either \ *startstring*\  or \ *endstring*\  match in the middle of a record.
 
 If neither \ **--recstart**\  nor \ **--recend**\  are given, then \ **--recend**\  defaults to '\n'. To have no record separator (e.g. for binary files) use \ **--recend ""**\ .
 
 \ **--recstart**\  and \ **--recend**\  are used with \ **--pipe**\ .
 
 Use \ **--regexp**\  to interpret \ **--recstart**\  and \ **--recend**\  as regular expressions. This is slow, however.
 
 Use \ **--remove-rec-sep**\  to remove \ **--recstart**\  and \ **--recend**\  before passing the block to the job.
 
 See also: \ **--pipe**\  \ **--regexp**\  \ **--remove-rec-sep**\ 
 


- \ **--regexp**\ 
 
 Use \ **--regexp**\  to interpret \ **--recstart**\  and \ **--recend**\  as regular expressions. This is slow, however.
 
 See also: \ **--pipe**\  \ **--regexp**\  \ **--remove-rec-sep**\  \ **--recstart**\  \ **--recend**\ 
 


- \ **--remove-rec-sep**\ 



- \ **--removerecsep**\ 



- \ **--rrs**\ 
 
 Remove record separator.
 
 Remove the text matched by \ **--recstart**\  and \ **--recend**\  before piping it to the command.
 
 Only used with \ **--pipe**\ /\ **--pipe-part**\ .
 
 See also: \ **--pipe**\  \ **--regexp**\  \ **--pipe-part**\  \ **--recstart**\  \ **--recend**\ 
 


- \ **--results**\  \ *name*\ 



- \ **--res**\  \ *name*\ 
 
 Save the output into files.
 
 \ **Simple string output dir**\ 
 
 If \ *name*\  does not contain replacement strings and does not end in \ **.csv/.tsv**\ , the output will be stored in a directory tree rooted at \ *name*\ .  Within this directory tree, each command will result in three files: \ *name*\ /<ARGS>/stdout and \ *name*\ /<ARGS>/stderr, \ *name*\ /<ARGS>/seq, where <ARGS> is a sequence of directories representing the header of the input source (if using \ **--header :**\ ) or the number of the input source and corresponding values.
 
 E.g:
 
 
 .. code-block:: perl
 
    parallel --header : --results foo echo {a} {b} \
      ::: a I II ::: b III IIII
 
 
 will generate the files:
 
 
 .. code-block:: perl
 
    foo/a/II/b/III/seq
    foo/a/II/b/III/stderr
    foo/a/II/b/III/stdout
    foo/a/II/b/IIII/seq
    foo/a/II/b/IIII/stderr
    foo/a/II/b/IIII/stdout
    foo/a/I/b/III/seq
    foo/a/I/b/III/stderr
    foo/a/I/b/III/stdout
    foo/a/I/b/IIII/seq
    foo/a/I/b/IIII/stderr
    foo/a/I/b/IIII/stdout
 
 
 and
 
 
 .. code-block:: perl
 
    parallel --results foo echo {1} {2} ::: I II ::: III IIII
 
 
 will generate the files:
 
 
 .. code-block:: perl
 
    foo/1/II/2/III/seq
    foo/1/II/2/III/stderr
    foo/1/II/2/III/stdout
    foo/1/II/2/IIII/seq
    foo/1/II/2/IIII/stderr
    foo/1/II/2/IIII/stdout
    foo/1/I/2/III/seq
    foo/1/I/2/III/stderr
    foo/1/I/2/III/stdout
    foo/1/I/2/IIII/seq
    foo/1/I/2/IIII/stderr
    foo/1/I/2/IIII/stdout
 
 
 \ **CSV file output**\ 
 
 If \ *name*\  ends in \ **.csv**\ /\ **.tsv**\  the output will be a CSV-file named \ *name*\ .
 
 \ **.csv**\  gives a comma separated value file. \ **.tsv**\  gives a TAB separated value file.
 
 \ **-.csv**\ /\ **-.tsv**\  are special: It will give the file on stdout (standard output).
 
 \ **JSON file output**\ 
 
 If \ *name*\  ends in \ **.json**\  the output will be a JSON-file named \ *name*\ .
 
 \ **-.json**\  is special: It will give the file on stdout (standard output).
 
 \ **Replacement string output file**\ 
 
 If \ *name*\  contains a replacement string and the replaced result does not end in /, then the standard output will be stored in a file named by this result. Standard error will be stored in the same file name with '.err' added, and the sequence number will be stored in the same file name with '.seq' added.
 
 E.g.
 
 
 .. code-block:: perl
 
    parallel --results my_{} echo ::: foo bar baz
 
 
 will generate the files:
 
 
 .. code-block:: perl
 
    my_bar
    my_bar.err
    my_bar.seq
    my_baz
    my_baz.err
    my_baz.seq
    my_foo
    my_foo.err
    my_foo.seq
 
 
 \ **Replacement string output dir**\ 
 
 If \ *name*\  contains a replacement string and the replaced result ends in /, then output files will be stored in the resulting dir.
 
 E.g.
 
 
 .. code-block:: perl
 
    parallel --results my_{}/ echo ::: foo bar baz
 
 
 will generate the files:
 
 
 .. code-block:: perl
 
    my_bar/seq
    my_bar/stderr
    my_bar/stdout
    my_baz/seq
    my_baz/stderr
    my_baz/stdout
    my_foo/seq
    my_foo/stderr
    my_foo/stdout
 
 
 See also: \ **--output-as-files**\  \ **--tag**\  \ **--header**\  \ **--joblog**\ 
 


- \ **--resume**\ 
 
 Resumes from the last unfinished job.
 
 By reading \ **--joblog**\  or the \ **--results**\  dir GNU \ **parallel**\  will figure out the last unfinished job and continue from there. As GNU \ **parallel**\  only looks at the sequence numbers in \ **--joblog**\  then the input, the command, and \ **--joblog**\  all have to remain unchanged; otherwise GNU \ **parallel**\  may run wrong commands.
 
 See also: \ **--joblog**\  \ **--results**\  \ **--resume-failed**\  \ **--retries**\ 
 


- \ **--resume-failed**\ 
 
 Retry all failed and resume from the last unfinished job.
 
 By reading \ **--joblog**\  GNU \ **parallel**\  will figure out the failed jobs and run those again. After that it will resume last unfinished job and continue from there. As GNU \ **parallel**\  only looks at the sequence numbers in \ **--joblog**\  then the input, the command, and \ **--joblog**\  all have to remain unchanged; otherwise GNU \ **parallel**\  may run wrong commands.
 
 See also: \ **--joblog**\  \ **--resume**\  \ **--retry-failed**\  \ **--retries**\ 
 


- \ **--retry-failed**\ 
 
 Retry all failed jobs in joblog.
 
 By reading \ **--joblog**\  GNU \ **parallel**\  will figure out the failed jobs and run those again.
 
 \ **--retry-failed**\  ignores the command and arguments on the command line: It only looks at the joblog.
 
 \ **Differences between --resume, --resume-failed, --retry-failed**\ 
 
 In this example \ **exit {= $_%=2 =}**\  will cause every other job to fail.
 
 
 .. code-block:: perl
 
    timeout -k 1 4 parallel --joblog log -j10 \
      'sleep {}; exit {= $_%=2 =}' ::: {10..1}
 
 
 4 jobs completed. 2 failed:
 
 
 .. code-block:: perl
 
    Seq	[...]	Exitval	Signal	Command
    10	[...]	1	0	sleep 1; exit 1
    9	[...]	0	0	sleep 2; exit 0
    8	[...]	1	0	sleep 3; exit 1
    7	[...]	0	0	sleep 4; exit 0
 
 
 \ **--resume**\  does not care about the Exitval, but only looks at Seq. If the Seq is run, it will not be run again. So if needed, you can change the command for the seqs not run yet:
 
 
 .. code-block:: perl
 
    parallel --resume --joblog log -j10 \
      'sleep .{}; exit {= $_%=2 =}' ::: {10..1}
  
    Seq	[...]	Exitval	Signal	Command
    [... as above ...]
    1	[...]	0	0	sleep .10; exit 0
    6	[...]	1	0	sleep .5; exit 1
    5	[...]	0	0	sleep .6; exit 0
    4	[...]	1	0	sleep .7; exit 1
    3	[...]	0	0	sleep .8; exit 0
    2	[...]	1	0	sleep .9; exit 1
 
 
 \ **--resume-failed**\  cares about the Exitval, but also only looks at Seq to figure out which commands to run. Again this means you can change the command, but not the arguments. It will run the failed seqs and the seqs not yet run:
 
 
 .. code-block:: perl
 
    parallel --resume-failed --joblog log -j10 \
      'echo {};sleep .{}; exit {= $_%=3 =}' ::: {10..1}
  
    Seq	[...]	Exitval	Signal	Command
    [... as above ...]
    10	[...]	1	0	echo 1;sleep .1; exit 1
    8	[...]	0	0	echo 3;sleep .3; exit 0
    6	[...]	2	0	echo 5;sleep .5; exit 2
    4	[...]	1	0	echo 7;sleep .7; exit 1
    2	[...]	0	0	echo 9;sleep .9; exit 0
 
 
 \ **--retry-failed**\  cares about the Exitval, but takes the command from the joblog. It ignores any arguments or commands given on the command line:
 
 
 .. code-block:: perl
 
    parallel --retry-failed --joblog log -j10 this part is ignored
  
    Seq	[...]	Exitval	Signal	Command
    [... as above ...]
    10	[...]	1	0	echo 1;sleep .1; exit 1
    6	[...]	2	0	echo 5;sleep .5; exit 2
    4	[...]	1	0	echo 7;sleep .7; exit 1
 
 
 See also: \ **--joblog**\  \ **--resume**\  \ **--resume-failed**\  \ **--retries**\ 
 


- \ **--retries**\  \ *n*\ 
 
 Try failing jobs \ *n*\  times.
 
 If a job fails, retry it on another computer on which it has not failed. Do this \ *n*\  times. If there are fewer than \ *n*\  computers in \ **--sshlogin**\  GNU \ **parallel**\  will re-use all the computers. This is useful if some jobs fail for no apparent reason (such as network failure).
 
 \ *n*\ =0 means infinite.
 
 See also: \ **--term-seq**\  \ **--sshlogin**\ 
 


- \ **--return**\  \ *filename*\ 
 
 Transfer files from remote computers.
 
 \ **--return**\  is used with \ **--sshlogin**\  when the arguments are files on the remote computers. When processing is done the file \ *filename*\  will be transferred from the remote computer using \ **rsync**\  and will be put relative to the default login dir. E.g.
 
 
 .. code-block:: perl
 
    echo foo/bar.txt | parallel --return {.}.out \
      --sshlogin server.example.com touch {.}.out
 
 
 This will transfer the file \ *$HOME/foo/bar.out*\  from the computer \ *server.example.com*\  to the file \ *foo/bar.out*\  after running \ **touch foo/bar.out**\  on \ *server.example.com*\ .
 
 
 .. code-block:: perl
 
    parallel -S server --trc out/./{}.out touch {}.out ::: in/file
 
 
 This will transfer the file \ *in/file.out*\  from the computer \ *server.example.com*\  to the files \ *out/in/file.out*\  after running \ **touch in/file.out**\  on \ *server*\ .
 
 
 .. code-block:: perl
 
    echo /tmp/foo/bar.txt | parallel --return {.}.out \
      --sshlogin server.example.com touch {.}.out
 
 
 This will transfer the file \ */tmp/foo/bar.out*\  from the computer \ *server.example.com*\  to the file \ */tmp/foo/bar.out*\  after running \ **touch /tmp/foo/bar.out**\  on \ *server.example.com*\ .
 
 Multiple files can be transferred by repeating the option multiple times:
 
 
 .. code-block:: perl
 
    echo /tmp/foo/bar.txt | parallel \
      --sshlogin server.example.com \
      --return {.}.out --return {.}.out2 touch {.}.out {.}.out2
 
 
 \ **--return**\  is ignored when used with \ **--sshlogin :**\  or when not used with \ **--sshlogin**\ .
 
 For details on transferring see \ **--transferfile**\ .
 
 See also: \ **--transfer**\  \ **--transferfile**\  \ **--sshlogin**\  \ **--cleanup**\  \ **--workdir**\ 
 


- \ **--round-robin**\ 



- \ **--round**\ 
 
 Distribute chunks of standard input in a round robin fashion.
 
 Normally \ **--pipe**\  will give a single block to each instance of the command. With \ **--round-robin**\  all blocks will at random be written to commands already running. This is useful if the command takes a long time to initialize.
 
 \ **--keep-order**\  will not work with \ **--round-robin**\  as it is impossible to track which input block corresponds to which output.
 
 \ **--round-robin**\  implies \ **--pipe**\ , except if \ **--pipe-part**\  is given.
 
 See the section: SPREADING BLOCKS OF DATA.
 
 See also: \ **--bin**\  \ **--group-by**\  \ **--shard**\ 
 


- \ **--rpl**\  '\ *tag*\  \ *perl expression*\ '
 
 Define replacement string.
 
 Use \ *tag*\  as a replacement string for \ *perl expression*\ . This makes it possible to define your own replacement strings. GNU \ **parallel**\ 's 7 replacement strings are implemented as:
 
 
 .. code-block:: perl
 
    --rpl '{} '
    --rpl '{#} 1 $_=$job->seq()'
    --rpl '{%} 1 $_=$job->slot()'
    --rpl '{/} s:.*/::'
    --rpl '{//} $Global::use{"File::Basename"} ||=
                eval "use File::Basename; 1;"; $_ = dirname($_);'
    --rpl '{/.} s:.*/::; s:\.[^/.]+$::;'
    --rpl '{.} s:\.[^/.]+$::'
 
 
 The \ **--plus**\  replacement strings are implemented as:
 
 
 .. code-block:: perl
 
    --rpl '{+/} s:/[^/]*$:: || s:.*$::'
    --rpl '{+.} s:.*\.:: || s:.*$::'
    --rpl '{+..} s:.*\.([^/.]+\.[^/.]+)$:$1: || s:.*$::'
    --rpl '{+...} s:.*\.([^/.]+\.[^/.]+\.[^/.]+)$:$1: || s:.*$::'
    --rpl '{..} s:\.[^/.]+\.[^/.]+$::'
    --rpl '{...} s:\.[^/.]+\.[^/.]+\.[^/.]+$::'
    --rpl '{/..} s:.*/::; s:\.[^/.]+\.[^/.]+$::'
    --rpl '{/...} s:.*/::; s:\.[^/.]+\.[^/.]+\.[^/.]+$::'
    --rpl '{choose_k}
           for $t (2..$#arg){ if($arg[$t-1] ge $arg[$t]) { skip() } }'
    --rpl '{##} 1 $_=total_jobs()'
    --rpl '{0%} 1 $f=1+int((log($Global::max_jobs_running||1)/
                            log(10))); $_=sprintf("%0${f}d",slot())'
    --rpl '{0#} 1 $f=1+int((log(total_jobs())/log(10)));
                $_=sprintf("%0${f}d",seq())'
  
    --rpl '{:-([^}]+?)} $_ ||= $$1'
    --rpl '{:(\d+?)} substr($_,0,$$1) = ""'
    --rpl '{:(\d+?):(\d+?)} $_ = substr($_,$$1,$$2);'
    --rpl '{#([^#}][^}]*?)} $nongreedy=::make_regexp_ungreedy($$1);
                            s/^$nongreedy(.*)/$1/;'
    --rpl '{##([^#}][^}]*?)} s/^$$1//;'
    --rpl '{%([^}]+?)} $nongreedy=::make_regexp_ungreedy($$1);
                       s/(.*)$nongreedy$/$1/;'
    --rpl '{%%([^}]+?)} s/$$1$//;'
    --rpl '{/([^}]+?)/([^}]*?)} s/$$1/$$2/;'
    --rpl '{^([^}]+?)} s/^($$1)/uc($1)/e;'
    --rpl '{^^([^}]+?)} s/($$1)/uc($1)/eg;'
    --rpl '{,([^}]+?)} s/^($$1)/lc($1)/e;'
    --rpl '{,,([^}]+?)} s/($$1)/lc($1)/eg;'
  
    --rpl '{slot} 1 $_="\${PARALLEL_JOBSLOT}";uq()'
    --rpl '{host} 1 $_="\${PARALLEL_SSHHOST}";uq()'
    --rpl '{sshlogin} 1 $_="\${PARALLEL_SSHLOGIN}";uq()'
    --rpl '{hgrp} 1 $_="\${PARALLEL_HOSTGROUPS}";uq()'
    --rpl '{agrp} 1 $_="\${PARALLEL_ARGHOSTGROUPS}";uq()'
 
 
 If the user defined replacement string starts with '{' it can also be used as a positional replacement string (like \ **{2.}**\ ).
 
 It is recommended to only change $_ but you have full access to all of GNU \ **parallel**\ 's internal functions and data structures.
 
