NAME
SYNOPSIS
DESCRIPTION
OPTIONS
EXAMPLE: Working as xargs -n1. Argument appending
EXAMPLE: Reading arguments from command line
EXAMPLE: Inserting multiple arguments
EXAMPLE: Context replace
EXAMPLE: Compute intensive jobs and substitution
EXAMPLE: Substitution and redirection
EXAMPLE: Composed commands
EXAMPLE: Removing file extension when processing files
EXAMPLE: Removing two file extensions when processing files and
EXAMPLE: Download 10 images for each of the past 30 days
EXAMPLE: Rewriting a for-loop and a while-loop
EXAMPLE: Group output lines
EXAMPLE: Keep order of output same as order of input
EXAMPLE: Parallel grep
EXAMPLE: Using remote computers
EXAMPLE: Transferring of files
EXAMPLE: Distributing work to local and remote computers
EXAMPLE: Use multiple inputs in one command
EXAMPLE: Use a table as input
EXAMPLE: Working as cat | sh. Ressource inexpensive jobs and evaluation
EXAMPLE: Working as mutex and counting semaphore
QUOTING
LIST RUNNING JOBS
COMPLETE RUNNING JOBS BUT DO NOT START NEW JOBS
ENVIRONMENT VARIABLES
CONFIG FILE
EXIT STATUS
DIFFERENCES BETWEEN GNU Parallel AND ALTERNATIVES

EXAMPLES FROM ppss MANUAL

DIFFERENCES BETWEEN pexec AND GNU Parallel
DIFFERENCES BETWEEN xjobs AND GNU Parallel
DIFFERENCES BETWEEN prll AND GNU parallel
DIFFERENCES BETWEEN dxargs AND GNU Parallel
DIFFERENCES BETWEEN mdm/middleman AND GNU Parallel
DIFFERENCES BETWEEN xapply AND GNU Parallel
DIFFERENCES BETWEEN ClusterSSH AND GNU Parallel

BUGS

Quoting of newline
Startup speed

REPORTING BUGS
AUTHOR
LICENSE

Documentation license I
Documentation license II

DEPENDENCIES
SEE ALSO

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

parallel [options] [command [arguments]] :::: argfile(s)

parallel --semaphore [options] command

DESCRIPTION

GNU parallel is a shell tool for executing jobs in parallel locally or using remote computers. A job is typically 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.

If you use xargs 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. If you use ppss or pexec you will find GNU parallel will often make the command easier to read.

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.

Before looking at the options you may want to check out the examples after the list of options. That will give you an idea of what GNU parallel is capable of.

You can also watch the intro video for a quick introduction: http://www.youtube.com/watch?v=OpaiGYxkSuQ or at http://tinyogg.com/watch/TORaR/ and http://tinyogg.com/watch/hfxKj/

OPTIONS

command

Command to execute. If command or the following arguments contain {} every instance will be substituted with the input line. Setting a command also invokes --file.

If command is given, GNU parallel will behave similar to xargs. If command is not given GNU parallel will behave similar to cat | sh.

{}

Input line. This is the default replacement string and will normally be used for putting the argument in the command line. It can be changed with -I.

{.}

Input line without extension. This is a specialized replacement string with the extension removed. If the input line contains . after the last / the last . till 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.

{.} can be used the same places as {}. The replacement string {.} can be changed with -U.

{n} (beta testing)

Argument from argument file n or the n'th argument. See -a and -N.

{n} can be used the same places as {}.

{n.} (beta testing)

Argument from argument file n or the n'th argument without extension. Similar to {.}. See -a and -N.

{n.} can be used the same places as {.}.

::: arguments (beta testing)

Use arguments from the command line as input instead of from stdin (standard input). 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. See also --arg-sep.

stdin (standard input) will be passed to the first process run.

If --arg-file is set arguments from that file will be appended.

:::: argfiles (beta testing)

Another way to write -a argfile1 -a argfile2 ...

See -a.

--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).

--arg-file input-file

-a input-file

Read items from the file input-file instead of stdin (standard input). If you use this option, stdin is given to the first process run. Otherwise, stdin is redirected from /dev/null.

