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Updated: Wednesday, February 10, 2021
 
 

parallel (1)

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

PARALLEL(1)                        parallel                        PARALLEL(1)



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
       Start by watching the intro videos for a quick introduction:
       http://www.youtube.com/playlist?list=PL284C9FF2488BC6D1

       Then look at the EXAMPLEs after the list of OPTIONS (Use
       LESS=+/EXAMPLE\: man parallel). That will give you an idea of what GNU
       parallel is capable of.

       Then spend an hour walking through the tutorial (man
       parallel_tutorial). Your command line will love you for it.

       Finally you may want to look at the rest of this manual if you have
       special needs not already covered.

       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.

                The command cannot contain the character \257 (macron: X).

       {} (beta testing)
                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 -a, :::, 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.

       {.} (beta testing)
                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 . 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.

                The replacement string {.} can be changed with --er.

                To understand replacement strings see {}.

       {/} (beta testing)
                Basename of input line. This replacement string will be
                replaced by the input with the directory part removed.

                The replacement string {/} can be changed with
                --basenamereplace.

                To understand replacement strings see {}.

       {//} (beta testing)
                Dirname of input line. This replacement string will be
                replaced by the dir of the input line. See dirname(1).

                The replacement string {//} can be changed with
                --dirnamereplace.

                To understand replacement strings see {}.

       {/.} (beta testing)
                Basename of input line without extension. This replacement
                string will be replaced by the input with the directory and
                extension part removed. It is a combination of {/} and {.}.

                The replacement string {/.} can be changed with
                --basenameextensionreplace.

                To understand replacement strings see {}.

       {#} (beta testing)
                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.

                The replacement string {#} can be changed with --seqreplace.

                To understand replacement strings see {}.

       {%} (beta testing)
                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.

                The replacement string {%} can be changed with --slotreplace.

                To understand replacement strings see {}.

       {n} (beta testing)
                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 -a or ::::) or with the
                n'th argument (when used with -N). If n is negative it refers
                to the n'th last argument.

                To understand replacement strings see {}.

       {n.} (beta testing)
                Argument from input source n or the n'th argument without
                extension. It is a combination of {n} and {.}.

                This positional replacement string will be replaced by the
                input from input source n (when used with -a or ::::) or with
                the n'th argument (when used with -N). The input will have the
                extension removed.

                To understand positional replacement strings see {n}.

       {n/} (beta testing)
                Basename of argument from input source n or the n'th argument.
                It is a combination of {n} and {/}.

                This positional replacement string will be replaced by the
                input from input source n (when used with -a or ::::) or with
                the n'th argument (when used with -N). The input will have the
                directory (if any) removed.

                To understand positional replacement strings see {n}.

       {n//} (beta testing)
                Dirname of argument from input source n or the n'th argument.
                It 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 -a or ::::)
                or with the n'th argument (when used with -N). See dirname(1).

                To understand positional replacement strings see {n}.

       {n/.} (beta testing)
                Basename of argument from input source n or the n'th argument
                without extension.  It is a combination of {n}, {/}, and {.}.

                This positional replacement string will be replaced by the
                input from input source n (when used with -a or ::::) or with
                the n'th argument (when used with -N). The input will have the
                directory (if any) and extension removed.

                To understand positional replacement strings see {n}.

       {=perl expression=} (beta testing)
                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. A few convenience functions and data
                structures have been made:

                 Q(string)     shell quote a string

                 pQ(string)    perl quote a string

                 total_jobs()  number of jobs in total

                 slot()        slot number of job

                 seq()         sequence number of job

                 @arg          the arguments

                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=} (beta testing)
                Positional equivalent to {=perl expression=}. To understand
                positional replacement strings see {n}.

                See also: {=perl expression=} {n}.

       ::: arguments
                Use arguments from the command line as input source instead of
                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.

                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 :::: 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

       :::+ 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

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

                ::: and :::: can be mixed.

                See -a, ::: and --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.

       --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
                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 -a 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 and {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> >(zenity --progress --auto-kill) | wc

       --basefile file
       --bf file
                file will be transferred to each sshlogin before a jobs 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
                jobs.  Multiple --bf can be specified to transfer more
                basefiles. The file will be transferred the same way as
                --transferfile.