 Here are a few examples:
 
 
 .. code-block:: perl
 
    Is the job sequence even or odd?
    --rpl '{odd} $_ = seq() % 2 ? "odd" : "even"'
    Pad job sequence with leading zeros to get equal width
    --rpl '{0#} $f=1+int("".(log(total_jobs())/log(10)));
      $_=sprintf("%0${f}d",seq())'
    Job sequence counting from 0
    --rpl '{#0} $_ = seq() - 1'
    Job slot counting from 2
    --rpl '{%1} $_ = slot() + 1'
    Remove all extensions
    --rpl '{:} s:(\.[^/]+)*$::'
 
 
 You can have dynamic replacement strings by including parenthesis in the replacement string and adding a regular expression between the parenthesis. The matching string will be inserted as $$1:
 
 
 .. code-block:: perl
 
    parallel --rpl '{%(.*?)} s/$$1//' echo {%.tar.gz} ::: my.tar.gz
    parallel --rpl '{:%(.+?)} s:$$1(\.[^/]+)*$::' \
      echo {:%_file} ::: my_file.tar.gz
    parallel -n3 --rpl '{/:%(.*?)} s:.*/(.*)$$1(\.[^/]+)*$:$1:' \
      echo job {#}: {2} {2.} {3/:%_1} ::: a/b.c c/d.e f/g_1.h.i
 
 
 You can even use multiple matches:
 
 
 .. code-block:: perl
 
    parallel --rpl '{/(.+?)/(.*?)} s/$$1/$$2/;'
      echo {/replacethis/withthis} {/b/C} ::: a_replacethis_b
  
    parallel --rpl '{(.*?)/(.*?)} $_="$$2$_$$1"' \
      echo {swap/these} ::: -middle-
 
 
 See also: \ **{=**\ \ *perl expression*\ \ **=}**\  \ **--parens**\ 
 


- \ **--rsync-opts**\  \ *options*\ 
 
 Options to pass on to \ **rsync**\ .
 
 Setting \ **--rsync-opts**\  takes precedence over setting the environment variable $PARALLEL_RSYNC_OPTS.
 


- \ **--max-chars**\  \ *max-chars*\ 



- \ **-s**\  \ *max-chars*\ 
 
 Limit length of command.
 
 Use at most \ *max-chars*\  characters per command line, including the command and initial-arguments and the terminating nulls at the ends of the argument strings.  The largest allowed value is system-dependent, and is calculated as the argument length limit for exec, less the size of your environment.  The default value is the maximum.
 
 \ *max-chars*\  can be postfixed with K, M, G, T, P, k, m, g, t, or p (see UNIT PREFIX).
 
 Implies \ **-X**\  unless \ **-m**\  or \ **--xargs**\  is set.
 
 See also: \ **-X**\  \ **-m**\  \ **--xargs**\  \ **--max-line-length-allowed**\  \ **--show-limits**\ 
 


- \ **--show-limits**\ 
 
 Display limits given by the operating system.
 
 Display the limits on the command-line length which are imposed by the operating system and the \ **-s**\  option.  Pipe the input from /dev/null (and perhaps specify --no-run-if-empty) if you don't want GNU \ **parallel**\  to do anything.
 
 See also: \ **--max-chars**\  \ **--max-line-length-allowed**\  \ **--version**\ 
 


- \ **--semaphore**\ 
 
 Work as a counting semaphore.
 
 \ **--semaphore**\  will cause GNU \ **parallel**\  to start \ *command*\  in the background. When the number of jobs given by \ **--jobs**\  is reached, GNU \ **parallel**\  will wait for one of these to complete before starting another command.
 
 \ **--semaphore**\  implies \ **--bg**\  unless \ **--fg**\  is specified.
 
 The command \ **sem**\  is an alias for \ **parallel --semaphore**\ .
 
 See also: \ **man sem**\  \ **--bg**\  \ **--fg**\  \ **--semaphore-name**\  \ **--semaphore-timeout**\  \ **--wait**\ 
 


- \ **--semaphore-name**\  \ *name*\ 



- \ **--id**\  \ *name*\ 
 
 Use \ **name**\  as the name of the semaphore.
 
 The default is the name of the controlling tty (output from \ **tty**\ ).
 
 The default normally works as expected when used interactively, but when used in a script \ *name*\  should be set. \ *$$*\  or \ *my_task_name*\  are often a good value.
 
 The semaphore is stored in ~/.parallel/semaphores/
 
 Implies \ **--semaphore**\ .
 
 See also: \ **man sem**\  \ **--semaphore**\ 
 


- \ **--semaphore-timeout**\  \ *secs*\ 



- \ **--st**\  \ *secs*\ 
 
 If \ *secs*\  > 0: If the semaphore is not released within \ *secs*\  seconds, take it anyway.
 
 If \ *secs*\  < 0: If the semaphore is not released within \ *secs*\  seconds, exit.
 
 \ *secs*\  is in seconds, but can be postfixed with s, m, h, or d (see the section TIME POSTFIXES).
 
 Implies \ **--semaphore**\ .
 
 See also: \ **man sem**\ 
 


- \ **--seqreplace**\  \ *replace-str*\ 
 
 Use the replacement string \ *replace-str*\  instead of \ **{#}**\  for job sequence number.
 
 See also: \ **{#}**\ 
 


- \ **--session**\ 
 
 Record names in current environment in \ **$PARALLEL_IGNORED_NAMES**\  and exit.
 
 Only used with \ **env_parallel**\ . Aliases, functions, and variables with names in \ **$PARALLEL_IGNORED_NAMES**\  will not be copied.  So you should set variables/function you want copied \ *after*\  running \ **--session**\ .
 
 It is similar to \ **--record-env**\ , but only for this session.
 
 Only supported in \ **Ash, Bash, Dash, Ksh, Sh, and Zsh**\ .
 
 See also: \ **--env**\  \ **--record-env**\  \ **env_parallel**\ 
 


- \ **--shard**\  \ *shardexpr*\ 
 
 Use \ *shardexpr*\  as shard key and shard input to the jobs.
 
 \ *shardexpr*\  is [column number|column name] [perlexpression] e.g.:
 
 
 .. code-block:: perl
 
    3
    Address
    3 $_%=100
    Address s/\d//g
 
 
 Each input line is split using \ **--colsep**\ . The value of the column is put into $_, the perl expression is executed, the resulting value is hashed so that all lines of a given value is given to the same job slot.
 
 This is similar to sharding in databases.
 
 The performance is in the order of 100K rows per second. Faster if the \ *shardcol*\  is small (<10), slower if it is big (>100).
 
 \ **--shard**\  requires \ **--pipe**\  and a fixed numeric value for \ **--jobs**\ .
 
 See the section: SPREADING BLOCKS OF DATA.
 
 See also: \ **--bin**\  \ **--group-by**\  \ **--round-robin**\ 
 


- \ **--shebang**\ 



- \ **--hashbang**\ 
 
 GNU \ **parallel**\  can be called as a shebang (#!) command as the first line of a script. The content of the file will be treated as inputsource.
 
 Like this:
 
 
 .. code-block:: perl
 
    #!/usr/bin/parallel --shebang -r wget
  
    https://ftpmirror.gnu.org/parallel/parallel-20120822.tar.bz2
    https://ftpmirror.gnu.org/parallel/parallel-20130822.tar.bz2
    https://ftpmirror.gnu.org/parallel/parallel-20140822.tar.bz2
 
 
 \ **--shebang**\  must be set as the first option.
 
 On FreeBSD \ **env**\  is needed:
 
 
 .. code-block:: perl
 
    #!/usr/bin/env -S parallel --shebang -r wget
  
    https://ftpmirror.gnu.org/parallel/parallel-20120822.tar.bz2
    https://ftpmirror.gnu.org/parallel/parallel-20130822.tar.bz2
    https://ftpmirror.gnu.org/parallel/parallel-20140822.tar.bz2
 
 
 There are many limitations of shebang (#!) depending on your operating system. See details on https://www.in-ulm.de/~mascheck/various/shebang/
 
 See also: \ **--shebang-wrap**\ 
 


- \ **--shebang-wrap**\ 
 
 GNU \ **parallel**\  can parallelize scripts by wrapping the shebang line. If the program can be run like this:
 
 
 .. code-block:: perl
 
    cat arguments | parallel the_program
 
 
 then the script can be changed to:
 
 
 .. code-block:: perl
 
    #!/usr/bin/parallel --shebang-wrap /original/parser --options
 
 
 E.g.
 
 
 .. code-block:: perl
 
    #!/usr/bin/parallel --shebang-wrap /usr/bin/python
 
 
 If the program can be run like this:
 
 
 .. code-block:: perl
 
    cat data | parallel --pipe the_program
 
 
 then the script can be changed to:
 
 
 .. code-block:: perl
 
    #!/usr/bin/parallel --shebang-wrap --pipe /orig/parser --opts
 
 
 E.g.
 
 
 .. code-block:: perl
 
    #!/usr/bin/parallel --shebang-wrap --pipe /usr/bin/perl -w
 
 
 \ **--shebang-wrap**\  must be set as the first option.
 
 See also: \ **--shebang**\ 
 


- \ **--shell-completion**\  \ *shell*\  (beta testing)
 
 Generate shell completion code for interactive shells.
 
 Supported shells: bash zsh.
 
 Use \ *auto*\  as \ *shell*\  to automatically detect running shell.
 
 Activate the completion code with:
 
 
 .. code-block:: perl
 
    zsh% eval "$(parallel --shell-completion auto)"
    bash$ eval "$(parallel --shell-completion auto)"
 
 
 Or put this \`/usr/share/zsh/site-functions/_parallel\`, then \`compinit\` to generate \`~/.zcompdump\`:
 
 
 .. code-block:: perl
 
    #compdef parallel
    
    (( $+functions[_comp_parallel] )) ||
      eval "$(parallel --shell-completion auto)" &&
      _comp_parallel
 
 


- \ **--shell-quote**\ 
 
 Does not run the command but quotes it. Useful for making quoted composed commands for GNU \ **parallel**\ .
 
 Multiple \ **--shell-quote**\  with quote the string multiple times, so \ **parallel --shell-quote | parallel --shell-quote**\  can be written as \ **parallel --shell-quote --shell-quote**\ .
 
 See also: \ **--quote**\ 
 


- \ **--shuf**\ 
 
 Shuffle jobs.
 
 When having multiple input sources it is hard to randomize jobs. \ **--shuf**\  will generate all jobs, and shuffle them before running them. This is useful to get a quick preview of the results before running the full batch.
 
 Combined with \ **--halt soon,done=1%**\  you can run a random 1% sample of all jobs:
 
 
 .. code-block:: perl
 
    parallel --shuf --halt soon,done=1% echo ::: {1..100} ::: {1..100}
 
 
 See also: \ **--halt**\ 
 


- \ **--skip-first-line**\ 
 
 Do not use the first line of input (used by GNU \ **parallel**\  itself when called with \ **--shebang**\ ).
 


- \ **--sql**\  \ *DBURL*\  (obsolete)
 
 Use \ **--sql-master**\  instead.
 


- \ **--sql-master**\  \ *DBURL*\ 
 
 Submit jobs via SQL server. \ *DBURL*\  must point to a table, which will contain the same information as \ **--joblog**\ , the values from the input sources (stored in columns V1 .. Vn), and the output (stored in columns Stdout and Stderr).
 
 If \ *DBURL*\  is prepended with '+' GNU \ **parallel**\  assumes the table is already made with the correct columns and appends the jobs to it.
 
 If \ *DBURL*\  is not prepended with '+' the table will be dropped and created with the correct amount of V-columns unless
 
 \ **--sqlmaster**\  does not run any jobs, but it creates the values for the jobs to be run. One or more \ **--sqlworker**\  must be run to actually execute the jobs.
 
 If \ **--wait**\  is set, GNU \ **parallel**\  will wait for the jobs to complete.
 
 The format of a DBURL is:
 
 
 .. code-block:: perl
 
    [sql:]vendor://[[user][:pwd]@][host][:port]/[db]/table
 
 
 E.g.
 
 
 .. code-block:: perl
 
    sql:mysql://hr:hr@localhost:3306/hrdb/jobs
    mysql://scott:tiger@my.example.com/pardb/paralleljobs
    sql:oracle://scott:tiger@ora.example.com/xe/parjob
    postgresql://scott:tiger@pg.example.com/pgdb/parjob
    pg:///parjob
    sqlite3:///%2Ftmp%2Fpardb.sqlite/parjob
    csv:///%2Ftmp%2Fpardb/parjob
 
 
 Notice how / in the path of sqlite and CVS must be encoded as %2F. Except the last / in CSV which must be a /.
 
 It can also be an alias from ~/.sql/aliases:
 
 
 .. code-block:: perl
 
    :myalias mysql:///mydb/paralleljobs
 
 
 See also: \ **--sql-and-worker**\  \ **--sql-worker**\  \ **--joblog**\ 
 


- \ **--sql-and-worker**\  \ *DBURL*\ 
 
 Shorthand for: \ **--sql-master**\  \ *DBURL*\  \ **--sql-worker**\  \ *DBURL*\ .
 
 See also: \ **--sql-master**\  \ **--sql-worker**\ 
 


- \ **--sql-worker**\  \ *DBURL*\ 
 
 Execute jobs via SQL server. Read the input sources variables from the table pointed to by \ *DBURL*\ . The \ *command*\  on the command line should be the same as given by \ **--sqlmaster**\ .
 
 If you have more than one \ **--sqlworker**\  jobs may be run more than once.
 
 If \ **--sqlworker**\  runs on the local machine, the hostname in the SQL table will not be ':' but instead the hostname of the machine.
 
 See also: \ **--sql-master**\  \ **--sql-and-worker**\ 
 


- \ **--ssh**\  \ *sshcommand*\ 
 
 GNU \ **parallel**\  defaults to using \ **ssh**\  for remote access. This can be overridden with \ **--ssh**\ . It can also be set on a per server basis with \ **--sshlogin**\ .
 
 See also: \ **--sshlogin**\ 
 


- \ **--ssh-delay**\  \ *duration*\ 
 
 Delay starting next ssh by \ *duration*\ .
 
 GNU \ **parallel**\  will not start another ssh for the next \ *duration*\ .
 
 \ *duration*\  is in seconds, but can be postfixed with s, m, h, or d.
 
 See also: TIME POSTFIXES \ **--sshlogin**\  \ **--delay**\ 
 


- \ **--sshlogin**\  \ *[@hostgroups/][ncpus/]sshlogin[,[@hostgroups/][ncpus/]sshlogin[,...]]*\  (alpha testing)



- \ **--sshlogin**\  \ *@hostgroup*\  (alpha testing)



- \ **-S**\  \ *[@hostgroups/][ncpus/]sshlogin[,[@hostgroups/][ncpus/]sshlogin[,...]]*\  (alpha testing)



- \ **-S**\  \ *@hostgroup*\  (alpha testing)
 
 Distribute jobs to remote computers.
 
 The jobs will be run on a list of remote computers.
 
 If \ *hostgroups*\  is given, the \ *sshlogin*\  will be added to that hostgroup. Multiple hostgroups are separated by '+'. The \ *sshlogin*\  will always be added to a hostgroup named the same as \ *sshlogin*\ .
 
 If only the \ *@hostgroup*\  is given, only the sshlogins in that hostgroup will be used. Multiple \ *@hostgroup*\  can be given.
 
 GNU \ **parallel**\  will determine the number of CPUs on the remote computers and run the number of jobs as specified by \ **-j**\ .  If the number \ *ncpus*\  is given GNU \ **parallel**\  will use this number for number of CPUs on the host. Normally \ *ncpus*\  will not be needed.
 
 An \ *sshlogin*\  is of the form:
 
 
 .. code-block:: perl
 
    [sshcommand [options]] [username[:password]@]hostname
 
 
 If \ *password*\  is given, \ **sshpass**\  will be used. Otherwise the sshlogin must not require a password (\ **ssh-agent**\  and \ **ssh-copy-id**\  may help with that).
 
 The sshlogin ':' is special, it means 'no ssh' and will therefore run on the local computer.
 
 The sshlogin '..' is special, it read sshlogins from ~/.parallel/sshloginfile or $XDG_CONFIG_HOME/parallel/sshloginfile
 
 The sshlogin '-' is special, too, it read sshlogins from stdin (standard input).
 
 To specify more sshlogins separate the sshlogins by comma, newline (in the same string), or repeat the options multiple times.
 
 GNU \ **parallel**\  splits on , (comma) so if your sshlogin contains , (comma) you need to replace it with \, or ,,
 
 For examples: see \ **--sshloginfile**\ .
 
 The remote host must have GNU \ **parallel**\  installed.
 
 \ **--sshlogin**\  is known to cause problems with \ **-m**\  and \ **-X**\ .
 
 See also: \ **--basefile**\  \ **--transferfile**\  \ **--return**\  \ **--cleanup**\  \ **--trc**\  \ **--sshloginfile**\  \ **--workdir**\  \ **--filter-hosts**\  \ **--ssh**\ 
 


- \ **--sshloginfile**\  \ *filename*\ 



- \ **--slf**\  \ *filename*\ 
 
 File with sshlogins. The file consists of sshlogins on separate lines. Empty lines and lines starting with '#' are ignored. Example:
 
 
 .. code-block:: perl
 
    server.example.com
    username@server2.example.com
    8/my-8-cpu-server.example.com
    2/my_other_username@my-dualcore.example.net
    # This server has SSH running on port 2222
    ssh -p 2222 server.example.net
    4/ssh -p 2222 quadserver.example.net
    # Use a different ssh program
    myssh -p 2222 -l myusername hexacpu.example.net
    # Use a different ssh program with default number of CPUs
    //usr/local/bin/myssh -p 2222 -l myusername hexacpu
    # Use a different ssh program with 6 CPUs
    6//usr/local/bin/myssh -p 2222 -l myusername hexacpu
    # Assume 16 CPUs on the local computer
    16/:
    # Put server1 in hostgroup1
    @hostgroup1/server1
    # Put myusername@server2 in hostgroup1+hostgroup2
    @hostgroup1+hostgroup2/myusername@server2
    # Force 4 CPUs and put 'ssh -p 2222 server3' in hostgroup1
    @hostgroup1/4/ssh -p 2222 server3
 
 
 When using a different ssh program the last argument must be the hostname.
 
 Multiple \ **--sshloginfile**\  are allowed.
 
 GNU \ **parallel**\  will first look for the file in current dir; if that fails it look for the file in ~/.parallel.
 