If multiple -a are given, one line will be read from each of the files. The arguments can be accessed in the command as {1} .. {n}, so {1} will be a line from the first file, and {6} will refer to the line with the same line number from the 6th file.

--arg-file-sep sep-str (beta testing)

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

EXAMPLE: Working as xargs -n1. Argument appending

GNU parallel can work similar to xargs -n1.

To compress all html files using gzip run:

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

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

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

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

EXAMPLE: Reading arguments from command line

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

parallel gzip ::: *.html

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

parallel -j+0 lame {} -o {.}.mp3 ::: *.wav

EXAMPLE: Inserting multiple arguments

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

bash: /bin/mv: Argument list too long

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

ls | parallel mv {} destdir

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

ls | parallel -m mv {} destdir

EXAMPLE: Context replace

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

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

You could also do:

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

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

You could also run:

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

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

EXAMPLE: Compute intensive jobs and substitution

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

convert -geometry 120 foo.jpg thumb_foo.jpg

If the system has more than 1 CPU core it can be run with number-of-cpu-cores jobs in parallel (-j +0). This will do that for all jpg files in a directory:

ls *.jpg | parallel -j +0 convert -geometry 120 {} thumb_{}

To do it recursively use find:

find . -name '*.jpg' | parallel -j +0 convert -geometry 120 {} {}_thumb.jpg

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

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

find . -name '*.jpg' | parallel -j +0 convert -geometry 120 {} {.}_thumb.jpg

EXAMPLE: Substitution and redirection

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

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

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

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

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

EXAMPLE: Composed commands

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

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

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

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

Even small shell scripts can be run by GNU parallel:

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

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

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

EXAMPLE: Removing file extension when processing files

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

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

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

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

parallel -j+0 "zcat {} | bzip2 >{.}.bz2 && rm {}" ::: *.gz

Convert all WAV files to MP3 using LAME:

find sounddir -type f -name '*.wav' | parallel -j+0 lame {} -o {.}.mp3

EXAMPLE: Removing two file extensions when processing files and calling GNU Parallel from itself

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

ls *.tar.gz| parallel -U {tar} 'echo {tar}|parallel "mkdir -p {.} ; tar -C {.} -xf {.}.tar.gz"'

EXAMPLE: Download 10 images for each of the past 30 days

Let us assume a website stores images like:

   http://www.website.com/path/to/YYYYMMDD_##.jpg

where YYYYMMDD is the date and ## is the number 01-10. This will generate the past 30 days as YYYYMMDD:

seq 1 30 | parallel date -d '"today -{} days"' +%Y%m%d

Based on this we can let GNU parallel generate 10 wgets per day:

the above | parallel -I {o} seq -w 1 10 "|" parallel wget http://www.website.com/path/to/{o}_{}.jpg

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

for-loops like this:

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

and while-loops like this:

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

can be written like this:

cat list | parallel do_something | process_output

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

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

and while-loops like this:

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

can be written like this:

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

EXAMPLE: Group output lines

When runnning jobs that output data, you often do not want the output of multiple jobs to run together. GNU parallel defaults to grouping the output of each job, so the output is printed when the job finishes. If you want the output to be printed while the job is running you can use -u.

Compare the output of:

parallel traceroute ::: foss.org.my debian.org freenetproject.org

to the output of:

parallel -u traceroute ::: foss.org.my debian.org freenetproject.org

EXAMPLE: Keep order of output same as order of input

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

Append a string to every line in a text file:

cat textfile | parallel -k echo {} append_string

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

Another example is traceroute:

parallel traceroute ::: foss.org.my debian.org freenetproject.org

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

To keep the order the same as input run:

parallel -k traceroute ::: foss.org.my debian.org freenetproject.org

This will make sure the traceroute to foss.org.my will be printed first.

EXAMPLE: Parallel grep

grep -r greps recursively through directories. On multicore CPUs GNU parallel can often speed this up.

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

This will run 1.5 job per core, and give 1000 arguments to grep.

