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 examples of use in man parallel_examples. They 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:

$ 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:

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:

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:

(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:

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:

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:

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.:

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:

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.

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.

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:

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:

(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:

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:

parallel --pipe --fifo wc {}

Beware: If the fifo is never opened for reading, the job will block forever:

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:

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:

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:

tail -n +2 table.csv | \
  parallel --pipe --colsep , --group-by 1 -kN1 wc

Make GNU parallel treat the first line as a header:

cat table.csv | \
  parallel --pipe --colsep , --header : --group-by 1 -kN1 wc

Address column by column name:

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:

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 (beta testing)

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.

--header 0 is special: It will make replacement strings for files given with --arg-file or ::::. It will make {foo/bar} for the file foo/bar.

See also: --colsep --pipe --pipe-part --arg-file

  • --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:

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 -Ireplace-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:

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:

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:

# 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:

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:

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:

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 (alpha testing)

  • --lb (alpha 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:

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:

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:

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:

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:

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

  • -o

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 (beta 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:

{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:

{:-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

Set the environment variable varname to the jobslot number-1.

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 ,,:

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 endstringstartstring 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:

parallel --header : --results foo echo {a} {b} \
  ::: a I II ::: b III IIII

will generate the files:

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

parallel --results foo echo {1} {2} ::: I II ::: III IIII

will generate the files:

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.

parallel --results my_{} echo ::: foo bar baz

will generate the files:

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.

parallel --results my_{}/ echo ::: foo bar baz

will generate the files:

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.

timeout -k 1 4 parallel --joblog log -j10 \
  'sleep {}; exit {= $_%=2 =}' ::: {10..1}

4 jobs completed. 2 failed:

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:

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:

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:

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 (beta testing)

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.

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.

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.

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:

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:

--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:

--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:

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:

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:

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.:

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:

#!/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:

#!/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:

cat arguments | parallel the_program

then the script can be changed to:

#!/usr/bin/parallel --shebang-wrap /original/parser --options

E.g.

#!/usr/bin/parallel --shebang-wrap /usr/bin/python

If the program can be run like this:

cat data | parallel --pipe the_program

then the script can be changed to:

#!/usr/bin/parallel --shebang-wrap --pipe /orig/parser --opts

E.g.

#!/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

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:

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`:

#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:

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:

[sql:]vendor://[[user][:pwd]@][host][:port]/[db]/table

E.g.

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:

: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:

[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:

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:

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:

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:

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 :::.

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:

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?

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:

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 (beta testing)

Transfer files to remote computers.

Shorthand for: --transferfile {}.

See also: --transferfile.

  • --transferfile filename (beta testing)

  • --tf filename (beta testing)

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:

/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:

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:

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:

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 (beta testing)

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:

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:

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:

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

See: man parallel_examples

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:

( ) $ ` ' " < > ; | \

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

~ & # ! ? space * {

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

Often you can simply put \' around every ':

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

can be quoted:

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:

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:

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:

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:

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:

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:

seq 10 | parallel echo $$

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

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:

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

Prints: test $VAR

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

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:

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

It needs to be quoted like one of these:

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:

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:

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:

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:

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

Or for substituting output:

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:

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:

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.

4//usr/bin/specialssh user@host

becomes:

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.

4//usr/bin/specialssh user@host

becomes:

/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:

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:

cat list | parallel -j1 -k -v ls
cat list | parallel -j1 -k -v -S"myssh user@server" ls

can be written as:

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:

echo --tag -S .. --nonall > ~/.parallel/nonall_profile
parallel -J nonall_profile uptime

Example: Profile for running every command with -j-1 and nice

echo -j-1 nice > ~/.parallel/nice_profile
parallel -J nice_profile bzip2 -9 ::: *

Example: Profile for running a perl script before every command:

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:

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:

echo 1,2,3 | parallel -vkd, "echo 'a{}b'"

However, these will all work:

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 parallels 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:

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:

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:

Can't exec "command": No such file or directory

or:

open3: exec of by command failed

or:

/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:

  1. 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)