       --basenamereplace replace-str
       --bnr replace-str
                Use the replacement string replace-str instead of {/} for
                basename of input line.

       --basenameextensionreplace replace-str
       --bner replace-str
                Use the replacement string replace-str instead of {/.} for
                basename of input line without extension.

       --bg     Run command in background thus GNU parallel will not wait for
                completion of the command before exiting. This is the default
                if --semaphore is set.

                See also: --fg, man sem.

                Implies --semaphore.

       --bibtex
       --citation
                Print the BibTeX entry for GNU parallel and silence citation
                notice.

                If it is impossible for you to run --bibtex you can use
                --will-cite.

                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 should feel free to use --will-cite in
                scripts.

       --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, E, k, m, g, t, p, or e which
                would multiply the size with 1024, 1048576, 1073741824,
                1099511627776, 1125899906842624, 1152921504606846976, 1000,
                1000000, 1000000000, 1000000000000, 1000000000000000, or
                1000000000000000000 respectively.

                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-size should be bigger than N+1 records.

                size defaults to 1M.

                When using --pipepart 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 --pipepart -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 a large amount of data.

                See --pipe and --pipepart for use of this.

       --cat    Create a temporary file with content. Normally
                --pipe/--pipepart 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 --pipepart is used.

                See also --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
                created will not be removed - even if they are empty.

                With --return the file transferred from the remote computer
                will be removed on the remote computer.  Directories created
                will not be removed - even if they are empty.

                --cleanup is ignored when not used with --transferfile or
                --return.

       --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 access
                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 (see --link).

                  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:
                http://perldoc.perl.org/perlre.html

       --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.

       --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.

       --delimiter delim
       -d delim Input items are terminated by delim.  Quotes and backslash are
                not special; every character in the input is taken literally.
                Disables the end-of-file string, which is treated like any
                other argument. 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.  Multibyte characters
                are not supported.

       --dirnamereplace replace-str
       --dnr replace-str
                Use the replacement string replace-str instead of {//} for
                dirname of input line.

       -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.

       --delay secs
                Delay starting next job secs seconds. GNU parallel will pause
                secs seconds after starting each job. secs can be less than 1
                second.

       --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,
                --pipepart, --fifo and --cat). Do not count on this literaly,
                though, as the job may be scheduled on another computer or the
                local computer if : is in the list.

       --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.

       --env var
                Copy 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, see command.

                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.

       --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.

                See also --bg, man sem.

       --fifo   Create a temporary fifo with content. Normally --pipe and
                --pipepart 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 data is not read from the fifo, the job will block
                forever.

                Implies --pipe unless --pipepart is used.

                See also --cat.

       --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.

       --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 jobs 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 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

       --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
                give up 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 compability these also work:

                0      never

                1      soon,fail=1

                2      now,fail=1

                -1     soon,success=1

                -2     now,success=1

                1-99%  soon,fail=1-99%

       --header regexp
                Use regexp as header. For normal usage the matched header
                (typically the first line: --header '.*\n') will be split
                using --colsep (which will default to '\t') and column names
                can be used as replacement variables: {column name}, {column
                name/}, {column name//}, {column name/.}, {column name.},
                {=column name perl expression =}, ..

                For --pipe the matched header will be prepended to each
                output.

                --header : is an alias for --header '.*\n'.

                If regexp is a number, it is a fixed number of lines.

       --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.

       -I replace-str
                Use the replacement string replace-str instead of {}.

       --replace[=replace-str]
       -i[replace-str]
                This option is a synonym for -Ireplace-str if replace-str is
                specified, and for -I {} otherwise.  This option is
                deprecated; use -I instead.

       --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. Default is 100% which
                will run one job per CPU core on each machine.

                If --semaphore is set, the default is 1 thus making a mutex.

       --jobs +N
       -j +N
       --max-procs +N
       -P +N    Add N to the number of CPU cores.  Run this many jobs in
                parallel.  See also --use-cpus-instead-of-cores.