 The sshloginfile '..' is special, it read sshlogins from ~/.parallel/sshloginfile
 
 The sshloginfile '.' is special, it read sshlogins from /etc/parallel/sshloginfile
 
 The sshloginfile '-' is special, too, it read sshlogins from stdin (standard input).
 
 If the sshloginfile is changed it will be re-read when a job finishes though at most once per second. This makes it possible to add and remove hosts while running.
 
 This can be used to have a daemon that updates the sshloginfile to only contain servers that are up:
 
 
 .. code-block:: perl
 
      cp original.slf tmp2.slf
      while [ 1 ] ; do
        nice parallel --nonall -j0 -k --slf original.slf \
          --tag echo | perl 's/\t$//' > tmp.slf
        if diff tmp.slf tmp2.slf; then
          mv tmp.slf tmp2.slf
        fi
        sleep 10
      done &
      parallel --slf tmp2.slf ...
 
 
 See also: \ **--filter-hosts**\ 
 


- \ **--slotreplace**\  \ *replace-str*\ 
 
 Use the replacement string \ *replace-str*\  instead of \ **{%}**\  for job slot number.
 
 See also: \ **{%}**\ 
 


- \ **--silent**\ 
 
 Silent.
 
 The job to be run will not be printed. This is the default.  Can be reversed with \ **-v**\ .
 
 See also: \ **-v**\ 
 


- \ **--template**\  \ *file*\ =\ *repl*\ 



- \ **--tmpl**\  \ *file*\ =\ *repl*\ 
 
 Replace replacement strings in \ *file*\  and save it in \ *repl*\ .
 
 All replacement strings in the contents of \ *file*\  will be replaced. All replacement strings in the name \ *repl*\  will be replaced.
 
 With \ **--cleanup**\  the new file will be removed when the job is done.
 
 If \ *my.tmpl*\  contains this:
 
 
 .. code-block:: perl
 
    Xval: {x}
    Yval: {y}
    FixedValue: 9
    # x with 2 decimals
    DecimalX: {=x $_=sprintf("%.2f",$_) =}
    TenX: {=x $_=$_*10 =}
    RandomVal: {=1 $_=rand() =}
 
 
 it can be used like this:
 
 
 .. code-block:: perl
 
    myprog() { echo Using "$@"; cat "$@"; }
    export -f myprog
    parallel --cleanup --header : --tmpl my.tmpl={#}.t myprog {#}.t \
      ::: x 1.234 2.345 3.45678 ::: y 1 2 3
 
 
 See also: \ **{}**\  \ **--cleanup**\ 
 


- \ **--tty**\ 
 
 Open terminal tty.
 
 If GNU \ **parallel**\  is used for starting a program that accesses the tty (such as an interactive program) then this option may be needed. It will default to starting only one job at a time (i.e. \ **-j1**\ ), not buffer the output (i.e. \ **-u**\ ), and it will open a tty for the job.
 
 You can of course override \ **-j1**\  and \ **-u**\ .
 
 Using \ **--tty**\  unfortunately means that GNU \ **parallel**\  cannot kill the jobs (with \ **--timeout**\ , \ **--memfree**\ , or \ **--halt**\ ). This is due to GNU \ **parallel**\  giving each child its own process group, which is then killed. Process groups are dependant on the tty.
 
 See also: \ **--ungroup**\  \ **--open-tty**\ 
 


- \ **--tag**\ 
 
 Tag lines with arguments.
 
 Each output line will be prepended with the arguments and TAB (\t). When combined with \ **--onall**\  or \ **--nonall**\  the lines will be prepended with the sshlogin instead.
 
 \ **--tag**\  is ignored when using \ **-u**\ .
 
 See also: \ **--tagstring**\  \ **--ctag**\ 
 


- \ **--tagstring**\  \ *str*\ 
 
 Tag lines with a string.
 
 Each output line will be prepended with \ *str*\  and TAB (\t). \ *str*\  can contain replacement strings such as \ **{}**\ .
 
 \ **--tagstring**\  is ignored when using \ **-u**\ , \ **--onall**\ , and \ **--nonall**\ .
 
 See also: \ **--tag**\  \ **--ctagstring**\ 
 


- \ **--tee**\ 
 
 Pipe all data to all jobs.
 
 Used with \ **--pipe**\ /\ **--pipe-part**\  and \ **:::**\ .
 
 
 .. code-block:: perl
 
    seq 1000 | parallel --pipe --tee -v wc {} ::: -w -l -c
 
 
 How many numbers in 1..1000 contain 0..9, and how many bytes do they fill:
 
 
 .. code-block:: perl
 
    seq 1000 | parallel --pipe --tee --tag \
      'grep {1} | wc {2}' ::: {0..9} ::: -l -c
 
 
 How many words contain a..z and how many bytes do they fill?
 
 
 .. code-block:: perl
 
    parallel -a /usr/share/dict/words --pipe-part --tee --tag \
      'grep {1} | wc {2}' ::: {a..z} ::: -l -c
 
 
 See also: \ **:::**\  \ **--pipe**\  \ **--pipe-part**\ 
 


- \ **--term-seq**\  \ *sequence*\ 
 
 Termination sequence.
 
 When a job is killed due to \ **--timeout**\ , \ **--memfree**\ , \ **--halt**\ , or abnormal termination of GNU \ **parallel**\ , \ *sequence*\  determines how the job is killed. The default is:
 
 
 .. code-block:: perl
 
      TERM,200,TERM,100,TERM,50,KILL,25
 
 
 which sends a TERM signal, waits 200 ms, sends another TERM signal, waits 100 ms, sends another TERM signal, waits 50 ms, sends a KILL signal, waits 25 ms, and exits. GNU \ **parallel**\  detects if a process dies before the waiting time is up.
 
 See also: \ **--halt**\  \ **--timeout**\  \ **--memfree**\ 
 


- \ **--tmpdir**\  \ *dirname*\ 
 
 Directory for temporary files.
 
 GNU \ **parallel**\  normally buffers output into temporary files in /tmp. By setting \ **--tmpdir**\  you can use a different dir for the files. Setting \ **--tmpdir**\  is equivalent to setting $TMPDIR.
 
 See also: \ **--compress**\  \ **$TMPDIR**\  \ **$PARALLEL_REMOTE_TMPDIR**\ 
 


- \ **--tmux**\  (Long beta testing)
 
 Use \ **tmux**\  for output. Start a \ **tmux**\  session and run each job in a window in that session. No other output will be produced.
 
 See also: \ **--tmuxpane**\ 
 


- \ **--tmuxpane**\  (Long beta testing)
 
 Use \ **tmux**\  for output but put output into panes in the first window. Useful if you want to monitor the progress of less than 100 concurrent jobs.
 
 See also: \ **--tmux**\ 
 


- \ **--timeout**\  \ *duration*\ 
 
 Time out for command. If the command runs for longer than \ *duration*\  seconds it will get killed as per \ **--term-seq**\ .
 
 If \ *duration*\  is followed by a % then the timeout will dynamically be computed as a percentage of the median average runtime of successful jobs. Only values > 100% will make sense.
 
 \ *duration*\  is in seconds, but can be postfixed with s, m, h, or d.
 
 See also: TIME POSTFIXES \ **--term-seq**\  \ **--retries**\ 
 


- \ **--verbose**\ 



- \ **-t**\ 
 
 Print the job to be run on stderr (standard error).
 
 See also: \ **-v**\  \ **--interactive**\ 
 


- \ **--transfer**\ 
 
 Transfer files to remote computers.
 
 Shorthand for: \ **--transferfile {}**\ .
 
 See also: \ **--transferfile**\ .
 


- \ **--transferfile**\  \ *filename*\ 



- \ **--tf**\  \ *filename*\ 
 
 Transfer \ *filename*\  to remote computers.
 
 \ **--transferfile**\  is used with \ **--sshlogin**\  to transfer files to the remote computers. The files will be transferred using \ **rsync**\  and will be put relative to the work dir.
 
 The \ *filename*\  will normally contain a replacement string.
 
 If the path contains /./ the remaining path will be relative to the work dir (for details: see \ **rsync**\ ). If the work dir is \ **/home/user**\ , the transferring will be as follows:
 
 
 .. code-block:: perl
 
    /tmp/foo/bar   => /tmp/foo/bar
    tmp/foo/bar    => /home/user/tmp/foo/bar
    /tmp/./foo/bar => /home/user/foo/bar
    tmp/./foo/bar  => /home/user/foo/bar
 
 
 \ *Examples*\ 
 
 This will transfer the file \ *foo/bar.txt*\  to the computer \ *server.example.com*\  to the file \ *$HOME/foo/bar.txt*\  before running \ **wc foo/bar.txt**\  on \ *server.example.com*\ :
 
 
 .. code-block:: perl
 
    echo foo/bar.txt | parallel --transferfile {} \
      --sshlogin server.example.com wc
 
 
 This will transfer the file \ */tmp/foo/bar.txt*\  to the computer \ *server.example.com*\  to the file \ */tmp/foo/bar.txt*\  before running \ **wc /tmp/foo/bar.txt**\  on \ *server.example.com*\ :
 
 
 .. code-block:: perl
 
    echo /tmp/foo/bar.txt | parallel --transferfile {} \
      --sshlogin server.example.com wc
 
 
 This will transfer the file \ */tmp/foo/bar.txt*\  to the computer \ *server.example.com*\  to the file \ *foo/bar.txt*\  before running \ **wc ./foo/bar.txt**\  on \ *server.example.com*\ :
 
 
 .. code-block:: perl
 
    echo /tmp/./foo/bar.txt | parallel --transferfile {} \
      --sshlogin server.example.com wc {= s:.*/\./:./: =}
 
 
 \ **--transferfile**\  is often used with \ **--return**\  and \ **--cleanup**\ . A shorthand for \ **--transferfile {}**\  is \ **--transfer**\ .
 
 \ **--transferfile**\  is ignored when used with \ **--sshlogin :**\  or when not used with \ **--sshlogin**\ .
 
 See also: \ **--workdir**\  \ **--sshlogin**\  \ **--basefile**\  \ **--return**\  \ **--cleanup**\ 
 


- \ **--trc**\  \ *filename*\ 
 
 Transfer, Return, Cleanup. Shorthand for: \ **--transfer**\  \ **--return**\  \ *filename*\  \ **--cleanup**\ 
 
 See also: \ **--transfer**\  \ **--return**\  \ **--cleanup**\ 
 


- \ **--trim**\  <n|l|r|lr|rl>
 
 Trim white space in input.
 
 
 - n
  
  No trim. Input is not modified. This is the default.
  
 
 
 - l
  
  Left trim. Remove white space from start of input. E.g. " a bc " -> "a bc ".
  
 
 
 - r
  
  Right trim. Remove white space from end of input. E.g. " a bc " -> " a bc".
  
 
 
 - lr
 
 
 
 - rl
  
  Both trim. Remove white space from both start and end of input. E.g. " a bc " -> "a bc". This is the default if \ **--colsep**\  is used.
  
 
 
 See also: \ **--no-run-if-empty**\  \ **{}**\  \ **--colsep**\ 
 


- \ **--ungroup**\ 



- \ **-u**\ 
 
 Ungroup output.
 
 Output is printed as soon as possible and bypasses GNU \ **parallel**\  internal processing. This may cause output from different commands to be mixed thus should only be used if you do not care about the output. Compare these:
 
 
 .. code-block:: perl
 
    seq 4 | parallel -j0 \
      'sleep {};echo -n start{};sleep {};echo {}end'
    seq 4 | parallel -u -j0 \
      'sleep {};echo -n start{};sleep {};echo {}end'
 
 
 It also disables \ **--tag**\ . GNU \ **parallel**\  outputs faster with \ **-u**\ . Compare the speeds of these:
 
 
 .. code-block:: perl
 
    parallel seq ::: 300000000 >/dev/null
    parallel -u seq ::: 300000000 >/dev/null
    parallel --line-buffer seq ::: 300000000 >/dev/null
 
 
 Can be reversed with \ **--group**\ .
 
 See also: \ **--line-buffer**\  \ **--group**\ 
 


- \ **--extensionreplace**\  \ *replace-str*\ 



- \ **--er**\  \ *replace-str*\ 
 
 Use the replacement string \ *replace-str*\  instead of \ **{.}**\  for input line without extension.
 
 See also: \ **{.}**\ 
 


- \ **--use-sockets-instead-of-threads**\ 
 
 See also: \ **--use-cores-instead-of-threads**\ 
 


- \ **--use-cores-instead-of-threads**\ 



- \ **--use-cpus-instead-of-cores**\  (obsolete)
 
 Determine how GNU \ **parallel**\  counts the number of CPUs.
 
 GNU \ **parallel**\  uses this number when the number of jobslots (\ **--jobs**\ ) is computed relative to the number of CPUs (e.g. 100% or +1).
 
 CPUs can be counted in three different ways:
 
 
 - sockets
  
  The number of filled CPU sockets (i.e. the number of physical chips).
  
 
 
 - cores
  
  The number of physical cores (i.e. the number of physical compute cores).
  
 
 
 - threads
  
  The number of hyperthreaded cores (i.e. the number of virtual cores - with some of them possibly being hyperthreaded)
  
 
 
 Normally the number of CPUs is computed as the number of CPU threads. With \ **--use-sockets-instead-of-threads**\  or \ **--use-cores-instead-of-threads**\  you can force it to be computed as the number of filled sockets or number of cores instead.
 
 Most users will not need these options.
 
 \ **--use-cpus-instead-of-cores**\  is a (misleading) alias for \ **--use-sockets-instead-of-threads**\  and is kept for backwards compatibility.
 
 See also: \ **--number-of-threads**\  \ **--number-of-cores**\  \ **--number-of-sockets**\ 
 


- \ **-v**\ 
 
 Verbose.
 
 Print the job to be run on stdout (standard output). Can be reversed with \ **--silent**\ .
 
 Use \ **-v**\  \ **-v**\  to print the wrapping ssh command when running remotely.
 
 See also: \ **-t**\ 
 


- \ **--version**\ 



- \ **-V**\ 
 
 Print the version GNU \ **parallel**\  and exit.
 


- \ **--workdir**\  \ *mydir*\ 



- \ **--wd**\  \ *mydir*\ 
 
 Jobs will be run in the dir \ *mydir*\ . The default is the current dir for the local machine, and the login dir for remote computers.
 
 Files transferred using \ **--transferfile**\  and \ **--return**\  will be relative to \ *mydir*\  on remote computers.
 
 The special \ *mydir*\  value \ **...**\  will create working dirs under \ **~/.parallel/tmp/**\ . If \ **--cleanup**\  is given these dirs will be removed.
 
 The special \ *mydir*\  value \ **.**\  uses the current working dir.  If the current working dir is beneath your home dir, the value \ **.**\  is treated as the relative path to your home dir. This means that if your home dir is different on remote computers (e.g. if your login is different) the relative path will still be relative to your home dir.
 
 To see the difference try:
 
 
 .. code-block:: perl
 
    parallel -S server pwd ::: ""
    parallel --wd . -S server pwd ::: ""
    parallel --wd ... -S server pwd ::: ""
 
 
 \ *mydir*\  can contain GNU \ **parallel**\ 's replacement strings.
 


- \ **--wait**\ 
 
 Wait for all commands to complete.
 
 Used with \ **--semaphore**\  or \ **--sqlmaster**\ .
 
 See also: \ **man sem**\ 
 


- \ **-X**\ 
 
 Multiple arguments with context replace. Insert as many arguments as the command line length permits. If multiple jobs are being run in parallel: distribute the arguments evenly among the jobs. Use \ **-j1**\  to avoid this.
 
 If \ **{}**\  is not used the arguments will be appended to the line.  If \ **{}**\  is used as part of a word (like \ *pic{}.jpg*\ ) then the whole word will be repeated. If \ **{}**\  is used multiple times each \ **{}**\  will be replaced with the arguments.
 
 Normally \ **-X**\  will do the right thing, whereas \ **-m**\  can give unexpected results if \ **{}**\  is used as part of a word.
 
 Support for \ **-X**\  with \ **--sshlogin**\  is limited and may fail.
 
 See also: \ **-m**\ 
 


- \ **--exit**\ 



- \ **-x**\ 
 
 Exit if the size (see the \ **-s**\  option) is exceeded.
 


- \ **--xargs**\ 
 
 Multiple arguments. Insert as many arguments as the command line length permits.
 
 If \ **{}**\  is not used the arguments will be appended to the line.  If \ **{}**\  is used multiple times each \ **{}**\  will be replaced with all the arguments.
 
 Support for \ **--xargs**\  with \ **--sshlogin**\  is limited and may fail.
 
 See also: \ **-X**\ 
 



********
EXAMPLES
********


EXAMPLE: Working as xargs -n1. Argument appending
=================================================


GNU \ **parallel**\  can work similar to \ **xargs -n1**\ .

To compress all html files using \ **gzip**\  run:


.. code-block:: perl

   find . -name '*.html' | parallel gzip --best


If the file names may contain a newline use \ **-0**\ . Substitute FOO BAR with FUBAR in all files in this dir and subdirs:


.. code-block:: perl

   find . -type f -print0 | \
     parallel -q0 perl -i -pe 's/FOO BAR/FUBAR/g'


Note \ **-q**\  is needed because of the space in 'FOO BAR'.