EXAMPLE: Using remote computers

To run commands on a remote computer SSH needs to be set up and you must be able to login without entering a password (ssh-agent may be handy).

To run echo on server.example.com:

  seq 1 10 | parallel --sshlogin server.example.com echo

To run commands on more than one remote computer run:

  seq 1 10 | parallel --sshlogin server.example.com,server2.example.net echo

Or:

  seq 1 10 | parallel --sshlogin server.example.com \
    --sshlogin server2.example.net echo

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

  seq 1 10 | parallel --sshlogin server.example.com \
    --sshlogin foo@server2.example.net echo

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

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

Then run:

  seq 1 10 | parallel --sshloginfile mycomputers echo

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

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

GNU parallel will try to determine the number of CPU cores on each of the remote computers, so -j+0 will run one job per CPU core - even if the remote computers do not have the same number of CPU cores.

If the number of CPU cores on the remote servers is not identified correctly the number of CPU cores can be added in front. Here the server has 8 CPU cores.

  seq 1 10 | parallel --sshlogin 8/server.example.com echo

EXAMPLE: Transferring of files

To recompress gzipped files with bzip2 using a remote server run:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

EXAMPLE: Distributing work to local and remote computers

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

  parallel --trc {.}.ogg -j+0 -S server2,: \
  'mpg321 -w - {} | oggenc -q0 - -o {.}.ogg' ::: *.mp3

EXAMPLE: Use multiple inputs in one command

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

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

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

EXAMPLE: Use a table as input

Content of table_file.tsv:

  foo<TAB>bar
  baz <TAB> quux

To run:

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

you can run:

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

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

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

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

GNU parallel can work similar to cat | sh.

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

The content of the file jobs_to_run:

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

To run 100 processes simultaneously do:

parallel -j 100 < jobs_to_run

As there is not a command the option --command is default because the jobs needs to be evaluated by the shell.

EXAMPLE: Working as mutex and counting semaphore

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

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

Run 10 jobs in parallel in the background:

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

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

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

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

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

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

QUOTING

For more advanced use quoting may be an issue. The following will print the filename for each line that has exactly 2 columns:

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

This can be done by GNU parallel using:

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

Notice how \'s, "'s, and $'s needs 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 shell interpret special characters. For example this will not work:

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

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

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

If you get errors like:

sh: -c: line 0: syntax error near unexpected token

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: To avoid dealing with the quoting problems it may be easier just to write a small script and have GNU parallel call that script.

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.

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 halfcompleted jobs you should send the signal SIGTERM to GNU parallel:

killall -TERM 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_PID - unimplemented

The environment variable $PARALLEL_PID is set by GNU parallel and is visible to the jobs started from GNU parallel. 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 -TERM $PARALLEL_PID. This only works on the local computer.

$PARALLEL

The environment variable $PARALLEL will be used as default options for GNU parallel. However, because some options take arguments the options need to be split into groups in which only the last option takes an argument. Each group of options should be put on a line of its own.

Example:

cat list | parallel -j1 -k -v ls

can be written as:

cat list | PARALLEL="-kvj1" parallel ls

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

can be written as:

cat list | PARALLEL="-kvj1

-Smyssh user@server" parallel echo

Notice the newline in the middle is needed because both -S and -j take an argument and thus both need to be at the end of a group.

CONFIG FILE

The file ~/.parallel/config will be read if it exists. It should be formatted like the environment variable $PARALLEL. Lines starting with '#' will be ignored.

Options on the command line takes precedence over the environment variable $PARALLEL which takes precedence over the file ~/.parallel/config.

EXIT STATUS

If --halt-on-error 0 or not specified:

All jobs ran without error.
-253
Some of the jobs failed. The exit status gives the number of failed jobs
More than 253 jobs failed.
Other error.

If --halt-on-error 1 or 2: Exit status of the failing job.

DIFFERENCES BETWEEN GNU Parallel AND ALTERNATIVES

There are a lot programs with some of the functionality of GNU parallel. GNU parallel strives to include the best of the functionality without sacrifying ease of use.