       --jobs -N
       -j -N
       --max-procs -N
       -P -N    Subtract N from the number of CPU cores.  Run this many jobs
                in parallel.  If the evaluated number is less than 1 then 1
                will be used.  See also --use-cpus-instead-of-cores.

       --jobs N%
       -j N%
       --max-procs N%
       -P N%    Multiply N% with the number of CPU cores.  Run this many jobs
                in parallel. See also --use-cpus-instead-of-cores.

       --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.

                If used with --pipe --roundrobin and the same input, the
                jobslots will get the same blocks in the same order in every
                run.

       -L max-lines
                When used with --pipe: Read records of max-lines.

                When used otherwise: Use at most max-lines 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.

                Implies -X unless -m, --xargs, or --pipe is set.

       --max-lines[=max-lines]
       -l[max-lines]
                When used with --pipe: Read records of max-lines.

                When used otherwise: Synonym for the -L option.  Unlike -L,
                the max-lines argument is optional.  If max-lines 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.

       --line-buffer
       --lb     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 than both --group and
                --ungroup, but can be faster than --group if the CPU is not
                the limiting factor.

                See also: --group --ungroup

       --xapply
       --link   Link input sources. Read multiple input sources like 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 (beta testing)
                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.

       --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.

       --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 for context replace. If in doubt use -X as that
                will most likely do what is needed.

       -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.

                See also -X for context replace. If in doubt use -X as that
                will most likely do what is needed.

       --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 which would
                multiply the size with 1024, 1048576, 1073741824,
                1099511627776, 1125899906842624, 1000, 1000000, 1000000000,
                1000000000000, or 1000000000000000, respectively.

                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,
                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.

       --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.

       --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.

       --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.

       --output-as-files
       --outputasfiles
       --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. 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.

                If --recstart is given this will be used to split at record
                start.

                If --recend is given this will be used to split at record end.

                If both --recstart and --recend are given both will have to
                match to find a split position.

                If neither --recstart nor --recend are given --recend defaults
                to '\n'. To have no record separator use --recend "".

                --files is often used with --pipe.

                --pipe maxes out at around 1 GB/s input, and 100 MB/s output.
                If performance is important use --pipepart.

                See also: --recstart, --recend, --fifo, --cat, --pipepart,
                --files.

       --pipepart
                Pipe parts of a physical file. --pipepart works similar to
                --pipe, but is much faster.

                --pipepart has a few limitations:

                o  The file must be a normal file or a block device
                   (technically it must be seekable) and must be given using
                   -a or ::::. The file cannot be a pipe or a fifo as they are
                   not seekable.

                   If using a block device with lot of NUL bytes, remember to
                   set --recend ''.

                o  Record counting (-N) and line counting (-L/-l) do not work.

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

       --plus (beta testing)
                Activate additional replacement strings: {+/} {+.} {+..}
                {+...} {..} {...} {/..} {/...} {##}. The idea being that
                '{+foo}' matches the opposite of '{foo}' and {} = {+/}/{/} =
                {.}.{+.} = {+/}/{/.}.{+.} = {..}.{+..} = {+/}/{/..}.{+..} =
                {...}.{+...} = {+/}/{/...}.{+...}

                {##} is the number of jobs to be run. It is incompatible with
                -X/-m/--xargs.

                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
                  {:num1:num2}  characters from num1 to num2
                  {#str}        remove prefix str
                  {%str}        remove postfix str
                  {/str1/str2}  replace str1 with str2
                  {^str}        uppercase str if found at the start
                  {^^str}       uppercase str
                  {,str}        lowercase str if found at the start
                  {,,str}       lowercase str

       --progress
                Show progress of computations. List the computers involved in
                the task with number of CPU cores 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 and --bar.

       --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.

                Implies -X unless -m is set.

       --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.

                When used with --pipe -N is the number of records to read.
                This is somewhat slower than --block.

       --max-line-length-allowed
                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).

       --number-of-cpus
                Print the number of physical CPUs and exit (used by GNU
                parallel itself to determine the number of physical CPUs on
                remote computers).

       --number-of-cores
                Print the number of CPU cores and exit (used by GNU parallel
                itself to determine the number of CPU cores on remote
                computers).