EXAMPLE: Simple network scanner
===============================


\ **prips**\  can generate IP-addresses from CIDR notation. With GNU \ **parallel**\  you can build a simple network scanner to see which addresses respond to \ **ping**\ :


.. code-block:: perl

   prips 130.229.16.0/20 | \
     parallel --timeout 2 -j0 \
       'ping -c 1 {} >/dev/null && echo {}' 2>/dev/null



EXAMPLE: Reading arguments from command line
============================================


GNU \ **parallel**\  can take the arguments from command line instead of stdin (standard input). To compress all html files in the current dir using \ **gzip**\  run:


.. code-block:: perl

   parallel gzip --best ::: *.html


To convert \*.wav to \*.mp3 using LAME running one process per CPU run:


.. code-block:: perl

   parallel lame {} -o {.}.mp3 ::: *.wav



EXAMPLE: Inserting multiple arguments
=====================================


When moving a lot of files like this: \ **mv \*.log destdir**\  you will sometimes get the error:


.. code-block:: perl

   bash: /bin/mv: Argument list too long


because there are too many files. You can instead do:


.. code-block:: perl

   ls | grep -E '\.log$' | parallel mv {} destdir


This will run \ **mv**\  for each file. It can be done faster if \ **mv**\  gets as many arguments that will fit on the line:


.. code-block:: perl

   ls | grep -E '\.log$' | parallel -m mv {} destdir


In many shells you can also use \ **printf**\ :


.. code-block:: perl

   printf '%s\0' *.log | parallel -0 -m mv {} destdir



EXAMPLE: Context replace
========================


To remove the files \ *pict0000.jpg*\  .. \ *pict9999.jpg*\  you could do:


.. code-block:: perl

   seq -w 0 9999 | parallel rm pict{}.jpg


You could also do:


.. code-block:: perl

   seq -w 0 9999 | perl -pe 's/(.*)/pict$1.jpg/' | parallel -m rm


The first will run \ **rm**\  10000 times, while the last will only run \ **rm**\  as many times needed to keep the command line length short enough to avoid \ **Argument list too long**\  (it typically runs 1-2 times).

You could also run:


.. code-block:: perl

   seq -w 0 9999 | parallel -X rm pict{}.jpg


This will also only run \ **rm**\  as many times needed to keep the command line length short enough.


EXAMPLE: Compute intensive jobs and substitution
================================================


If ImageMagick is installed this will generate a thumbnail of a jpg file:


.. code-block:: perl

   convert -geometry 120 foo.jpg thumb_foo.jpg


This will run with number-of-cpus jobs in parallel for all jpg files in a directory:


.. code-block:: perl

   ls *.jpg | parallel convert -geometry 120 {} thumb_{}


To do it recursively use \ **find**\ :


.. code-block:: perl

   find . -name '*.jpg' | \
     parallel convert -geometry 120 {} {}_thumb.jpg


Notice how the argument has to start with \ **{}**\  as \ **{}**\  will include path (e.g. running \ **convert -geometry 120 ./foo/bar.jpg thumb_./foo/bar.jpg**\  would clearly be wrong). The command will generate files like ./foo/bar.jpg_thumb.jpg.

Use \ **{.}**\  to avoid the extra .jpg in the file name. This command will make files like ./foo/bar_thumb.jpg:


.. code-block:: perl

   find . -name '*.jpg' | \
     parallel convert -geometry 120 {} {.}_thumb.jpg



EXAMPLE: Substitution and redirection
=====================================


This will generate an uncompressed version of .gz-files next to the .gz-file:


.. code-block:: perl

   parallel zcat {} ">"{.} ::: *.gz


Quoting of > is necessary to postpone the redirection. Another solution is to quote the whole command:


.. code-block:: perl

   parallel "zcat {} >{.}" ::: *.gz


Other special shell characters (such as \* ; $ > < | >> <<) also need to be put in quotes, as they may otherwise be interpreted by the shell and not given to GNU \ **parallel**\ .


EXAMPLE: Composed commands
==========================


A job can consist of several commands. This will print the number of files in each directory:


.. code-block:: perl

   ls | parallel 'echo -n {}" "; ls {}|wc -l'


To put the output in a file called <name>.dir:


.. code-block:: perl

   ls | parallel '(echo -n {}" "; ls {}|wc -l) >{}.dir'


Even small shell scripts can be run by GNU \ **parallel**\ :


.. code-block:: perl

   find . | parallel 'a={}; name=${a##*/};' \
     'upper=$(echo "$name" | tr "[:lower:]" "[:upper:]");'\
     'echo "$name - $upper"'
 
   ls | parallel 'mv {} "$(echo {} | tr "[:upper:]" "[:lower:]")"'


Given a list of URLs, list all URLs that fail to download. Print the line number and the URL.


.. code-block:: perl

   cat urlfile | parallel "wget {} 2>/dev/null || grep -n {} urlfile"


Create a mirror directory with the same filenames except all files and symlinks are empty files.


.. code-block:: perl

   cp -rs /the/source/dir mirror_dir
   find mirror_dir -type l | parallel -m rm {} '&&' touch {}


Find the files in a list that do not exist


.. code-block:: perl

   cat file_list | parallel 'if [ ! -e {} ] ; then echo {}; fi'



EXAMPLE: Composed command with perl replacement string
======================================================


You have a bunch of file. You want them sorted into dirs. The dir of each file should be named the first letter of the file name.


.. code-block:: perl

   parallel 'mkdir -p {=s/(.).*/$1/=}; mv {} {=s/(.).*/$1/=}' ::: *



EXAMPLE: Composed command with multiple input sources
=====================================================


You have a dir with files named as 24 hours in 5 minute intervals: 00:00, 00:05, 00:10 .. 23:55. You want to find the files missing:


.. code-block:: perl

   parallel [ -f {1}:{2} ] "||" echo {1}:{2} does not exist \
     ::: {00..23} ::: {00..55..5}



EXAMPLE: Calling Bash functions
===============================


If the composed command is longer than a line, it becomes hard to read. In Bash you can use functions. Just remember to \ **export -f**\  the function.


.. code-block:: perl

   doit() {
     echo Doing it for $1
     sleep 2
     echo Done with $1
   }
   export -f doit
   parallel doit ::: 1 2 3
 
   doubleit() {
     echo Doing it for $1 $2
     sleep 2
     echo Done with $1 $2
   }
   export -f doubleit
   parallel doubleit ::: 1 2 3 ::: a b


To do this on remote servers you need to transfer the function using \ **--env**\ :


.. code-block:: perl

   parallel --env doit -S server doit ::: 1 2 3
   parallel --env doubleit -S server doubleit ::: 1 2 3 ::: a b


If your environment (aliases, variables, and functions) is small you can copy the full environment without having to \ **export -f**\  anything. See \ **env_parallel**\ .


EXAMPLE: Function tester
========================


To test a program with different parameters:


.. code-block:: perl

   tester() {
     if (eval "$@") >&/dev/null; then
       perl -e 'printf "\033[30;102m[ OK ]\033[0m @ARGV\n"' "$@"
     else
       perl -e 'printf "\033[30;101m[FAIL]\033[0m @ARGV\n"' "$@"
     fi
   }
   export -f tester
   parallel tester my_program ::: arg1 arg2
   parallel tester exit ::: 1 0 2 0


If \ **my_program**\  fails a red FAIL will be printed followed by the failing command; otherwise a green OK will be printed followed by the command.


EXAMPLE: Continously show the latest line of output
===================================================


It can be useful to monitor the output of running jobs.

This shows the most recent output line until a job finishes. After which the output of the job is printed in full:


.. code-block:: perl

   parallel '{} | tee >(cat >&3)' ::: 'command 1' 'command 2' \
     3> >(perl -ne '$|=1;chomp;printf"%.'$COLUMNS's\r",$_." "x100')



EXAMPLE: Log rotate
===================


Log rotation renames a logfile to an extension with a higher number: log.1 becomes log.2, log.2 becomes log.3, and so on. The oldest log is removed. To avoid overwriting files the process starts backwards from the high number to the low number.  This will keep 10 old versions of the log:


.. code-block:: perl

   seq 9 -1 1 | parallel -j1 mv log.{} log.'{= $_++ =}'
   mv log log.1



EXAMPLE: Removing file extension when processing files
======================================================


When processing files removing the file extension using \ **{.}**\  is often useful.

Create a directory for each zip-file and unzip it in that dir:


.. code-block:: perl

   parallel 'mkdir {.}; cd {.}; unzip ../{}' ::: *.zip


Recompress all .gz files in current directory using \ **bzip2**\  running 1 job per CPU in parallel:


.. code-block:: perl

   parallel "zcat {} | bzip2 >{.}.bz2 && rm {}" ::: *.gz


Convert all WAV files to MP3 using LAME:


.. code-block:: perl

   find sounddir -type f -name '*.wav' | parallel lame {} -o {.}.mp3


Put all converted in the same directory:


.. code-block:: perl

   find sounddir -type f -name '*.wav' | \
     parallel lame {} -o mydir/{/.}.mp3



EXAMPLE: Removing strings from the argument
===========================================


If you have directory with tar.gz files and want these extracted in the corresponding dir (e.g foo.tar.gz will be extracted in the dir foo) you can do:


.. code-block:: perl

   parallel --plus 'mkdir {..}; tar -C {..} -xf {}' ::: *.tar.gz


If you want to remove a different ending, you can use {%string}:


.. code-block:: perl

   parallel --plus echo {%_demo} ::: mycode_demo keep_demo_here


You can also remove a starting string with {#string}


.. code-block:: perl

   parallel --plus echo {#demo_} ::: demo_mycode keep_demo_here


To remove a string anywhere you can use regular expressions with {/regexp/replacement} and leave the replacement empty:


.. code-block:: perl

   parallel --plus echo {/demo_/} ::: demo_mycode remove_demo_here



EXAMPLE: Download 24 images for each of the past 30 days
========================================================


Let us assume a website stores images like:


.. code-block:: perl

   https://www.example.com/path/to/YYYYMMDD_##.jpg


where YYYYMMDD is the date and ## is the number 01-24. This will download images for the past 30 days:


.. code-block:: perl

   getit() {
     date=$(date -d "today -$1 days" +%Y%m%d)
     num=$2
     echo wget https://www.example.com/path/to/${date}_${num}.jpg
   }
   export -f getit
   
   parallel getit ::: $(seq 30) ::: $(seq -w 24)


\ **$(date -d "today -$1 days" +%Y%m%d)**\  will give the dates in YYYYMMDD with \ **$1**\  days subtracted.


EXAMPLE: Download world map from NASA
=====================================


NASA provides tiles to download on earthdata.nasa.gov. Download tiles for Blue Marble world map and create a 10240x20480 map.


.. code-block:: perl

   base=https://map1a.vis.earthdata.nasa.gov/wmts-geo/wmts.cgi
   service="SERVICE=WMTS&REQUEST=GetTile&VERSION=1.0.0"
   layer="LAYER=BlueMarble_ShadedRelief_Bathymetry"
   set="STYLE=&TILEMATRIXSET=EPSG4326_500m&TILEMATRIX=5"
   tile="TILEROW={1}&TILECOL={2}"
   format="FORMAT=image%2Fjpeg"
   url="$base?$service&$layer&$set&$tile&$format"
 
   parallel -j0 -q wget "$url" -O {1}_{2}.jpg ::: {0..19} ::: {0..39}
   parallel eval convert +append {}_{0..39}.jpg line{}.jpg ::: {0..19}
   convert -append line{0..19}.jpg world.jpg



EXAMPLE: Download Apollo-11 images from NASA using jq
=====================================================


Search NASA using their API to get JSON for images related to 'apollo 11' and has 'moon landing' in the description.

The search query returns JSON containing URLs to JSON containing collections of pictures. One of the pictures in each of these collection is \ *large*\ .

\ **wget**\  is used to get the JSON for the search query. \ **jq**\  is then used to extract the URLs of the collections. \ **parallel**\  then calls \ **wget**\  to get each collection, which is passed to \ **jq**\  to extract the URLs of all images. \ **grep**\  filters out the \ *large*\  images, and \ **parallel**\  finally uses \ **wget**\  to fetch the images.


.. code-block:: perl

   base="https://images-api.nasa.gov/search"
   q="q=apollo 11"
   description="description=moon landing"
   media_type="media_type=image"
   wget -O - "$base?$q&$description&$media_type" |
     jq -r .collection.items[].href |
     parallel wget -O - |
     jq -r .[] |
     grep large |
     parallel wget



EXAMPLE: Download video playlist in parallel
============================================


\ **youtube-dl**\  is an excellent tool to download videos. It can, however, not download videos in parallel. This takes a playlist and downloads 10 videos in parallel.


.. code-block:: perl

   url='youtu.be/watch?v=0wOf2Fgi3DE&list=UU_cznB5YZZmvAmeq7Y3EriQ'
   export url
   youtube-dl --flat-playlist "https://$url" |
     parallel --tagstring {#} --lb -j10 \
       youtube-dl --playlist-start {#} --playlist-end {#} '"https://$url"'



EXAMPLE: Prepend last modified date (ISO8601) to file name
==========================================================



.. code-block:: perl

   parallel mv {} '{= $a=pQ($_); $b=$_;' \
     '$_=qx{date -r "$a" +%FT%T}; chomp; $_="$_ $b" =}' ::: *


\ **{=**\  and \ **=}**\  mark a perl expression. \ **pQ**\  perl-quotes the string. \ **date +%FT%T**\  is the date in ISO8601 with time.


EXAMPLE: Save output in ISO8601 dirs
====================================


Save output from \ **ps aux**\  every second into dirs named yyyy-mm-ddThh:mm:ss+zz:zz.


.. code-block:: perl

   seq 1000 | parallel -N0 -j1 --delay 1 \
     --results '{= $_=`date -Isec`; chomp=}/' ps aux



EXAMPLE: Digital clock with "blinking" :
========================================


The : in a digital clock blinks. To make every other line have a ':' and the rest a ' ' a perl expression is used to look at the 3rd input source. If the value modulo 2 is 1: Use ":" otherwise use " ":


.. code-block:: perl

   parallel -k echo {1}'{=3 $_=$_%2?":":" "=}'{2}{3} \
     ::: {0..12} ::: {0..5} ::: {0..9}



EXAMPLE: Aggregating content of files
=====================================


This:


.. code-block:: perl

   parallel --header : echo x{X}y{Y}z{Z} \> x{X}y{Y}z{Z} \
   ::: X {1..5} ::: Y {01..10} ::: Z {1..5}


will generate the files x1y01z1 .. x5y10z5. If you want to aggregate the output grouping on x and z you can do this:


.. code-block:: perl

   parallel eval 'cat {=s/y01/y*/=} > {=s/y01//=}' ::: *y01*


For all values of x and z it runs commands like:


.. code-block:: perl

   cat x1y*z1 > x1z1


So you end up with x1z1 .. x5z5 each containing the content of all values of y.


EXAMPLE: Breadth first parallel web crawler/mirrorer
====================================================


This script below will crawl and mirror a URL in parallel.  It downloads first pages that are 1 click down, then 2 clicks down, then 3; instead of the normal depth first, where the first link link on each page is fetched first.

Run like this:


.. code-block:: perl

   PARALLEL=-j100 ./parallel-crawl http://gatt.org.yeslab.org/


Remove the \ **wget**\  part if you only want a web crawler.

It works by fetching a page from a list of URLs and looking for links in that page that are within the same starting URL and that have not already been seen. These links are added to a new queue. When all the pages from the list is done, the new queue is moved to the list of URLs and the process is started over until no unseen links are found.


.. code-block:: perl

   #!/bin/bash
 
   # E.g. http://gatt.org.yeslab.org/
   URL=$1
   # Stay inside the start dir
   BASEURL=$(echo $URL | perl -pe 's:#.*::; s:(//.*/)[^/]*:$1:')
   URLLIST=$(mktemp urllist.XXXX)
   URLLIST2=$(mktemp urllist.XXXX)
   SEEN=$(mktemp seen.XXXX)
 
   # Spider to get the URLs
   echo $URL >$URLLIST
   cp $URLLIST $SEEN
 
   while [ -s $URLLIST ] ; do
     cat $URLLIST |
       parallel lynx -listonly -image_links -dump {} \; \
         wget -qm -l1 -Q1 {} \; echo Spidered: {} \>\&2 |
         perl -ne 's/#.*//; s/\s+\d+.\s(\S+)$/$1/ and
           do { $seen{$1}++ or print }' |
       grep -F $BASEURL |
       grep -v -x -F -f $SEEN | tee -a $SEEN > $URLLIST2
     mv $URLLIST2 $URLLIST
   done
 
   rm -f $URLLIST $URLLIST2 $SEEN



EXAMPLE: Process files from a tar file while unpacking
======================================================


If the files to be processed are in a tar file then unpacking one file and processing it immediately may be faster than first unpacking all files.


.. code-block:: perl

   tar xvf foo.tgz | perl -ne 'print $l;$l=$_;END{print $l}' | \
     parallel echo


The Perl one-liner is needed to make sure the file is complete before handing it to GNU \ **parallel**\ .