SUMMARY TABLE

The following features are in some of the comparable tools:

Execution E1. Running jobs in parallel E2. List running jobs E3. Finish running jobs, but do not start new jobs E4. Number of running jobs can depend on number of cpus E5. Finish running jobs, but do not start new jobs after first failure

Outputs O1. Grouping output so output from different jobs do not mix O2. Send stderr to stderr O3. Send stdout to stdout O4. Order of output can be same as order of input O5. Stdout only from the command O6. Stderr only from the command

Inputs I1. Arguments can be read from stdin I2. Arguments can be read from a file I3. Arguments can be read from multiple files I4. Arguments can be read from command line I5. Arguments can be read from a table I6. Line oriented input as default (Quoting of special chars not needed)

Manipulation of input M1. Composed command M2. Multiple arguments can fill up an execution line M3. Arguments can be put anywhere in the execution line M4. Multiple arguments can be put anywhere in the execution line M5. Arguments can be replaced with context M6. Input can be treated as complete execution line

Remote execution R1. Jobs can be run on remote computers R2. Basefiles can be transferred R3. Argument files can be transferred R4. Result files can be transferred R5. Cleanup of transferred files R6. No config files needed R7. Do not run more than SSHD's MaxStartup can handle R8. Configurable SSH command

Semaphore S1. Possibility to work as a mutex S2. Possibility to work as a counting semaphore

Legend - = no x = not applicable ID = yes

As every new version of the programs are not tested the table may be outdated. Please file a bug-report if you find errors.

parallel: E1 E2 E3 E4 E5 O1 O2 O3 O4 O5 O6 I1 I2 I3 I4 I5 I6 M1 M2 M3 M4 M5 M6 R1 R2 R3 R4 R5 R6 R7 R8 S1 S2

xargs: E1 - - - - - O2 O3 - O5 O6 I1 I2 - - - - - M2 M3 - - - - - - - - x - - -

find -exec: - - - x - x O2 O3 O4 O5 O6 - - - - - - - M2 M3 - - - - - - - - - - x x

make -j: E1 - - - E5 O1 O2 O3 - x O6 - - - - - - - - - - - - - - - - - - - -

ppss: E1 E2 ?E3 E4 - O1 - - x - - I1 I2 - - - I6 M1 - M3 - - M6 R1 R2 R3 R4 - - ?R7 - -

pexec: E1 - - E4 - O1 O2 O3 - O5 O6 I1 I2 - I4 I5 - M1 - M3 - - M6 R1 - - - - R6 - S1 -

xjobs: TODO

prll: TODO

dxargs: TODO

mdm/middelman: TODO

xapply: TODO

ClusterSSH: TODO

DIFFERENCES BETWEEN xargs AND GNU Parallel

xargs offer some of the same possibilites as GNU parallel.

xargs deals badly with special characters (such as space, ' and "). To see the problem try this:

  touch important_file
  touch 'not important_file'
  ls not* | xargs rm
  mkdir -p '12" records'
  ls | xargs rmdir

You can specify -0 or -d "\n", but many input generators are not optimized for using NUL as separator but are optimized for newline as separator. E.g head, tail, awk, ls, echo, sed, tar -v, perl (-0 and \0 instead of \n), locate (requires using -0), find (requires using -print0), grep (requires user to use -z or -Z), sort (requires using -z).

So GNU parallel's newline separation can be emulated with:

cat | xargs -d "\n" -n1 command

xargs can run a given number of jobs in parallel, but has no support for running number-of-cpu-cores jobs in parallel.

xargs has no support for grouping the output, therefore output may run together, e.g. the first half of a line is from one process and the last half of the line is from another process. The example Parallel grep cannot be done reliably with xargs because of this.

xargs has no support for keeping the order of the output, therefore if running jobs in parallel using xargs the output of the second job cannot be postponed till the first job is done.

xargs has no support for running jobs on remote computers.

xargs has no support for context replace, so you will have to create the arguments.

If you use a replace string in xargs (-I) you can not force xargs to use more than one argument.