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

       --nice niceness
                Run the command at this niceness. For simple commands you can
                just add nice in front of the command. But if the command
                consists of more sub commands (Like: ls|wc) then prepending
                nice will not always work. --nice will make sure all sub
                commands are niced - even on remote servers.

       --interactive
       -p       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.

       --parens parensstring
                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. See the section
                PROFILE FILES for examples.

                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: config

       --quote
       -q       Quote command.  This will quote the command line so special
                characters are not interpreted by the shell. See the section
                QUOTING. Most people will never need this.  Quoting is
                disabled by default.

       --no-run-if-empty
       -r       If the stdin (standard input) only contains whitespace, do not
                run the command.

                If used with --pipe this is slow.

       --noswap 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 may give better results, so try using that first.

       --record-env
                Record current environment variables in
                ~/.parallel/ignored_vars. This is useful before using --env _.

                See also --env.

       --recstart startstring
       --recend endstring
                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 use --recend "".

                --recstart and --recend are used with --pipe.

                Use --regexp to interpret --recstart and --recend as regular
                expressions. This is slow, however.

       --regexp Use --regexp to interpret --recstart and --recend as regular
                expressions. This is slow, however.

       --remove-rec-sep
       --removerecsep
       --rrs    Remove the text matched by --recstart and --recend before
                piping it to the command.

                Only used with --pipe.

       --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).

                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 --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
                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).

       --return filename
                Transfer files from remote computers. --return is used with
                --sshlogin when the arguments are files on the remote
                computers. When processing is done the file filename will be
                transferred from the remote computer using rsync and will be
                put relative to the default login dir. E.g.

                  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 often used with --transferfile and --cleanup.

                --return is ignored when used with --sshlogin : or when not
                used with --sshlogin.

       --round-robin
       --round  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 --pipepart is given.

       --rpl 'tag perl expression' (beta testing)
                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:/[^/]*$::'
                  --rpl '{+.} s:.*\.::'
                  --rpl '{+..} s:.*\.([^.]*\.):$1:'
                  --rpl '{+...} s:.*\.([^.]*\.[^.]*\.):$1:'
                  --rpl '{..} s:\.[^/.]+$::; s:\.[^/.]+$::'
                  --rpl '{...} s:\.[^/.]+$::; s:\.[^/.]+$::; s:\.[^/.]+$::'
                  --rpl '{/..} s:.*/::; s:\.[^/.]+$::; s:\.[^/.]+$::'
                  --rpl '{/...} s:.*/::;s:\.[^/.]+$::;s:\.[^/.]+$::;s:\.[^/.]+$::'
                  --rpl '{##} $_=total_jobs()'
                  --rpl '{:-(.+?)} $_ ||= $$1'
                  --rpl '{:(\d+?)} substr($_,0,$$1) = ""'
                  --rpl '{:(\d+?):(\d+?)} $_ = substr($_,$$1,$$2);'
                  --rpl '{#([^#].*?)} s/^$$1//;'
                  --rpl '{%(.+?)} s/$$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;'

                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

       --max-chars=max-chars
       -s max-chars
                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.

                Implies -X unless -m is set.

       --show-limits
                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.

       --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.

                --semaphore implies --semaphorename `tty` unless
                --semaphorename is specified.

                Used with --fg, --wait, and --semaphorename.

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

                See also man sem.

       --semaphorename name
       --id name
                Use name as the name of the semaphore. 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.

       --semaphoretimeout 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.

                Implies --semaphore.

                See also man sem.

       --seqreplace replace-str
                Use the replacement string replace-str instead of {#} for job
                sequence number.

       --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
                http://www.in-ulm.de/~mascheck/various/shebang/

       --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.

       --shellquote
                Does not run the command but quotes it. Useful for making
                quoted composed commands for GNU parallel.

       --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.

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

       --sql DBURL (obsolete)
                Use --sqlmaster instead.

       --sqlmaster 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:///pardb/parjob

                It can also be an alias from ~/.sql/aliases:

                  :myalias mysql:///mydb/paralleljobs

       --sqlandworker DBURL
                Shorthand for: --sqlmaster DBURL --sqlworker DBURL.

       --sqlworker 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.

       --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 (see --sshlogin).