EXAMPLE: Rewriting a for-loop and a while-read-loop
===================================================


for-loops like this:


.. code-block:: perl

   (for x in `cat list` ; do
     do_something $x
   done) | process_output


and while-read-loops like this:


.. code-block:: perl

   cat list | (while read x ; do
     do_something $x
   done) | process_output


can be written like this:


.. code-block:: perl

   cat list | parallel do_something | process_output


For example: Find which host name in a list has IP address 1.2.3 4:


.. code-block:: perl

   cat hosts.txt | parallel -P 100 host | grep 1.2.3.4


If the processing requires more steps the for-loop like this:


.. code-block:: perl

   (for x in `cat list` ; do
     no_extension=${x%.*};
     do_step1 $x scale $no_extension.jpg
     do_step2 <$x $no_extension
   done) | process_output


and while-loops like this:


.. code-block:: perl

   cat list | (while read x ; do
     no_extension=${x%.*};
     do_step1 $x scale $no_extension.jpg
     do_step2 <$x $no_extension
   done) | process_output


can be written like this:


.. code-block:: perl

   cat list | parallel "do_step1 {} scale {.}.jpg ; do_step2 <{} {.}" |\
     process_output


If the body of the loop is bigger, it improves readability to use a function:


.. code-block:: perl

   (for x in `cat list` ; do
     do_something $x
     [... 100 lines that do something with $x ...]
   done) | process_output
 
   cat list | (while read x ; do
     do_something $x
     [... 100 lines that do something with $x ...]
   done) | process_output


can both be rewritten as:


.. code-block:: perl

   doit() {
     x=$1
     do_something $x
     [... 100 lines that do something with $x ...]
   }
   export -f doit
   cat list | parallel doit



EXAMPLE: Rewriting nested for-loops
===================================


Nested for-loops like this:


.. code-block:: perl

   (for x in `cat xlist` ; do
     for y in `cat ylist` ; do
       do_something $x $y
     done
   done) | process_output


can be written like this:


.. code-block:: perl

   parallel do_something {1} {2} :::: xlist ylist | process_output


Nested for-loops like this:


.. code-block:: perl

   (for colour in red green blue ; do
     for size in S M L XL XXL ; do
       echo $colour $size
     done
   done) | sort


can be written like this:


.. code-block:: perl

   parallel echo {1} {2} ::: red green blue ::: S M L XL XXL | sort



EXAMPLE: Finding the lowest difference between files
====================================================


\ **diff**\  is good for finding differences in text files. \ **diff | wc -l**\  gives an indication of the size of the difference. To find the differences between all files in the current dir do:


.. code-block:: perl

   parallel --tag 'diff {1} {2} | wc -l' ::: * ::: * | sort -nk3


This way it is possible to see if some files are closer to other files.


EXAMPLE: for-loops with column names
====================================


When doing multiple nested for-loops it can be easier to keep track of the loop variable if is is named instead of just having a number. Use \ **--header :**\  to let the first argument be an named alias for the positional replacement string:


.. code-block:: perl

   parallel --header : echo {colour} {size} \
     ::: colour red green blue ::: size S M L XL XXL


This also works if the input file is a file with columns:


.. code-block:: perl

   cat addressbook.tsv | \
     parallel --colsep '\t' --header : echo {Name} {E-mail address}



EXAMPLE: All combinations in a list
===================================


GNU \ **parallel**\  makes all combinations when given two lists.

To make all combinations in a single list with unique values, you repeat the list and use replacement string \ **{choose_k}**\ :


.. code-block:: perl

   parallel --plus echo {choose_k} ::: A B C D ::: A B C D
 
   parallel --plus echo 2{2choose_k} 1{1choose_k} ::: A B C D ::: A B C D


\ **{choose_k}**\  works for any number of input sources:


.. code-block:: perl

   parallel --plus echo {choose_k} ::: A B C D ::: A B C D ::: A B C D


Where \ **{choose_k}**\  does not care about order, \ **{uniq}**\  cares about order. It simply skips jobs where values from different input sources are the same:


.. code-block:: perl

   parallel --plus echo {uniq} ::: A B C  ::: A B C  ::: A B C
   parallel --plus echo {1uniq}+{2uniq}+{3uniq} ::: A B C  ::: A B C  ::: A B C



EXAMPLE: From a to b and b to c
===============================


Assume you have input like:


.. code-block:: perl

   aardvark
   babble
   cab
   dab
   each


and want to run combinations like:


.. code-block:: perl

   aardvark babble
   babble cab
   cab dab
   dab each


If the input is in the file in.txt:


.. code-block:: perl

   parallel echo {1} - {2} ::::+ <(head -n -1 in.txt) <(tail -n +2 in.txt)


If the input is in the array $a here are two solutions:


.. code-block:: perl

   seq $((${#a[@]}-1)) | \
     env_parallel --env a echo '${a[{=$_--=}]} - ${a[{}]}'
   parallel echo {1} - {2} ::: "${a[@]::${#a[@]}-1}" :::+ "${a[@]:1}"



EXAMPLE: Count the differences between all files in a dir
=========================================================


Using \ **--results**\  the results are saved in /tmp/diffcount\*.


.. code-block:: perl

   parallel --results /tmp/diffcount "diff -U 0 {1} {2} | \
     tail -n +3 |grep -v '^@'|wc -l" ::: * ::: *


To see the difference between file A and file B look at the file '/tmp/diffcount/1/A/2/B'.


EXAMPLE: Speeding up fast jobs
==============================


Starting a job on the local machine takes around 3-10 ms. This can be a big overhead if the job takes very few ms to run. Often you can group small jobs together using \ **-X**\  which will make the overhead less significant. Compare the speed of these:


.. code-block:: perl

   seq -w 0 9999 | parallel touch pict{}.jpg
   seq -w 0 9999 | parallel -X touch pict{}.jpg


If your program cannot take multiple arguments, then you can use GNU \ **parallel**\  to spawn multiple GNU \ **parallel**\ s:


.. code-block:: perl

   seq -w 0 9999999 | \
     parallel -j10 -q -I,, --pipe parallel -j0 touch pict{}.jpg


If \ **-j0**\  normally spawns 252 jobs, then the above will try to spawn 2520 jobs. On a normal GNU/Linux system you can spawn 32000 jobs using this technique with no problems. To raise the 32000 jobs limit raise /proc/sys/kernel/pid_max to 4194303.

If you do not need GNU \ **parallel**\  to have control over each job (so no need for \ **--retries**\  or \ **--joblog**\  or similar), then it can be even faster if you can generate the command lines and pipe those to a shell. So if you can do this:


.. code-block:: perl

   mygenerator | sh


Then that can be parallelized like this:


.. code-block:: perl

   mygenerator | parallel --pipe --block 10M sh


E.g.


.. code-block:: perl

   mygenerator() {
     seq 10000000 | perl -pe 'print "echo This is fast job number "';
   }
   mygenerator | parallel --pipe --block 10M sh


The overhead is 100000 times smaller namely around 100 nanoseconds per job.


EXAMPLE: Using shell variables
==============================


When using shell variables you need to quote them correctly as they may otherwise be interpreted by the shell.

Notice the difference between:


.. code-block:: perl

   ARR=("My brother's 12\" records are worth <\$\$\$>"'!' Foo Bar)
   parallel echo ::: ${ARR[@]} # This is probably not what you want


and:


.. code-block:: perl

   ARR=("My brother's 12\" records are worth <\$\$\$>"'!' Foo Bar)
   parallel echo ::: "${ARR[@]}"


When using variables in the actual command that contains special characters (e.g. space) you can quote them using \ **'"$VAR"'**\  or using "'s and \ **-q**\ :


.. code-block:: perl

   VAR="My brother's 12\" records are worth <\$\$\$>"
   parallel -q echo "$VAR" ::: '!'
   export VAR
   parallel echo '"$VAR"' ::: '!'


If \ **$VAR**\  does not contain ' then \ **"'$VAR'"**\  will also work (and does not need \ **export**\ ):


.. code-block:: perl

   VAR="My 12\" records are worth <\$\$\$>"
   parallel echo "'$VAR'" ::: '!'


If you use them in a function you just quote as you normally would do:


.. code-block:: perl

   VAR="My brother's 12\" records are worth <\$\$\$>"
   export VAR
   myfunc() { echo "$VAR" "$1"; }
   export -f myfunc
   parallel myfunc ::: '!'



EXAMPLE: Group output lines
===========================


When running jobs that output data, you often do not want the output of multiple jobs to run together. GNU \ **parallel**\  defaults to grouping the output of each job, so the output is printed when the job finishes. If you want full lines to be printed while the job is running you can use \ **--line-buffer**\ . If you want output to be printed as soon as possible you can use \ **-u**\ .

Compare the output of:


.. code-block:: perl

   parallel wget --limit-rate=100k \
     https://ftpmirror.gnu.org/parallel/parallel-20{}0822.tar.bz2 \
     ::: {12..16}
   parallel --line-buffer wget --limit-rate=100k \
     https://ftpmirror.gnu.org/parallel/parallel-20{}0822.tar.bz2 \
     ::: {12..16}
   parallel -u wget --limit-rate=100k \
     https://ftpmirror.gnu.org/parallel/parallel-20{}0822.tar.bz2 \
     ::: {12..16}



EXAMPLE: Tag output lines
=========================


GNU \ **parallel**\  groups the output lines, but it can be hard to see where the different jobs begin. \ **--tag**\  prepends the argument to make that more visible:


.. code-block:: perl

   parallel --tag wget --limit-rate=100k \
     https://ftpmirror.gnu.org/parallel/parallel-20{}0822.tar.bz2 \
     ::: {12..16}


\ **--tag**\  works with \ **--line-buffer**\  but not with \ **-u**\ :


.. code-block:: perl

   parallel --tag --line-buffer wget --limit-rate=100k \
     https://ftpmirror.gnu.org/parallel/parallel-20{}0822.tar.bz2 \
     ::: {12..16}


Check the uptime of the servers in \ *~/.parallel/sshloginfile*\ :


.. code-block:: perl

   parallel --tag -S .. --nonall uptime



EXAMPLE: Colorize output
========================


Give each job a new color. Most terminals support ANSI colors with the escape code "\033[30;3Xm" where 0 <= X <= 7:


.. code-block:: perl

     seq 10 | \
       parallel --tagstring '\033[30;3{=$_=++$::color%8=}m' seq {}
     parallel --rpl '{color} $_="\033[30;3".(++$::color%8)."m"' \
       --tagstring {color} seq {} ::: {1..10}


To get rid of the initial \t (which comes from \ **--tagstring**\ ):


.. code-block:: perl

     ... | perl -pe 's/\t//'



EXAMPLE: Keep order of output same as order of input
====================================================


Normally the output of a job will be printed as soon as it completes. Sometimes you want the order of the output to remain the same as the order of the input. This is often important, if the output is used as input for another system. \ **-k**\  will make sure the order of output will be in the same order as input even if later jobs end before earlier jobs.

Append a string to every line in a text file:


.. code-block:: perl

   cat textfile | parallel -k echo {} append_string


If you remove \ **-k**\  some of the lines may come out in the wrong order.

Another example is \ **traceroute**\ :


.. code-block:: perl

   parallel traceroute ::: qubes-os.org debian.org freenetproject.org


will give traceroute of qubes-os.org, debian.org and freenetproject.org, but it will be sorted according to which job completed first.

To keep the order the same as input run:


.. code-block:: perl

   parallel -k traceroute ::: qubes-os.org debian.org freenetproject.org


This will make sure the traceroute to qubes-os.org will be printed first.

A bit more complex example is downloading a huge file in chunks in parallel: Some internet connections will deliver more data if you download files in parallel. For downloading files in parallel see: "EXAMPLE: Download 10 images for each of the past 30 days". But if you are downloading a big file you can download the file in chunks in parallel.

To download byte 10000000-19999999 you can use \ **curl**\ :


.. code-block:: perl

   curl -r 10000000-19999999 https://example.com/the/big/file >file.part


To download a 1 GB file we need 100 10MB chunks downloaded and combined in the correct order.


.. code-block:: perl

   seq 0 99 | parallel -k curl -r \
     {}0000000-{}9999999 https://example.com/the/big/file > file



EXAMPLE: Parallel grep
======================


\ **grep -r**\  greps recursively through directories. GNU \ **parallel**\  can often speed this up.


.. code-block:: perl

   find . -type f | parallel -k -j150% -n 1000 -m grep -H -n STRING {}


This will run 1.5 job per CPU, and give 1000 arguments to \ **grep**\ .

There are situations where the above will be slower than \ **grep -r**\ :


- 
 
 If data is already in RAM. The overhead of starting jobs and buffering output may outweigh the benefit of running in parallel.
 


- 
 
 If the files are big. If a file cannot be read in a single seek, the disk may start thrashing.
 


The speedup is caused by two factors:


- 
 
 On rotating harddisks small files often require a seek for each file. By searching for more files in parallel, the arm may pass another wanted file on its way.
 


- 
 
 NVMe drives often perform better by having multiple command running in parallel.
 



EXAMPLE: Grepping n lines for m regular expressions.
====================================================


The simplest solution to grep a big file for a lot of regexps is:


.. code-block:: perl

   grep -f regexps.txt bigfile


Or if the regexps are fixed strings:


.. code-block:: perl

   grep -F -f regexps.txt bigfile


There are 3 limiting factors: CPU, RAM, and disk I/O.

RAM is easy to measure: If the \ **grep**\  process takes up most of your free memory (e.g. when running \ **top**\ ), then RAM is a limiting factor.

CPU is also easy to measure: If the \ **grep**\  takes >90% CPU in \ **top**\ , then the CPU is a limiting factor, and parallelization will speed this up.

It is harder to see if disk I/O is the limiting factor, and depending on the disk system it may be faster or slower to parallelize. The only way to know for certain is to test and measure.

Limiting factor: RAM
--------------------


The normal \ **grep -f regexps.txt bigfile**\  works no matter the size of bigfile, but if regexps.txt is so big it cannot fit into memory, then you need to split this.

\ **grep -F**\  takes around 100 bytes of RAM and \ **grep**\  takes about 500 bytes of RAM per 1 byte of regexp. So if regexps.txt is 1% of your RAM, then it may be too big.

If you can convert your regexps into fixed strings do that. E.g. if the lines you are looking for in bigfile all looks like:


.. code-block:: perl

   ID1 foo bar baz Identifier1 quux
   fubar ID2 foo bar baz Identifier2


then your regexps.txt can be converted from:


.. code-block:: perl

   ID1.*Identifier1
   ID2.*Identifier2


into:


.. code-block:: perl

   ID1 foo bar baz Identifier1
   ID2 foo bar baz Identifier2


This way you can use \ **grep -F**\  which takes around 80% less memory and is much faster.

If it still does not fit in memory you can do this:


.. code-block:: perl

   parallel --pipe-part -a regexps.txt --block 1M grep -F -f - -n bigfile | \
     sort -un | perl -pe 's/^\d+://'


The 1M should be your free memory divided by the number of CPU threads and divided by 200 for \ **grep -F**\  and by 1000 for normal \ **grep**\ . On GNU/Linux you can do:


.. code-block:: perl

   free=$(awk '/^((Swap)?Cached|MemFree|Buffers):/ { sum += $2 }
               END { print sum }' /proc/meminfo)
   percpu=$((free / 200 / $(parallel --number-of-threads)))k
 
   parallel --pipe-part -a regexps.txt --block $percpu --compress \
     grep -F -f - -n bigfile | \
     sort -un | perl -pe 's/^\d+://'


If you can live with duplicated lines and wrong order, it is faster to do:


.. code-block:: perl

   parallel --pipe-part -a regexps.txt --block $percpu --compress \
     grep -F -f - bigfile



Limiting factor: CPU
--------------------


If the CPU is the limiting factor parallelization should be done on the regexps:


.. code-block:: perl

   cat regexps.txt | parallel --pipe -L1000 --round-robin --compress \
     grep -f - -n bigfile | \
     sort -un | perl -pe 's/^\d+://'


The command will start one \ **grep**\  per CPU and read \ *bigfile*\  one time per CPU, but as that is done in parallel, all reads except the first will be cached in RAM. Depending on the size of \ *regexps.txt*\  it may be faster to use \ **--block 10m**\  instead of \ **-L1000**\ .

Some storage systems perform better when reading multiple chunks in parallel. This is true for some RAID systems and for some network file systems. To parallelize the reading of \ *bigfile*\ :


.. code-block:: perl

   parallel --pipe-part --block 100M -a bigfile -k --compress \
     grep -f regexps.txt


This will split \ *bigfile*\  into 100MB chunks and run \ **grep**\  on each of these chunks. To parallelize both reading of \ *bigfile*\  and \ *regexps.txt*\  combine the two using \ **--cat**\ :


.. code-block:: perl

   parallel --pipe-part --block 100M -a bigfile --cat cat regexps.txt \
     \| parallel --pipe -L1000 --round-robin grep -f - {}


If a line matches multiple regexps, the line may be duplicated.


Bigger problem
--------------


If the problem is too big to be solved by this, you are probably ready for Lucene.



EXAMPLE: Using remote computers
===============================


To run commands on a remote computer SSH needs to be set up and you must be able to login without entering a password (The commands \ **ssh-copy-id**\ , \ **ssh-agent**\ , and \ **sshpass**\  may help you do that).

If you need to login to a whole cluster, you typically do not want to accept the host key for every host. You want to accept them the first time and be warned if they are ever changed. To do that:


.. code-block:: perl

   # Add the servers to the sshloginfile
   (echo servera; echo serverb) > .parallel/my_cluster
   # Make sure .ssh/config exist
   touch .ssh/config
   cp .ssh/config .ssh/config.backup
   # Disable StrictHostKeyChecking temporarily
   (echo 'Host *'; echo StrictHostKeyChecking no) >> .ssh/config
   parallel --slf my_cluster --nonall true
   # Remove the disabling of StrictHostKeyChecking
   mv .ssh/config.backup .ssh/config


The servers in \ **.parallel/my_cluster**\  are now added in \ **.ssh/known_hosts**\ .

To run \ **echo**\  on \ **server.example.com**\ :


.. code-block:: perl

   seq 10 | parallel --sshlogin server.example.com echo


To run commands on more than one remote computer run:


.. code-block:: perl

   seq 10 | parallel --sshlogin s1.example.com,s2.example.net echo


Or:


.. code-block:: perl

   seq 10 | parallel --sshlogin server.example.com \
     --sshlogin server2.example.net echo


If the login username is \ *foo*\  on \ *server2.example.net*\  use:


.. code-block:: perl

   seq 10 | parallel --sshlogin server.example.com \
     --sshlogin foo@server2.example.net echo


If your list of hosts is \ *server1-88.example.net*\  with login \ *foo*\ :


.. code-block:: perl

   seq 10 | parallel -Sfoo@server{1..88}.example.net echo


To distribute the commands to a list of computers, make a file \ *mycomputers*\  with all the computers:


.. code-block:: perl

   server.example.com
   foo@server2.example.com
   server3.example.com


Then run:


.. code-block:: perl

   seq 10 | parallel --sshloginfile mycomputers echo


To include the local computer add the special sshlogin ':' to the list:


.. code-block:: perl

   server.example.com
   foo@server2.example.com
   server3.example.com
   :


GNU \ **parallel**\  will try to determine the number of CPUs on each of the remote computers, and run one job per CPU - even if the remote computers do not have the same number of CPUs.