Quoting in xargs works like -q in GNU parallel. This means composed commands and redirection require using bash -c.

ls | parallel "wc {} > {}.wc"

becomes

ls | xargs -d "\n" -P9 -I {} bash -c "wc {} > {}.wc"

and

ls | parallel "echo {}; ls {}|wc"

becomes

ls | xargs -d "\n" -P9 -I {} bash -c "echo {}; ls {}|wc"

DIFFERENCES BETWEEN find -exec AND GNU Parallel

find -exec offer some of the same possibilites as GNU parallel.

find -exec only works on files. So processing other input (such as hosts or URLs) will require creating these inputs as files. find -exec has no support for running commands in parallel.

DIFFERENCES BETWEEN make -j AND GNU Parallel

make -j can run jobs in parallel, but requires a crafted Makefile to do this. That results in extra quoting to get filename containing newline to work correctly.

make -j has no support for grouping the output, therefore output may run together, e.g. the first half of a line is from one process and the last half of the line is from another process. The example Parallel grep cannot be done reliably with make -j because of this.

(Very early versions of GNU parallel were coincidently implemented using make -j).

DIFFERENCES BETWEEN ppss AND GNU Parallel

ppss is also a tool for running jobs in parallel.

The output of ppss is status information and thus not useful for using as input for another command. The output from the jobs are put into files.

The argument replace string ($ITEM) cannot be changed. Arguments must be quoted - thus arguments containing special characters (space '"&!*) may cause problems. More than one argument is not supported. File names containing newlines are not processed correctly. When reading input from a file null cannot be used terminator. ppss needs to read the whole input file before starting any jobs.

Output and status information is stored in ppss_dir and thus requires cleanup when completed. If the dir is not removed before running ppss again it may cause nothing to happen as ppss thinks the task is already done. GNU parallel will normally not need cleaning up if running locally and will only need cleaning up if stopped abnormally and running remote (--cleanup may not complete if stopped abnormally). The example Parallel grep would require extra postprocessing if written using ppss.

For remote systems PPSS requires 3 steps: config, deploy, and start. GNU parallel only requires one step.

EXAMPLES FROM ppss MANUAL

Here are the examples from ppss's manual page with the equivalent using GNU parallel:

1 ./ppss.sh standalone -d /path/to/files -c 'gzip '

1 find /path/to/files -type f | parallel -j+0 gzip

2 ./ppss.sh standalone -d /path/to/files -c 'cp "$ITEM" /destination/dir '

2 find /path/to/files -type f | parallel -j+0 cp {} /destination/dir

3 ./ppss.sh standalone -f list-of-urls.txt -c 'wget -q '

3 parallel -a list-of-urls.txt wget -q

4 ./ppss.sh standalone -f list-of-urls.txt -c 'wget -q "$ITEM"'

4 parallel -a list-of-urls.txt wget -q {}

5 ./ppss config -C config.cfg -c 'encode.sh ' -d /source/dir -m 192.168.1.100 -u ppss -k ppss-key.key -S ./encode.sh -n nodes.txt -o /some/output/dir --upload --download ; ./ppss deploy -C config.cfg ; ./ppss start -C config

5 # parallel does not use configs. If you want a different username put it in nodes.txt: user@hostname

5 find source/dir -type f | parallel --sshloginfile nodes.txt --trc {.}.mp3 lame -a {} -o {.}.mp3 --preset standard --quiet

6 ./ppss stop -C config.cfg

6 killall -TERM parallel

7 ./ppss pause -C config.cfg

7 Press: CTRL-Z or killall -SIGTSTP parallel

8 ./ppss continue -C config.cfg

8 Enter: fg or killall -SIGCONT parallel

9 ./ppss.sh status -C config.cfg

9 killall -SIGUSR2 parallel

DIFFERENCES BETWEEN pexec AND GNU Parallel

pexec is also a tool for running jobs in parallel.