       --sshdelay secs
                Delay starting next ssh by secs seconds. GNU parallel will
                pause secs seconds after starting each ssh. secs can be less
                than 1 seconds.

       -S
       [@hostgroups/][ncores/]sshlogin[,[@hostgroups/][ncores/]sshlogin[,...]]
       -S @hostgroup
       --sshlogin
       [@hostgroups/][ncores/]sshlogin[,[@hostgroups/][ncores/]sshlogin[,...]]
       --sshlogin @hostgroup
                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 CPU cores on the
                remote computers and run the number of jobs as specified by
                -j.  If the number ncores is given GNU parallel will use this
                number for number of CPU cores on the host. Normally ncores
                will not be needed.

                An sshlogin is of the form:

                  [sshcommand [options]] [username@]hostname

                The sshlogin must not require a password (ssh-agent, ssh-copy-
                id, and sshpass 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

                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.

                For examples: see --sshloginfile.

                The remote host must have GNU parallel installed.

                --sshlogin is known to cause problems with -m and -X.

                --sshlogin is often used with --transferfile, --return,
                --cleanup, and --trc.

       --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-core-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 cores
                  //usr/local/bin/myssh -p 2222 -l myusername hexacpu
                  # Use a different ssh program with 6 cores
                  6//usr/local/bin/myssh -p 2222 -l myusername hexacpu
                  # Assume 16 cores on the local computer
                  16/:
                  # Put server1 in hostgroup1
                  @hostgroup1/server1
                  # Put myusername@server2 in hostgroup1+hostgroup2
                  @hostgroup1+hostgroup2/myusername@server2
                  # Force 4 cores 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 ...

       --slotreplace replace-str
                Use the replacement string replace-str instead of {%} for job
                slot number.

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

       --tty    Open terminal tty. If GNU parallel is used for starting an
                interactive program then this option may be needed. It will
                start 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. When the job is
                done, the next job will get the tty.

                You can of course override -j1 and -u.

       --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.

       --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.

       --tee    Pipe all data to all jobs. Used with --pipe/--pipepart 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 --pipepart --tee --tag \
                    'grep {1} | wc {2}' ::: {a..z} ::: -l -c

       --termseq 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.

       --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.

       --tmux   Use tmux for output. Start a tmux session and run each job in
                a window in that session. No other output will be produced.

       --tmuxpane
                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.

       --timeout duration
                Time out for command. If the command runs for longer than
                duration seconds it will get killed as per --termseq.

                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 normally in seconds, but can be floats postfixed
                with s, m, h, or d which would multiply the float by 1, 60,
                3600, or 86400. Thus these are equivalent: --timeout 100000
                and --timeout 1d3.5h16.6m4s.

       --verbose
       -t       Print the job to be run on stderr (standard error).

                See also -v, -p.

       --transfer
                Transfer files to remote computers. Shorthand for:
                --transferfile {}.

       --transferfile filename
       --tf filename
                --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 default work dir. If the
                path contains /./ the remaining path will be relative to the
                work dir. E.g.

                  echo foo/bar.txt | parallel --transferfile {} \
                    --sshlogin server.example.com wc

                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 /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 /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 {= s:.*/./:./: =}

                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.

                --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.

       --trc filename
                Transfer, Return, Cleanup. Shorthand for:

                --transferfile {} --return filename --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.

       --ungroup
       -u       Ungroup output.  Output is printed as soon as possible and by
                passes 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.

       --use-cpus-instead-of-cores
                Count the number of physical CPUs instead of CPU cores. When
                computing how many jobs to run simultaneously relative to the
                number of CPU cores you can ask GNU parallel to instead look
                at the number of physical CPUs. This will make sense for
                computers that have hyperthreading as two jobs running on one
                CPU with hyperthreading will run slower than two jobs running
                on two physical CPUs. Some multi-core CPUs can run faster if
                only one thread is running per physical CPU. Most users will
                not need this option.

       -v       Verbose.  Print the job to be run on stdout (standard output).
                Can be reversed with --silent. See also -t.

                Use -v -v to print the wrapping ssh command when running
                remotely.