If the number of CPUs on the remote computers is not identified correctly the number of CPUs can be added in front. Here the computer has 8 CPUs.


.. code-block:: perl

   seq 10 | parallel --sshlogin 8/server.example.com echo



EXAMPLE: Transferring of files
==============================


To recompress gzipped files with \ **bzip2**\  using a remote computer run:


.. code-block:: perl

   find logs/ -name '*.gz' | \
     parallel --sshlogin server.example.com \
     --transfer "zcat {} | bzip2 -9 >{.}.bz2"


This will list the .gz-files in the \ *logs*\  directory and all directories below. Then it will transfer the files to \ *server.example.com*\  to the corresponding directory in \ *$HOME/logs*\ . On \ *server.example.com*\  the file will be recompressed using \ **zcat**\  and \ **bzip2**\  resulting in the corresponding file with \ *.gz*\  replaced with \ *.bz2*\ .

If you want the resulting bz2-file to be transferred back to the local computer add \ *--return {.}.bz2*\ :


.. code-block:: perl

   find logs/ -name '*.gz' | \
     parallel --sshlogin server.example.com \
     --transfer --return {.}.bz2 "zcat {} | bzip2 -9 >{.}.bz2"


After the recompressing is done the \ *.bz2*\ -file is transferred back to the local computer and put next to the original \ *.gz*\ -file.

If you want to delete the transferred files on the remote computer add \ *--cleanup*\ . This will remove both the file transferred to the remote computer and the files transferred from the remote computer:


.. code-block:: perl

   find logs/ -name '*.gz' | \
     parallel --sshlogin server.example.com \
     --transfer --return {.}.bz2 --cleanup "zcat {} | bzip2 -9 >{.}.bz2"


If you want run on several computers add the computers to \ *--sshlogin*\  either using ',' or multiple \ *--sshlogin*\ :


.. code-block:: perl

   find logs/ -name '*.gz' | \
     parallel --sshlogin server.example.com,server2.example.com \
     --sshlogin server3.example.com \
     --transfer --return {.}.bz2 --cleanup "zcat {} | bzip2 -9 >{.}.bz2"


You can add the local computer using \ *--sshlogin :*\ . This will disable the removing and transferring for the local computer only:


.. code-block:: perl

   find logs/ -name '*.gz' | \
     parallel --sshlogin server.example.com,server2.example.com \
     --sshlogin server3.example.com \
     --sshlogin : \
     --transfer --return {.}.bz2 --cleanup "zcat {} | bzip2 -9 >{.}.bz2"


Often \ *--transfer*\ , \ *--return*\  and \ *--cleanup*\  are used together. They can be shortened to \ *--trc*\ :


.. code-block:: perl

   find logs/ -name '*.gz' | \
     parallel --sshlogin server.example.com,server2.example.com \
     --sshlogin server3.example.com \
     --sshlogin : \
     --trc {.}.bz2 "zcat {} | bzip2 -9 >{.}.bz2"


With the file \ *mycomputers*\  containing the list of computers it becomes:


.. code-block:: perl

   find logs/ -name '*.gz' | parallel --sshloginfile mycomputers \
     --trc {.}.bz2 "zcat {} | bzip2 -9 >{.}.bz2"


If the file \ *~/.parallel/sshloginfile*\  contains the list of computers the special short hand \ *-S ..*\  can be used:


.. code-block:: perl

   find logs/ -name '*.gz' | parallel -S .. \
     --trc {.}.bz2 "zcat {} | bzip2 -9 >{.}.bz2"



EXAMPLE: Advanced file transfer
===============================


Assume you have files in in/\*, want them processed on server, and transferred back into /other/dir:


.. code-block:: perl

   parallel -S server --trc /other/dir/./{/}.out \
     cp {/} {/}.out ::: in/./*



EXAMPLE: Distributing work to local and remote computers
========================================================


Convert \*.mp3 to \*.ogg running one process per CPU on local computer and server2:


.. code-block:: perl

   parallel --trc {.}.ogg -S server2,: \
     'mpg321 -w - {} | oggenc -q0 - -o {.}.ogg' ::: *.mp3



EXAMPLE: Running the same command on remote computers
=====================================================


To run the command \ **uptime**\  on remote computers you can do:


.. code-block:: perl

   parallel --tag --nonall -S server1,server2 uptime


\ **--nonall**\  reads no arguments. If you have a list of jobs you want to run on each computer you can do:


.. code-block:: perl

   parallel --tag --onall -S server1,server2 echo ::: 1 2 3


Remove \ **--tag**\  if you do not want the sshlogin added before the output.

If you have a lot of hosts use '-j0' to access more hosts in parallel.


EXAMPLE: Running 'sudo' on remote computers
===========================================


Put the password into passwordfile then run:


.. code-block:: perl

   parallel --ssh 'cat passwordfile | ssh' --nonall \
     -S user@server1,user@server2 sudo -S ls -l /root



EXAMPLE: Using remote computers behind NAT wall
===============================================


If the workers are behind a NAT wall, you need some trickery to get to them.

If you can \ **ssh**\  to a jumphost, and reach the workers from there, then the obvious solution would be this, but it \ **does not work**\ :


.. code-block:: perl

   parallel --ssh 'ssh jumphost ssh' -S host1 echo ::: DOES NOT WORK


It does not work because the command is dequoted by \ **ssh**\  twice where as GNU \ **parallel**\  only expects it to be dequoted once.

You can use a bash function and have GNU \ **parallel**\  quote the command:


.. code-block:: perl

   jumpssh() { ssh -A jumphost ssh $(parallel --shellquote ::: "$@"); }
   export -f jumpssh
   parallel --ssh jumpssh -S host1 echo ::: this works


Or you can instead put this in \ **~/.ssh/config**\ :


.. code-block:: perl

   Host host1 host2 host3
     ProxyCommand ssh jumphost.domain nc -w 1 %h 22


It requires \ **nc(netcat)**\  to be installed on jumphost. With this you can simply:


.. code-block:: perl

   parallel -S host1,host2,host3 echo ::: This does work


No jumphost, but port forwards
------------------------------


If there is no jumphost but each server has port 22 forwarded from the firewall (e.g. the firewall's port 22001 = port 22 on host1, 22002 = host2, 22003 = host3) then you can use \ **~/.ssh/config**\ :


.. code-block:: perl

   Host host1.v
     Port 22001
   Host host2.v
     Port 22002
   Host host3.v
     Port 22003
   Host *.v
     Hostname firewall


And then use host{1..3}.v as normal hosts:


.. code-block:: perl

   parallel -S host1.v,host2.v,host3.v echo ::: a b c



No jumphost, no port forwards
-----------------------------


If ports cannot be forwarded, you need some sort of VPN to traverse the NAT-wall. TOR is one options for that, as it is very easy to get working.

You need to install TOR and setup a hidden service. In \ **torrc**\  put:


.. code-block:: perl

   HiddenServiceDir /var/lib/tor/hidden_service/
   HiddenServicePort 22 127.0.0.1:22


Then start TOR: \ **/etc/init.d/tor restart**\ 

The TOR hostname is now in \ **/var/lib/tor/hidden_service/hostname**\  and is something similar to \ **izjafdceobowklhz.onion**\ . Now you simply prepend \ **torsocks**\  to \ **ssh**\ :


.. code-block:: perl

   parallel --ssh 'torsocks ssh' -S izjafdceobowklhz.onion \
     -S zfcdaeiojoklbwhz.onion,auclucjzobowklhi.onion echo ::: a b c


If not all hosts are accessible through TOR:


.. code-block:: perl

   parallel -S 'torsocks ssh izjafdceobowklhz.onion,host2,host3' \
     echo ::: a b c


See more \ **ssh**\  tricks on https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Proxies_and_Jump_Hosts



EXAMPLE: Use sshpass with ssh
=============================


If you cannot use passwordless login, you may be able to use \ **sshpass**\ :


.. code-block:: perl

   export SSHPASS=MyPa$$w0rd
   seq 10 |
     parallel -S '4/sshpass -e ssh user-with-password@server' echo
   seq 10 |
     parallel --ssh 'sshpass -e ssh' -S 4/user-with-password@server' echo



EXAMPLE: Use outrun instead of ssh
==================================


\ **outrun**\  lets you run a command on a remote server. \ **outrun**\  sets up a connection to access files at the source server, and automatically transfers files. \ **outrun**\  must be installed on the remote system.

You can use \ **outrun**\  in an sshlogin this way:


.. code-block:: perl

   parallel -S 'outrun user@server' command


or:


.. code-block:: perl

   parallel --ssh outrun -S server command



EXAMPLE: Slurm cluster
======================


The Slurm Workload Manager is used in many clusters.

Here is a simple example of using GNU \ **parallel**\  to call \ **srun**\ :


.. code-block:: perl

   #!/bin/bash
   
   #SBATCH --time 00:02:00
   #SBATCH --ntasks=4
   #SBATCH --job-name GnuParallelDemo
   #SBATCH --output gnuparallel.out
   
   module purge
   module load gnu_parallel
   
   my_parallel="parallel --delay .2 -j $SLURM_NTASKS"
   my_srun="srun --export=all --exclusive -n1 --cpus-per-task=1 --cpu-bind=cores"
   $my_parallel "$my_srun" echo This is job {} ::: {1..20}



EXAMPLE: Parallelizing rsync
============================


\ **rsync**\  is a great tool, but sometimes it will not fill up the available bandwidth. Running multiple \ **rsync**\  in parallel can fix this.


.. code-block:: perl

   cd src-dir
   find . -type f |
     parallel -j10 -X rsync -zR -Ha ./{} fooserver:/dest-dir/


Adjust \ **-j10**\  until you find the optimal number.

\ **rsync -R**\  will create the needed subdirectories, so all files are not put into a single dir. The \ **./**\  is needed so the resulting command looks similar to:


.. code-block:: perl

   rsync -zR ././sub/dir/file fooserver:/dest-dir/


The \ **/./**\  is what \ **rsync -R**\  works on.

If you are unable to push data, but need to pull them and the files are called digits.png (e.g. 000000.png) you might be able to do:


.. code-block:: perl

   seq -w 0 99 | parallel rsync -Havessh fooserver:src/*{}.png destdir/



EXAMPLE: Use multiple inputs in one command
===========================================


Copy files like foo.es.ext to foo.ext:


.. code-block:: perl

   ls *.es.* | perl -pe 'print; s/\.es//' | parallel -N2 cp {1} {2}


The perl command spits out 2 lines for each input. GNU \ **parallel**\  takes 2 inputs (using \ **-N2**\ ) and replaces {1} and {2} with the inputs.

Count in binary:


.. code-block:: perl

   parallel -k echo ::: 0 1 ::: 0 1 ::: 0 1 ::: 0 1 ::: 0 1 ::: 0 1


Print the number on the opposing sides of a six sided die:


.. code-block:: perl

   parallel --link -a <(seq 6) -a <(seq 6 -1 1) echo
   parallel --link echo :::: <(seq 6) <(seq 6 -1 1)


Convert files from all subdirs to PNG-files with consecutive numbers (useful for making input PNG's for \ **ffmpeg**\ ):


.. code-block:: perl

   parallel --link -a <(find . -type f | sort) \
     -a <(seq $(find . -type f|wc -l)) convert {1} {2}.png


Alternative version:


.. code-block:: perl

   find . -type f | sort | parallel convert {} {#}.png



EXAMPLE: Use a table as input
=============================


Content of table_file.tsv:


.. code-block:: perl

   foo<TAB>bar
   baz <TAB> quux


To run:


.. code-block:: perl

   cmd -o bar -i foo
   cmd -o quux -i baz


you can run:


.. code-block:: perl

   parallel -a table_file.tsv --colsep '\t' cmd -o {2} -i {1}


Note: The default for GNU \ **parallel**\  is to remove the spaces around the columns. To keep the spaces:


.. code-block:: perl

   parallel -a table_file.tsv --trim n --colsep '\t' cmd -o {2} -i {1}



EXAMPLE: Output to database
===========================


GNU \ **parallel**\  can output to a database table and a CSV-file:


.. code-block:: perl

   dburl=csv:///%2Ftmp%2Fmydir
   dbtableurl=$dburl/mytable.csv
   parallel --sqlandworker $dbtableurl seq ::: {1..10}


It is rather slow and takes up a lot of CPU time because GNU \ **parallel**\  parses the whole CSV file for each update.

A better approach is to use an SQLite-base and then convert that to CSV:


.. code-block:: perl

   dburl=sqlite3:///%2Ftmp%2Fmy.sqlite
   dbtableurl=$dburl/mytable
   parallel --sqlandworker $dbtableurl seq ::: {1..10}
   sql $dburl '.headers on' '.mode csv' 'SELECT * FROM mytable;'


This takes around a second per job.

If you have access to a real database system, such as PostgreSQL, it is even faster:


.. code-block:: perl

   dburl=pg://user:pass@host/mydb
   dbtableurl=$dburl/mytable
   parallel --sqlandworker $dbtableurl seq ::: {1..10}
   sql $dburl \
     "COPY (SELECT * FROM mytable) TO stdout DELIMITER ',' CSV HEADER;"


Or MySQL:


.. code-block:: perl

   dburl=mysql://user:pass@host/mydb
   dbtableurl=$dburl/mytable
   parallel --sqlandworker $dbtableurl seq ::: {1..10}
   sql -p -B $dburl "SELECT * FROM mytable;" > mytable.tsv
   perl -pe 's/"/""/g; s/\t/","/g; s/^/"/; s/$/"/;
     %s=("\\" => "\\", "t" => "\t", "n" => "\n");
     s/\\([\\tn])/$s{$1}/g;' mytable.tsv



EXAMPLE: Output to CSV-file for R
=================================


If you have no need for the advanced job distribution control that a database provides, but you simply want output into a CSV file that you can read into R or LibreCalc, then you can use \ **--results**\ :


.. code-block:: perl

   parallel --results my.csv seq ::: 10 20 30
   R
   > mydf <- read.csv("my.csv");
   > print(mydf[2,])
   > write(as.character(mydf[2,c("Stdout")]),'')



EXAMPLE: Use XML as input
=========================


The show Aflyttet on Radio 24syv publishes an RSS feed with their audio podcasts on: http://arkiv.radio24syv.dk/audiopodcast/channel/4466232

Using \ **xpath**\  you can extract the URLs for 2019 and download them using GNU \ **parallel**\ :


.. code-block:: perl

   wget -O - http://arkiv.radio24syv.dk/audiopodcast/channel/4466232 | \
     xpath -e "//pubDate[contains(text(),'2019')]/../enclosure/@url" | \
     parallel -u wget '{= s/ url="//; s/"//; =}'



EXAMPLE: Run the same command 10 times
======================================


If you want to run the same command with the same arguments 10 times in parallel you can do:


.. code-block:: perl

   seq 10 | parallel -n0 my_command my_args



EXAMPLE: Working as cat | sh. Resource inexpensive jobs and evaluation
======================================================================


GNU \ **parallel**\  can work similar to \ **cat | sh**\ .

A resource inexpensive job is a job that takes very little CPU, disk I/O and network I/O. Ping is an example of a resource inexpensive job. wget is too - if the webpages are small.

The content of the file jobs_to_run:


.. code-block:: perl

   ping -c 1 10.0.0.1
   wget http://example.com/status.cgi?ip=10.0.0.1
   ping -c 1 10.0.0.2
   wget http://example.com/status.cgi?ip=10.0.0.2
   ...
   ping -c 1 10.0.0.255
   wget http://example.com/status.cgi?ip=10.0.0.255


To run 100 processes simultaneously do:


.. code-block:: perl

   parallel -j 100 < jobs_to_run


As there is not a \ *command*\  the jobs will be evaluated by the shell.


EXAMPLE: Call program with FASTA sequence
=========================================


FASTA files have the format:


.. code-block:: perl

   >Sequence name1
   sequence
   sequence continued
   >Sequence name2
   sequence
   sequence continued
   more sequence


To call \ **myprog**\  with the sequence as argument run:


.. code-block:: perl

   cat file.fasta |
     parallel --pipe -N1 --recstart '>' --rrs \
       'read a; echo Name: "$a"; myprog $(tr -d "\n")'



EXAMPLE: Call program with interleaved FASTQ records
====================================================


FASTQ files have the format:


.. code-block:: perl

   @M10991:61:000000000-A7EML:1:1101:14011:1001 1:N:0:28
   CTCCTAGGTCGGCATGATGGGGGAAGGAGAGCATGGGAAGAAATGAGAGAGTAGCAAGG
   +
   #8BCCGGGGGFEFECFGGGGGGGGG@;FFGGGEG@FF<EE<@FFC,CEGCCGGFF<FGF


Interleaved FASTQ starts with a line like these:


.. code-block:: perl

   @HWUSI-EAS100R:6:73:941:1973#0/1
   @EAS139:136:FC706VJ:2:2104:15343:197393 1:Y:18:ATCACG
   @EAS139:136:FC706VJ:2:2104:15343:197393 1:N:18:1


where '/1' and ' 1:' determines this is read 1.

This will cut big.fq into one chunk per CPU thread and pass it on stdin (standard input) to the program fastq-reader:


.. code-block:: perl

   parallel --pipe-part -a big.fq --block -1 --regexp \
     --recend '\n' --recstart '@.*(/1| 1:.*)\n[A-Za-z\n\.~]' \
     fastq-reader



EXAMPLE: Processing a big file using more CPUs
==============================================


To process a big file or some output you can use \ **--pipe**\  to split up the data into blocks and pipe the blocks into the processing program.