Here are the examples from pexec's info page with the equivalent using GNU parallel:

1 pexec -o sqrt-%s.dat -p "$(seq 10)" -e NUM -n 4 -c -- \ 'echo "scale=10000;sqrt($NUM)" | bc'

1 seq 10 | parallel -j4 'echo "scale=10000;sqrt({})" | bc > sqrt-{}.dat'

2 pexec -p "$(ls myfiles*.ext)" -i %s -o %s.sort -- sort

2 ls myfiles*.ext | parallel sort {} ">{}.sort"

3 pexec -f image.list -n auto -e B -u star.log -c -- \ 'fistar $B.fits -f 100 -F id,x,y,flux -o $B.star'

3 parallel -a image.list -j+0 \ 'fistar {}.fits -f 100 -F id,x,y,flux -o {}.star' 2>star.log

4 pexec -r *.png -e IMG -c -o - -- \ 'convert $IMG ${IMG%.png}.jpeg ; "echo $IMG: done"'

4 ls *.png | parallel 'convert {} {.}.jpeg; echo {}: done'

5 pexec -r *.png -i %s -o %s.jpg -c 'pngtopnm | pnmtojpeg'

5 ls *.png | parallel 'pngtopnm < {} | pnmtojpeg > {}.jpg'

6 for p in *.png ; do echo ${p%.png} ; done | \ pexec -f - -i %s.png -o %s.jpg -c 'pngtopnm | pnmtojpeg'

6 ls *.png | parallel 'pngtopnm < {} | pnmtojpeg > {.}.jpg'

7 LIST=$(for p in *.png ; do echo ${p%.png} ; done) pexec -r $LIST -i %s.png -o %s.jpg -c 'pngtopnm | pnmtojpeg'

7 ls *.png | parallel 'pngtopnm < {} | pnmtojpeg > {.}.jpg'

8 pexec -n 8 -r *.jpg -y unix -e IMG -c \ 'pexec -j -m blockread -d $IMG | \ jpegtopnm | pnmscale 0.5 | pnmtojpeg | \ pexec -j -m blockwrite -s th_$IMG'

8 Combining GNU parallel and GNU sem.

8 ls *jpg | parallel -j8 'sem --id blockread cat {} | jpegtopnm |' \ 'pnmscale 0.5 | pnmtojpeg | sem --id blockwrite cat > th_{}'

8 If reading and writing is done to the same disk, this may be faster as only one process will be either reading or writing:

8 ls *jpg | parallel -j8 'sem --id diskio cat {} | jpegtopnm |' \ 'pnmscale 0.5 | pnmtojpeg | sem --id diskio cat > th_{}'

DIFFERENCES BETWEEN xjobs AND GNU Parallel

xjobs is also a tool for running jobs in parallel. It only supports running jobs on your local computer.

xjobs deals badly with special characters just like xargs. See the section DIFFERENCES BETWEEN xargs AND GNU Parallel.

Here are the examples from xjobs's man page with the equivalent using GNU parallel:

1 ls -1 *.zip | xjobs unzip

1 ls *.zip | parallel unzip

2 ls -1 *.zip | xjobs -n unzip

2 ls *.zip | parallel unzip >/dev/null

3 find . -name '*.bak' | xjobs gzip

3 find . -name '*.bak' | parallel gzip

4 ls -1 *.jar | sed 's/$.*$/\1 > \1.idx/' | xjobs jar tf

4 ls *.jar | parallel jar tf {} '>' {}.idx

5 xjobs -s script

5 cat script | parallel

6 mkfifo /var/run/my_named_pipe; xjobs -s /var/run/my_named_pipe & echo unzip 1.zip >> /var/run/my_named_pipe; echo tar cf /backup/myhome.tar /home/me >> /var/run/my_named_pipe

6 mkfifo /var/run/my_named_pipe; cat /var/run/my_named_pipe | parallel & echo unzip 1.zip >> /var/run/my_named_pipe; echo tar cf /backup/myhome.tar /home/me >> /var/run/my_named_pipe

DIFFERENCES BETWEEN prll AND GNU parallel

prll is also a tool for running jobs in parallel. It does not support running jobs on remote computers.

prll encourages using BASH aliases and BASH functions instead of scripts. GNU parallel will never support running aliases and functions (see why http://www.perlmonks.org/index.pl?node_id=484296) but scripts or composed commands work just fine.

prll generates a lot of status information on STDERR which makes it harder to use the STDERR output of the job directly as input for another program.