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

       --workdir mydir
       --wd mydir
                Files transferred using --transferfile and --return will be
                relative to mydir on remote computers, and the command will be
                executed in the dir mydir.

                The special mydir value ... will create working dirs under
                ~/.parallel/tmp/ on the remote computers. 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.

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 --best

       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 --best ::: *.html

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

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

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

         bash: /bin/mv: Argument list too long

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

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

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

         ls | grep -E '\.log$' | 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

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

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

       To do it recursively use find:

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

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

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

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

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

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

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

         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"'

         ls | parallel 'mv {} "$(echo {} | tr "[:upper:]" "[:lower:]")"'

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

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

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

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

       Find the files in a list that do not exist

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

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

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

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

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

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

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

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

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

EXAMPLE: Function tester
       To test a program with different parameters:

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

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

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

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

EXAMPLE: Removing file extension when processing files
       When processing files removing the file extension using {.} is often
       useful.

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

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

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

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

       Convert all WAV files to MP3 using LAME:

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

       Put all converted in the same directory:

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

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

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

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

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

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

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

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

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

EXAMPLE: Download 24 images for each of the past 30 days
       Let us assume a website stores images like:

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

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

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

         parallel getit ::: $(seq 30) ::: $(seq -w 24)

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

EXAMPLE: Copy files as last modified date (ISO8601) with added random digits
         find . | parallel cp {} '../destdir/{= $a=int(10000*rand); $_=pQ($_);
           $_=`date -r "$_" +%FT%T"$a"`; chomp; =}'

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

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

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

EXAMPLE: Aggregating content of files
       This:

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

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

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

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

         cat x1y*z1 > x1z1

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

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

       Run like this:

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

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

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

         #!/bin/bash

         # E.g. http://gatt.org.yeslab.org/
         URL=$1
         # Stay inside the start dir
         BASEURL=$(echo $URL | perl -pe 's:#.*::; s:(//.*/)[^/]*:$1:')
         URLLIST=$(mktemp urllist.XXXX)
         URLLIST2=$(mktemp urllist.XXXX)
         SEEN=$(mktemp seen.XXXX)

         # Spider to get the URLs
         echo $URL >$URLLIST
         cp $URLLIST $SEEN

         while [ -s $URLLIST ] ; do
           cat $URLLIST |
             parallel lynx -listonly -image_links -dump {} \; \
               wget -qm -l1 -Q1 {} \; echo Spidered: {} \>\&2 |
               perl -ne 's/#.*//; s/\s+\d+.\s(\S+)$/$1/ and
                 do { $seen{$1}++ or print }' |
             grep -F $BASEURL |
             grep -v -x -F -f $SEEN | tee -a $SEEN > $URLLIST2
           mv $URLLIST2 $URLLIST
         done

         rm -f $URLLIST $URLLIST2 $SEEN

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

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

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

EXAMPLE: Rewriting a for-loop and a while-read-loop
       for-loops like this:

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

       and while-read-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

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

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

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

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

       and while-loops like this:

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

       can be written like this:

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

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

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

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

       can both be rewritten as:

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

EXAMPLE: Rewriting nested for-loops
       Nested for-loops like this:

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

       can be written like this:

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

       Nested for-loops like this:

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

       can be written like this:

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

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

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

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

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

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

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

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

EXAMPLE: From a to b and b to c
       Assume you have input like:

         aardvark
         babble
         cab
         dab
         each

       and want to run combinations like:

         aardvark babble
         babble cab
         cab dab
         dab each

       If the input is in the file in.txt:

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

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

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

EXAMPLE: Count the differences between all files in a dir
       Using --results the results are saved in /tmp/diffcount*.