If the program is \ **gzip -9**\  you can do:


.. code-block:: perl

   cat bigfile | parallel --pipe --recend '' -k gzip -9 > bigfile.gz


This will split \ **bigfile**\  into blocks of 1 MB and pass that to \ **gzip -9**\  in parallel. One \ **gzip**\  will be run per CPU. The output of \ **gzip -9**\  will be kept in order and saved to \ **bigfile.gz**\ 

\ **gzip**\  works fine if the output is appended, but some processing does not work like that - for example sorting. For this GNU \ **parallel**\  can put the output of each command into a file. This will sort a big file in parallel:


.. code-block:: perl

   cat bigfile | parallel --pipe --files sort |\
     parallel -Xj1 sort -m {} ';' rm {} >bigfile.sort


Here \ **bigfile**\  is split into blocks of around 1MB, each block ending in '\n' (which is the default for \ **--recend**\ ). Each block is passed to \ **sort**\  and the output from \ **sort**\  is saved into files. These files are passed to the second \ **parallel**\  that runs \ **sort -m**\  on the files before it removes the files. The output is saved to \ **bigfile.sort**\ .

GNU \ **parallel**\ 's \ **--pipe**\  maxes out at around 100 MB/s because every byte has to be copied through GNU \ **parallel**\ . But if \ **bigfile**\  is a real (seekable) file GNU \ **parallel**\  can by-pass the copying and send the parts directly to the program:


.. code-block:: perl

   parallel --pipe-part --block 100m -a bigfile --files sort |\
     parallel -Xj1 sort -m {} ';' rm {} >bigfile.sort



EXAMPLE: Grouping input lines
=============================


When processing with \ **--pipe**\  you may have lines grouped by a value. Here is \ *my.csv*\ :


.. code-block:: perl

    Transaction Customer Item
 	1	a	53
 	2	b	65
 	3	b	82
 	4	c	96
 	5	c	67
 	6	c	13
 	7	d	90
 	8	d	43
 	9	d	91
 	10	d	84
 	11	e	72
 	12	e	102
 	13	e	63
 	14	e	56
 	15	e	74


Let us assume you want GNU \ **parallel**\  to process each customer. In other words: You want all the transactions for a single customer to be treated as a single record.

To do this we preprocess the data with a program that inserts a record separator before each customer (column 2 = $F[1]). Here we first make a 50 character random string, which we then use as the separator:


.. code-block:: perl

   sep=`perl -e 'print map { ("a".."z","A".."Z")[rand(52)] } (1..50);'`
   cat my.csv | \
      perl -ape '$F[1] ne $l and print "'$sep'"; $l = $F[1]' | \
      parallel --recend $sep --rrs --pipe -N1 wc


If your program can process multiple customers replace \ **-N1**\  with a reasonable \ **--blocksize**\ .


EXAMPLE: Running more than 250 jobs workaround
==============================================


If you need to run a massive amount of jobs in parallel, then you will likely hit the filehandle limit which is often around 250 jobs. If you are super user you can raise the limit in /etc/security/limits.conf but you can also use this workaround. The filehandle limit is per process. That means that if you just spawn more GNU \ **parallel**\ s then each of them can run 250 jobs. This will spawn up to 2500 jobs:


.. code-block:: perl

   cat myinput |\
     parallel --pipe -N 50 --round-robin -j50 parallel -j50 your_prg


This will spawn up to 62500 jobs (use with caution - you need 64 GB RAM to do this, and you may need to increase /proc/sys/kernel/pid_max):


.. code-block:: perl

   cat myinput |\
     parallel --pipe -N 250 --round-robin -j250 parallel -j250 your_prg



EXAMPLE: Working as mutex and counting semaphore
================================================


The command \ **sem**\  is an alias for \ **parallel --semaphore**\ .

A counting semaphore will allow a given number of jobs to be started in the background.  When the number of jobs are running in the background, GNU \ **sem**\  will wait for one of these to complete before starting another command. \ **sem --wait**\  will wait for all jobs to complete.

Run 10 jobs concurrently in the background:


.. code-block:: perl

   for i in *.log ; do
     echo $i
     sem -j10 gzip $i ";" echo done
   done
   sem --wait


A mutex is a counting semaphore allowing only one job to run. This will edit the file \ *myfile*\  and prepends the file with lines with the numbers 1 to 3.


.. code-block:: perl

   seq 3 | parallel sem sed -i -e '1i{}' myfile


As \ *myfile*\  can be very big it is important only one process edits the file at the same time.

Name the semaphore to have multiple different semaphores active at the same time:


.. code-block:: perl

   seq 3 | parallel sem --id mymutex sed -i -e '1i{}' myfile



EXAMPLE: Mutex for a script
===========================


Assume a script is called from cron or from a web service, but only one instance can be run at a time. With \ **sem**\  and \ **--shebang-wrap**\  the script can be made to wait for other instances to finish. Here in \ **bash**\ :


.. code-block:: perl

   #!/usr/bin/sem --shebang-wrap -u --id $0 --fg /bin/bash
   
   echo This will run
   sleep 5
   echo exclusively


Here \ **perl**\ :


.. code-block:: perl

   #!/usr/bin/sem --shebang-wrap -u --id $0 --fg /usr/bin/perl
   
   print "This will run ";
   sleep 5;
   print "exclusively\n";


Here \ **python**\ :


.. code-block:: perl

   #!/usr/local/bin/sem --shebang-wrap -u --id $0 --fg /usr/bin/python
   
   import time
   print "This will run ";
   time.sleep(5)
   print "exclusively";



EXAMPLE: Start editor with filenames from stdin (standard input)
================================================================


You can use GNU \ **parallel**\  to start interactive programs like emacs or vi:


.. code-block:: perl

   cat filelist | parallel --tty -X emacs
   cat filelist | parallel --tty -X vi


If there are more files than will fit on a single command line, the editor will be started again with the remaining files.


EXAMPLE: Running sudo
=====================


\ **sudo**\  requires a password to run a command as root. It caches the access, so you only need to enter the password again if you have not used \ **sudo**\  for a while.

The command:


.. code-block:: perl

   parallel sudo echo ::: This is a bad idea


is no good, as you would be prompted for the sudo password for each of the jobs. You can either do:


.. code-block:: perl

   sudo echo This
   parallel sudo echo ::: is a good idea


or:


.. code-block:: perl

   sudo parallel echo ::: This is a good idea


This way you only have to enter the sudo password once.


EXAMPLE: GNU Parallel as queue system/batch manager
===================================================


GNU \ **parallel**\  can work as a simple job queue system or batch manager. The idea is to put the jobs into a file and have GNU \ **parallel**\  read from that continuously. As GNU \ **parallel**\  will stop at end of file we use \ **tail**\  to continue reading:


.. code-block:: perl

   true >jobqueue; tail -n+0 -f jobqueue | parallel


To submit your jobs to the queue:


.. code-block:: perl

   echo my_command my_arg >> jobqueue


You can of course use \ **-S**\  to distribute the jobs to remote computers:


.. code-block:: perl

   true >jobqueue; tail -n+0 -f jobqueue | parallel -S ..


Output only will be printed when reading the next input after a job has finished: So you need to submit a job after the first has finished to see the output from the first job.

If you keep this running for a long time, jobqueue will grow. A way of removing the jobs already run is by making GNU \ **parallel**\  stop when it hits a special value and then restart. To use \ **--eof**\  to make GNU \ **parallel**\  exit, \ **tail**\  also needs to be forced to exit:


.. code-block:: perl

   true >jobqueue;
   while true; do
     tail -n+0 -f jobqueue |
       (parallel -E StOpHeRe -S ..; echo GNU Parallel is now done;
        perl -e 'while(<>){/StOpHeRe/ and last};print <>' jobqueue > j2;
        (seq 1000 >> jobqueue &);
        echo Done appending dummy data forcing tail to exit)
     echo tail exited;
     mv j2 jobqueue
   done


In some cases you can run on more CPUs and computers during the night:


.. code-block:: perl

   # Day time
   echo 50% > jobfile
   cp day_server_list ~/.parallel/sshloginfile
   # Night time
   echo 100% > jobfile
   cp night_server_list ~/.parallel/sshloginfile
   tail -n+0 -f jobqueue | parallel --jobs jobfile -S ..


GNU \ **parallel**\  discovers if \ **jobfile**\  or \ **~/.parallel/sshloginfile**\  changes.


EXAMPLE: GNU Parallel as dir processor
======================================


If you have a dir in which users drop files that needs to be processed you can do this on GNU/Linux (If you know what \ **inotifywait**\  is called on other platforms file a bug report):


.. code-block:: perl

   inotifywait -qmre MOVED_TO -e CLOSE_WRITE --format %w%f my_dir |\
     parallel -u echo


This will run the command \ **echo**\  on each file put into \ **my_dir**\  or subdirs of \ **my_dir**\ .

You can of course use \ **-S**\  to distribute the jobs to remote computers:


.. code-block:: perl

   inotifywait -qmre MOVED_TO -e CLOSE_WRITE --format %w%f my_dir |\
     parallel -S ..  -u echo


If the files to be processed are in a tar file then unpacking one file and processing it immediately may be faster than first unpacking all files. Set up the dir processor as above and unpack into the dir.

Using GNU \ **parallel**\  as dir processor has the same limitations as using GNU \ **parallel**\  as queue system/batch manager.


EXAMPLE: Locate the missing package
===================================


If you have downloaded source and tried compiling it, you may have seen:


.. code-block:: perl

   $ ./configure
   [...]
   checking for something.h... no
   configure: error: "libsomething not found"


Often it is not obvious which package you should install to get that file. Debian has \`apt-file\` to search for a file. \`tracefile\` from https://gitlab.com/ole.tange/tangetools can tell which files a program tried to access. In this case we are interested in one of the last files:


.. code-block:: perl

   $ tracefile -un ./configure | tail | parallel -j0 apt-file search




************************
SPREADING BLOCKS OF DATA
************************


\ **--round-robin**\ , \ **--pipe-part**\ , \ **--shard**\ , \ **--bin**\  and \ **--group-by**\  are all specialized versions of \ **--pipe**\ .

In the following \ *n*\  is the number of jobslots given by \ **--jobs**\ . A record starts with \ **--recstart**\  and ends with \ **--recend**\ . It is typically a full line. A chunk is a number of full records that is approximately the size of a block. A block can contain half records, a chunk cannot.

\ **--pipe**\  starts one job per chunk. It reads blocks from stdin (standard input). It finds a record end near a block border and passes a chunk to the program.

\ **--pipe-part**\  starts one job per chunk - just like normal \ **--pipe**\ . It first finds record endings near all block borders in the file and then starts the jobs. By using \ **--block -1**\  it will set the block size to size-of-file/\ *n*\ . Used this way it will start \ *n*\  jobs in total.

\ **--round-robin**\  starts \ *n*\  jobs in total. It reads a block and passes a chunk to whichever job is ready to read. It does not parse the content except for identifying where a record ends to make sure it only passes full records.

\ **--shard**\  starts \ *n*\  jobs in total. It parses each line to read the value in the given column. Based on this value the line is passed to one of the \ *n*\  jobs. All lines having this value will be given to the same jobslot.

\ **--bin**\  works like \ **--shard**\  but the value of the column is the jobslot number it will be passed to. If the value is bigger than \ *n*\ , then \ *n*\  will be subtracted from the value until the values is smaller than or equal to \ *n*\ .

\ **--group-by**\  starts one job per chunk. Record borders are not given by \ **--recend**\ /\ **--recstart**\ . Instead a record is defined by a number of lines having the same value in a given column. So the value of a given column changes at a chunk border. With \ **--pipe**\  every line is parsed, with \ **--pipe-part**\  only a few lines are parsed to find the chunk border.

\ **--group-by**\  can be combined with \ **--round-robin**\  or \ **--pipe-part**\ .


**************
TIME POSTFIXES
**************


Arguments that give a duration are given in seconds, but can be expressed as floats postfixed with \ **s**\ , \ **m**\ , \ **h**\ , or \ **d**\  which would multiply the float by 1, 60, 60\*60, or 60\*60\*24. Thus these are equivalent: 100000 and 1d3.5h16.6m4s.


***********
UNIT PREFIX
***********


Many numerical arguments in GNU \ **parallel**\  can be postfixed with K, M, G, T, P, k, m, g, t, or p which would multiply the number with 1024, 1048576, 1073741824, 1099511627776, 1125899906842624, 1000, 1000000, 1000000000, 1000000000000, or 1000000000000000, respectively.

You can even give it as a math expression. E.g. 1000000 can be written as 1M-12\*2.024\*2k.


*******
QUOTING
*******


GNU \ **parallel**\  is very liberal in quoting. You only need to quote characters that have special meaning in shell:


.. code-block:: perl

   ( ) $ ` ' " < > ; | \


and depending on context these needs to be quoted, too:


.. code-block:: perl

   ~ & # ! ? space * {


Therefore most people will never need more quoting than putting '\' in front of the special characters.

Often you can simply put \' around every ':


.. code-block:: perl

   perl -ne '/^\S+\s+\S+$/ and print $ARGV,"\n"' file


can be quoted:


.. code-block:: perl

   parallel perl -ne \''/^\S+\s+\S+$/ and print $ARGV,"\n"'\' ::: file


However, when you want to use a shell variable you need to quote the $-sign. Here is an example using $PARALLEL_SEQ. This variable is set by GNU \ **parallel**\  itself, so the evaluation of the $ must be done by the sub shell started by GNU \ **parallel**\ :


.. code-block:: perl

   seq 10 | parallel -N2 echo seq:\$PARALLEL_SEQ arg1:{1} arg2:{2}


If the variable is set before GNU \ **parallel**\  starts you can do this:


.. code-block:: perl

   VAR=this_is_set_before_starting
   echo test | parallel echo {} $VAR


Prints: \ **test this_is_set_before_starting**\ 

It is a little more tricky if the variable contains more than one space in a row:


.. code-block:: perl

   VAR="two  spaces  between  each  word"
   echo test | parallel echo {} \'"$VAR"\'


Prints: \ **test two  spaces  between  each  word**\ 

If the variable should not be evaluated by the shell starting GNU \ **parallel**\  but be evaluated by the sub shell started by GNU \ **parallel**\ , then you need to quote it:


.. code-block:: perl

   echo test | parallel VAR=this_is_set_after_starting \; echo {} \$VAR


Prints: \ **test this_is_set_after_starting**\ 

It is a little more tricky if the variable contains space:


.. code-block:: perl

   echo test |\
     parallel VAR='"two  spaces  between  each  word"' echo {} \'"$VAR"\'


Prints: \ **test two  spaces  between  each  word**\ 

$$ is the shell variable containing the process id of the shell. This will print the process id of the shell running GNU \ **parallel**\ :


.. code-block:: perl

   seq 10 | parallel echo $$


And this will print the process ids of the sub shells started by GNU \ **parallel**\ .


.. code-block:: perl

   seq 10 | parallel echo \$\$


If the special characters should not be evaluated by the sub shell then you need to protect it against evaluation from both the shell starting GNU \ **parallel**\  and the sub shell:


.. code-block:: perl

   echo test | parallel echo {} \\\$VAR


Prints: \ **test $VAR**\ 

GNU \ **parallel**\  can protect against evaluation by the sub shell by using -q:


.. code-block:: perl

   echo test | parallel -q echo {} \$VAR


Prints: \ **test $VAR**\ 

This is particularly useful if you have lots of quoting. If you want to run a perl script like this:


.. code-block:: perl

   perl -ne '/^\S+\s+\S+$/ and print $ARGV,"\n"' file


It needs to be quoted like one of these:


.. code-block:: perl

   ls | parallel perl -ne '/^\\S+\\s+\\S+\$/\ and\ print\ \$ARGV,\"\\n\"'
   ls | parallel perl -ne \''/^\S+\s+\S+$/ and print $ARGV,"\n"'\'


Notice how spaces, \'s, "'s, and $'s need to be quoted. GNU \ **parallel**\  can do the quoting by using option -q:


.. code-block:: perl

   ls | parallel -q  perl -ne '/^\S+\s+\S+$/ and print $ARGV,"\n"'


However, this means you cannot make the sub shell interpret special characters. For example because of \ **-q**\  this WILL NOT WORK:


.. code-block:: perl

   ls *.gz | parallel -q "zcat {} >{.}"
   ls *.gz | parallel -q "zcat {} | bzip2 >{.}.bz2"


because > and | need to be interpreted by the sub shell.

If you get errors like:


.. code-block:: perl

   sh: -c: line 0: syntax error near unexpected token
   sh: Syntax error: Unterminated quoted string
   sh: -c: line 0: unexpected EOF while looking for matching `''
   sh: -c: line 1: syntax error: unexpected end of file
   zsh:1: no matches found:


then you might try using \ **-q**\ .

If you are using \ **bash**\  process substitution like \ **<(cat foo)**\  then you may try \ **-q**\  and prepending \ *command*\  with \ **bash -c**\ :


.. code-block:: perl

   ls | parallel -q bash -c 'wc -c <(echo {})'


Or for substituting output:


.. code-block:: perl

   ls | parallel -q bash -c \
     'tar c {} | tee >(gzip >{}.tar.gz) | bzip2 >{}.tar.bz2'


\ **Conclusion**\ : If this is confusing consider avoiding having to deal with quoting by writing a small script or a function (remember to \ **export -f**\  the function) and have GNU \ **parallel**\  call that.


*****************
LIST RUNNING JOBS
*****************


If you want a list of the jobs currently running you can run:


.. code-block:: perl

   killall -USR1 parallel


GNU \ **parallel**\  will then print the currently running jobs on stderr (standard error).