Here is the example from prll's man page with the equivalent using GNU parallel:

prll -s 'mogrify -flip $1' *.jpg

parallel mogrify -flip ::: *.jpg

DIFFERENCES BETWEEN dxargs AND GNU Parallel

dxargs is also a tool for running jobs in parallel.

dxargs does not deal well with more simultaneous jobs than SSHD's MaxStartup. dxargs is only built for remote run jobs, but does not support transferring of files.

DIFFERENCES BETWEEN mdm/middleman AND GNU Parallel

middleman(mdm) is also a tool for running jobs in parallel.

Here are the shellscripts of http://mdm.berlios.de/usage.html ported to GNU parallel:

seq 1 19 | parallel -j+0 buffon -o - | sort -n > result

cat files | parallel -j+0 cmd

find dir -execdir sem -j+0 cmd {} \;

DIFFERENCES BETWEEN xapply AND GNU Parallel

xapply can run jobs in parallel on the local computer.

Here are the examples from xapply's man page with the equivalent using GNU parallel:

1 xapply '(cd %1 && make all)' */

1 parallel 'cd {} && make all' ::: */

2 xapply -f 'diff %1 ../version5/%1' manifest | more

2 parallel diff {} ../version5/{} < manifest | more

3 xapply -p/dev/null -f 'diff %1 %2' manifest1 checklist1

3 parallel diff {1} {2} :::: manifest1 checklist1

4 xapply 'indent' *.c

4 parallel indent ::: *.c

5 find ~ksb/bin -type f ! -perm -111 -print | xapply -f -v 'chmod a+x' -

5 find ~ksb/bin -type f ! -perm -111 -print | parallel -v chmod a+x

6 find */ -... | fmt 960 1024 | xapply -f -i /dev/tty 'vi' -

6 sh <(find */ -... | parallel -s 1024 echo vi)

7 find ... | xapply -f -5 -i /dev/tty 'vi' - - - - -

7 sh <(find ... |parallel -n5 echo vi)

8 xapply -fn "" /etc/passwd

8 parallel -k echo < /etc/passwd

9 tr ':' '\012' < /etc/passwd | xapply -7 -nf 'chown %1 %6' - - - - - - -

9 tr ':' '\012' < /etc/passwd | parallel -N7 chown {1} {6}

10 xapply '[ -d %1/RCS ] || echo %1' */

10 parallel '[ -d {}/RCS ] || echo {}' ::: */

11 xapply -f '[ -f %1 ] && echo %1' List | ...

11 parallel '[ -f {} ] && echo {}' < List | ...

DIFFERENCES BETWEEN ClusterSSH AND GNU Parallel

ClusterSSH solves a different problem than GNU parallel.

ClusterSSH runs the same command with the same arguments on a list of machines - one per machine. This is typically used for administrating several machines that are almost identical.

GNU parallel runs the same (or different) commands with different arguments in parallel possibly using remote machines to help computing. If more than one machine is listed in -S GNU parallel may only use one of these (e.g. if there are 8 jobs to be run and one machine has 8 cores).

GNU parallel can be used as a poor-mans version of ClusterSSH:

cat hostlist | parallel ssh {} do_stuff

BUGS

Quoting of newline

Because of the way newline is quoted this will not work:

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

However, this will work:

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

Startup speed

GNU parallel is slow at starting up. Half of the startup time is spent finding the maximal length of a command line. Setting -s will remove this part of the startup time.

REPORTING BUGS

Report bugs to <bug-parallel@gnu.org>.

AUTHOR

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 <http://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 fdl.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;
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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 cc-by-sa.txt.

DEPENDENCIES

GNU parallel uses Perl, and the Perl modules Getopt::Long, IPC::Open3, Symbol, IO::File, POSIX, and File::Temp.