***********************************************
COMPLETE RUNNING JOBS BUT DO NOT START NEW JOBS
***********************************************


If you regret starting a lot of jobs you can simply break GNU \ **parallel**\ , but if you want to make sure you do not have half-completed jobs you should send the signal \ **SIGHUP**\  to GNU \ **parallel**\ :


.. code-block:: perl

   killall -HUP parallel


This will tell GNU \ **parallel**\  to not start any new jobs, but wait until the currently running jobs are finished before exiting.


*********************
ENVIRONMENT VARIABLES
*********************



- $PARALLEL_HOME
 
 Dir where GNU \ **parallel**\  stores config files, semaphores, and caches information between invocations. If set to a non-existent dir, the dir will be created.
 
 Default: $HOME/.parallel.
 


- $PARALLEL_ARGHOSTGROUPS
 
 When using \ **--hostgroups**\  GNU \ **parallel**\  sets this to the hostgroups of the job.
 
 Remember to quote the $, so it gets evaluated by the correct shell. Or use \ **--plus**\  and {agrp}.
 


- $PARALLEL_HOSTGROUPS
 
 When using \ **--hostgroups**\  GNU \ **parallel**\  sets this to the hostgroups of the sshlogin that the job is run on.
 
 Remember to quote the $, so it gets evaluated by the correct shell. Or use \ **--plus**\  and {hgrp}.
 


- $PARALLEL_JOBSLOT
 
 Set by GNU \ **parallel**\  and can be used in jobs run by GNU \ **parallel**\ . Remember to quote the $, so it gets evaluated by the correct shell. Or use \ **--plus**\  and {slot}.
 
 $PARALLEL_JOBSLOT is the jobslot of the job. It is equal to {%} unless the job is being retried. See {%} for details.
 


- $PARALLEL_PID
 
 Set by GNU \ **parallel**\  and can be used in jobs run by GNU \ **parallel**\ . Remember to quote the $, so it gets evaluated by the correct shell.
 
 This makes it possible for the jobs to communicate directly to GNU \ **parallel**\ .
 
 \ **Example:**\  If each of the jobs tests a solution and one of jobs finds the solution the job can tell GNU \ **parallel**\  not to start more jobs by: \ **kill -HUP $PARALLEL_PID**\ . This only works on the local computer.
 


- $PARALLEL_RSYNC_OPTS
 
 Options to pass on to \ **rsync**\ . Defaults to: -rlDzR.
 


- $PARALLEL_SHELL
 
 Use this shell for the commands run by GNU \ **parallel**\ :
 
 
 - 
  
  $PARALLEL_SHELL. If undefined use:
  
 
 
 - 
  
  The shell that started GNU \ **parallel**\ . If that cannot be determined:
  
 
 
 - 
  
  $SHELL. If undefined use:
  
 
 
 - 
  
  /bin/sh
  
 
 


- $PARALLEL_SSH
 
 GNU \ **parallel**\  defaults to using the \ **ssh**\  command for remote access. This can be overridden with $PARALLEL_SSH, which again can be overridden with \ **--ssh**\ . It can also be set on a per server basis (see \ **--sshlogin**\ ).
 


- $PARALLEL_SSHHOST
 
 Set by GNU \ **parallel**\  and can be used in jobs run by GNU \ **parallel**\ . Remember to quote the $, so it gets evaluated by the correct shell. Or use \ **--plus**\  and {host}.
 
 $PARALLEL_SSHHOST is the host part of an sshlogin line. E.g.
 
 
 .. code-block:: perl
 
    4//usr/bin/specialssh user@host
 
 
 becomes:
 
 
 .. code-block:: perl
 
    host
 
 


- $PARALLEL_SSHLOGIN
 
 Set by GNU \ **parallel**\  and can be used in jobs run by GNU \ **parallel**\ . Remember to quote the $, so it gets evaluated by the correct shell. Or use \ **--plus**\  and {sshlogin}.
 
 The value is the sshlogin line with number of threads removed. E.g.
 
 
 .. code-block:: perl
 
    4//usr/bin/specialssh user@host
 
 
 becomes:
 
 
 .. code-block:: perl
 
    /usr/bin/specialssh user@host
 
 


- $PARALLEL_SEQ
 
 Set by GNU \ **parallel**\  and can be used in jobs run by GNU \ **parallel**\ . Remember to quote the $, so it gets evaluated by the correct shell.
 
 $PARALLEL_SEQ is the sequence number of the job running.
 
 \ **Example:**\ 
 
 
 .. code-block:: perl
 
    seq 10 | parallel -N2 \
      echo seq:'$'PARALLEL_SEQ arg1:{1} arg2:{2}
 
 
 {#} is a shorthand for $PARALLEL_SEQ.
 


- $PARALLEL_TMUX
 
 Path to \ **tmux**\ . If unset the \ **tmux**\  in $PATH is used.
 


- $TMPDIR
 
 Directory for temporary files.
 
 See also: \ **--tmpdir**\ 
 


- $PARALLEL_REMOTE_TMPDIR
 
 Directory for temporary files on remote servers.
 
 See also: \ **--tmpdir**\ 
 


- $PARALLEL
 
 The environment variable $PARALLEL will be used as default options for GNU \ **parallel**\ . If the variable contains special shell characters (e.g. $, \*, or space) then these need to be to be escaped with \.
 
 \ **Example:**\ 
 
 
 .. code-block:: perl
 
    cat list | parallel -j1 -k -v ls
    cat list | parallel -j1 -k -v -S"myssh user@server" ls
 
 
 can be written as:
 
 
 .. code-block:: perl
 
    cat list | PARALLEL="-kvj1" parallel ls
    cat list | PARALLEL='-kvj1 -S myssh\ user@server' \
      parallel echo
 
 
 Notice the \ after 'myssh' is needed because 'myssh' and 'user@server' must be one argument.
 
 See also: \ **--profile**\ 
 



*****************************
DEFAULT PROFILE (CONFIG FILE)
*****************************


The global configuration file /etc/parallel/config, followed by user configuration file ~/.parallel/config (formerly known as .parallelrc) will be read in turn if they exist.  Lines starting with '#' will be ignored. The format can follow that of the environment variable $PARALLEL, but it is often easier to simply put each option on its own line.

Options on the command line take precedence, followed by the environment variable $PARALLEL, user configuration file ~/.parallel/config, and finally the global configuration file /etc/parallel/config.

Note that no file that is read for options, nor the environment variable $PARALLEL, may contain retired options such as \ **--tollef**\ .


*************
PROFILE FILES
*************


If \ **--profile**\  set, GNU \ **parallel**\  will read the profile from that file rather than the global or user configuration files. You can have multiple \ **--profiles**\ .

Profiles are searched for in \ **~/.parallel**\ . If the name starts with \ **/**\  it is seen as an absolute path. If the name starts with \ **./**\  it is seen as a relative path from current dir.

Example: Profile for running a command on every sshlogin in ~/.ssh/sshlogins and prepend the output with the sshlogin:


.. code-block:: perl

   echo --tag -S .. --nonall > ~/.parallel/nonall_profile
   parallel -J nonall_profile uptime


Example: Profile for running every command with \ **-j-1**\  and \ **nice**\ 


.. code-block:: perl

   echo -j-1 nice > ~/.parallel/nice_profile
   parallel -J nice_profile bzip2 -9 ::: *


Example: Profile for running a perl script before every command:


.. code-block:: perl

   echo "perl -e '\$a=\$\$; print \$a,\" \",'\$PARALLEL_SEQ',\" \";';" \
     > ~/.parallel/pre_perl
   parallel -J pre_perl echo ::: *


Note how the $ and " need to be quoted using \.

Example: Profile for running distributed jobs with \ **nice**\  on the remote computers:


.. code-block:: perl

   echo -S .. nice > ~/.parallel/dist
   parallel -J dist --trc {.}.bz2 bzip2 -9 ::: *



***********
EXIT STATUS
***********


Exit status depends on \ **--halt-on-error**\  if one of these is used: success=X, success=Y%, fail=Y%.


- 0
 
 All jobs ran without error. If success=X is used: X jobs ran without error. If success=Y% is used: Y% of the jobs ran without error.
 


- 1-100
 
 Some of the jobs failed. The exit status gives the number of failed jobs. If Y% is used the exit status is the percentage of jobs that failed.
 


- 101
 
 More than 100 jobs failed.
 


- 255
 
 Other error.
 


- -1 (In joblog and SQL table)
 
 Killed by Ctrl-C, timeout, not enough memory or similar.
 


- -2 (In joblog and SQL table)
 
 skip() was called in \ **{= =}**\ .
 


- -1000 (In SQL table)
 
 Job is ready to run (set by --sqlmaster).
 


- -1220 (In SQL table)
 
 Job is taken by worker (set by --sqlworker).
 


If fail=1 is used, the exit status will be the exit status of the failing job.


*************************************************
DIFFERENCES BETWEEN GNU Parallel AND ALTERNATIVES
*************************************************


See: \ **man parallel_alternatives**\ 


****
BUGS
****


Quoting of newline
==================


Because of the way newline is quoted this will not work:


.. code-block:: perl

   echo 1,2,3 | parallel -vkd, "echo 'a{}b'"


However, these will all work:


.. code-block:: perl

   echo 1,2,3 | parallel -vkd, echo a{}b
   echo 1,2,3 | parallel -vkd, "echo 'a'{}'b'"
   echo 1,2,3 | parallel -vkd, "echo 'a'"{}"'b'"



Speed
=====


Startup
-------


GNU \ **parallel**\  is slow at starting up - around 250 ms the first time and 150 ms after that.


Job startup
-----------


Starting a job on the local machine takes around 3-10 ms. This can be a big overhead if the job takes very few ms to run. Often you can group small jobs together using \ **-X**\  which will make the overhead less significant. Or you can run multiple GNU \ **parallel**\ s as described in \ **EXAMPLE: Speeding up fast jobs**\ .


SSH
---


When using multiple computers GNU \ **parallel**\  opens \ **ssh**\  connections to them to figure out how many connections can be used reliably simultaneously (Namely SSHD's MaxStartups). This test is done for each host in serial, so if your \ **--sshloginfile**\  contains many hosts it may be slow.

If your jobs are short you may see that there are fewer jobs running on the remote systems than expected. This is due to time spent logging in and out. \ **-M**\  may help here.


Disk access
-----------


A single disk can normally read data faster if it reads one file at a time instead of reading a lot of files in parallel, as this will avoid disk seeks. However, newer disk systems with multiple drives can read faster if reading from multiple files in parallel.

If the jobs are of the form read-all-compute-all-write-all, so everything is read before anything is written, it may be faster to force only one disk access at the time:


.. code-block:: perl

   sem --id diskio cat file | compute | sem --id diskio cat > file


If the jobs are of the form read-compute-write, so writing starts before all reading is done, it may be faster to force only one reader and writer at the time:


.. code-block:: perl

   sem --id read cat file | compute | sem --id write cat > file


If the jobs are of the form read-compute-read-compute, it may be faster to run more jobs in parallel than the system has CPUs, as some of the jobs will be stuck waiting for disk access.



--nice limits command length
============================


The current implementation of \ **--nice**\  is too pessimistic in the max allowed command length. It only uses a little more than half of what it could. This affects \ **-X**\  and \ **-m**\ . If this becomes a real problem for you, file a bug-report.


Aliases and functions do not work
=================================


If you get:


.. code-block:: perl

   Can't exec "command": No such file or directory


or:


.. code-block:: perl

   open3: exec of by command failed


or:


.. code-block:: perl

   /bin/bash: command: command not found


it may be because \ *command*\  is not known, but it could also be because \ *command*\  is an alias or a function. If it is a function you need to \ **export -f**\  the function first or use \ **env_parallel**\ . An alias will only work if you use \ **env_parallel**\ .


Database with MySQL fails randomly
==================================


The \ **--sql\***\  options may fail randomly with MySQL. This problem does not exist with PostgreSQL.



**************
REPORTING BUGS
**************


Report bugs to <parallel@gnu.org> or https://savannah.gnu.org/bugs/?func=additem&group=parallel

When you write your report, please keep in mind, that you must give the reader enough information to be able to run exactly what you run. So you need to include all data and programs that you use to show the problem.

See a perfect bug report on https://lists.gnu.org/archive/html/bug-parallel/2015-01/msg00000.html

Your bug report should always include:


- 
 
 The error message you get (if any). If the error message is not from GNU \ **parallel**\  you need to show why you think GNU \ **parallel**\  caused this.
 


- 
 
 The complete output of \ **parallel --version**\ . If you are not running the latest released version (see https://ftp.gnu.org/gnu/parallel/) you should specify why you believe the problem is not fixed in that version.
 


- 
 
 A minimal, complete, and verifiable example (See description on https://stackoverflow.com/help/mcve).
 
 It should be a complete example that others can run which shows the problem including all files needed to run the example. This should preferably be small and simple, so try to remove as many options as possible.
 
 A combination of \ **yes**\ , \ **seq**\ , \ **cat**\ , \ **echo**\ , \ **wc**\ , and \ **sleep**\  can reproduce most errors.
 
 If your example requires large files, see if you can make them with something like \ **seq 100000000**\  > \ **bigfile**\  or \ **yes | head -n 1000000000**\  > \ **file**\ . If you need multiple columns: \ **paste <(seq 1000) <(seq 1000 1999)**\ 
 
 If your example requires remote execution, see if you can use \ **localhost**\  - maybe using another login.
 
 If you have access to a different system (maybe a VirtualBox on your own machine), test if your MCVE shows the problem on that system. If it does not, read below.
 


- 
 
 The output of your example. If your problem is not easily reproduced by others, the output might help them figure out the problem.
 


- 
 
 Whether you have watched the intro videos (https://www.youtube.com/playlist?list=PL284C9FF2488BC6D1), walked through the tutorial (man parallel_tutorial), and read the EXAMPLE section in the man page (man parallel - search for EXAMPLE:).
 


Bug dependent on environment
============================


If you suspect the error is dependent on your environment or distribution, please see if you can reproduce the error on one of these VirtualBox images: https://sourceforge.net/projects/virtualboximage/files/ https://www.osboxes.org/virtualbox-images/

Specifying the name of your distribution is not enough as you may have installed software that is not in the VirtualBox images.

If you cannot reproduce the error on any of the VirtualBox images above, see if you can build a VirtualBox image on which you can reproduce the error. If not you should assume the debugging will be done through you. That will put a lot more burden on you and it is extra important you give any information that help. In general the problem will be fixed faster and with much less work for you if you can reproduce the error on a VirtualBox - even if you have to build a VirtualBox image.


In summary
==========


Your report must include:


- 
 
 \ **parallel --version**\ 
 


- 
 
 output + error message
 


- 
 
 full example including all files
 


- 
 
 VirtualBox image, if you cannot reproduce it on other systems
 




******
AUTHOR
******


When using GNU \ **parallel**\  for a publication please cite:

O. Tange (2011): GNU Parallel - The Command-Line Power Tool, ;login: The USENIX Magazine, February 2011:42-47.

This helps funding further development; and it won't cost you a cent. If you pay 10000 EUR you should feel free to use GNU Parallel without citing.

Copyright (C) 2007-10-18 Ole Tange, http://ole.tange.dk

Copyright (C) 2008-2010 Ole Tange, http://ole.tange.dk

Copyright (C) 2010-2022 Ole Tange, http://ole.tange.dk and Free Software Foundation, Inc.

Parts of the manual concerning \ **xargs**\  compatibility is inspired by the manual of \ **xargs**\  from GNU findutils 4.4.2.


*******
LICENSE
*******


This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or at your option any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program.  If not, see <https://www.gnu.org/licenses/>.

Documentation license I
=======================


Permission is granted to copy, distribute and/or modify this documentation under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.  A copy of the license is included in the file LICENSES/GFDL-1.3-or-later.txt.


Documentation license II
========================


You are free:


- \ **to Share**\ 
 
 to copy, distribute and transmit the work
 


- \ **to Remix**\ 
 
 to adapt the work
 


Under the following conditions:


- \ **Attribution**\ 
 
 You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work).
 


- \ **Share Alike**\ 
 
 If you alter, transform, or build upon this work, you may distribute the resulting work only under the same, similar or a compatible license.
 


With the understanding that:


- \ **Waiver**\ 
 
 Any of the above conditions can be waived if you get permission from the copyright holder.
 


- \ **Public Domain**\ 
 
 Where the work or any of its elements is in the public domain under applicable law, that status is in no way affected by the license.
 


- \ **Other Rights**\ 
 
 In no way are any of the following rights affected by the license:
 
 
 - 
  
  Your fair dealing or fair use rights, or other applicable copyright exceptions and limitations;
  
 
 
 - 
  
  The author's moral rights;
  
 
 
 - 
  
  Rights other persons may have either in the work itself or in how the work is used, such as publicity or privacy rights.
  
 
 



- \ **Notice**\ 
 
 For any reuse or distribution, you must make clear to others the license terms of this work.
 


A copy of the full license is included in the file as LICENCES/CC-BY-SA-4.0.txt



************
DEPENDENCIES
************


GNU \ **parallel**\  uses Perl, and the Perl modules Getopt::Long, IPC::Open3, Symbol, IO::File, POSIX, and File::Temp.

For \ **--csv**\  it uses the Perl module Text::CSV.

For remote usage it uses \ **rsync**\  with \ **ssh**\ .


********
SEE ALSO
********


\ **parallel_tutorial**\ (1), \ **env_parallel**\ (1), \ **parset**\ (1), \ **parsort**\ (1), \ **parallel_alternatives**\ (1), \ **parallel_design**\ (7), \ **niceload**\ (1), \ **sql**\ (1), \ **ssh**\ (1), \ **ssh-agent**\ (1), \ **sshpass**\ (1), \ **ssh-copy-id**\ (1), \ **rsync**\ (1)