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

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

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

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

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

         seq -w 0 999999 | parallel -j10 --pipe parallel -j0 touch pict{}.jpg

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

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

       Notice the difference between:

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

       and:

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

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

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

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

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

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

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

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

       Compare the output of:

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

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

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

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

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

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

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

EXAMPLE: Colorize output
       Give each job a new color. Most terminals support ANSI colors with the
       escape code "\033[30;XXm" where 30 <= XX <= 37:

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

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

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

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

       Append a string to every line in a text file:

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

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

       To keep the order the same as input run:

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

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

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

       To download byte 10000000-19999999 you can use curl:

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

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

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

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: Grepping n lines for m regular expressions.
       The simplest solution to grep a big file for a lot of regexps is:

         grep -f regexps.txt bigfile

       Or if the regexps are fixed strings:

         grep -F -f regexps.txt bigfile

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

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

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

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

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

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

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

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

       then your regexps.txt can be converted from:

         ID1.*Identifier1
         ID2.*Identifier2

       into:

         ID1 foo bar baz Identifier1
         ID2 foo bar baz Identifier2

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

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

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

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

         free=$(awk '/^((Swap)?Cached|MemFree|Buffers):/ { sum += $2 }
                     END { print sum }' /proc/meminfo)
         percpu=$((free / 200 / $(parallel --number-of-cores)))k

         parallel --pipepart -a regexps.txt --block $percpu --compress \
           grep -F -f - -n bigfile |
           sort -un | perl -pe 's/^\d+://'

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

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

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

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

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

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

         parallel --pipepart --block 100M -a bigfile -k --compress \
           grep -f regexp.txt

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

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

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

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

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

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

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

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

       To run echo on server.example.com:

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

       To run commands on more than one remote computer run:

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

       Or:

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

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

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

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

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

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

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

       Then run:

         seq 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, and 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 computers is not identified
       correctly the number of CPU cores can be added in front. Here the
       computer has 8 CPU cores.

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

EXAMPLE: Transferring of files
       To recompress gzipped files with bzip2 using a remote computer 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 computers add the computers 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"

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

         find logs/ -name '*.gz' | parallel -S .. \
           --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 -S server2,: \
           'mpg321 -w - {} | oggenc -q0 - -o {.}.ogg' ::: *.mp3

EXAMPLE: Running the same command on remote computers
       To run the command uptime on remote computers you can do:

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

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

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

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

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

EXAMPLE: Using remote computers behind NAT wall
       If the workers are behind a NAT wall, you need some trickery to get to
       them.

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

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

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

       So instead put this in ~/.ssh/config:

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

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

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

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

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

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

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

   No jumphost, no port forwards
       If ports cannot be forwarded, you need some sort of VPN to traverse the
       NAT-wall. TOR is one options for that, as it is very easy to get
       working.

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

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

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

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

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

       If not all hosts are accessible through TOR:

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

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

EXAMPLE: Parallelizing rsync
       rsync is a great tool, but sometimes it will not fill up the available
       bandwidth. This is often a problem when copying several big files over
       high speed connections.

       The following will start one rsync per big file in src-dir to dest-dir
       on the server fooserver:

         cd src-dir; find . -type f -size +100000 | \
           parallel -v ssh fooserver mkdir -p /dest-dir/{//}\; \
             rsync -s -Havessh {} fooserver:/dest-dir/{}

       The dirs created may end up with wrong permissions and smaller files
       are not being transferred. To fix those run rsync a final time:

         rsync -Havessh src-dir/ fooserver:/dest-dir/

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

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

EXAMPLE: Use multiple inputs in one command
       Copy files like foo.es.ext to foo.ext:

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

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

       Count in binary:

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

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

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

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

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

       Alternative version:

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

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

         DBURL=csv:///%2Ftmp%2Fmy.csv
         DBTABLEURL=$DBURL/mytable
         parallel --sqlandworker $DBTABLEURL seq ::: {1..10}

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

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

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

       This takes around a second per job.

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

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

       Or MySQL:

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

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

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

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

       Using xpath you can extract the URLs for 2016 and download them using
       GNU parallel:

         wget -O - http://arkiv.radio24syv.dk/audiopodcast/channel/4466232 |
           xpath -e "//ancestor::pubDate[contains(text(),'2016')]/../enclosure/@url" |
           parallel -u wget '{= s/ url="//; s/"//; =}'

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

         seq 10 | parallel -n0 my_command my_args

EXAMPLE: Working as cat | sh. Resource inexpensive jobs and evaluation
       GNU parallel can work similar to cat | sh.

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

       The content of the file jobs_to_run:

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

       To run 100 processes simultaneously do:

         parallel -j 100 < jobs_to_run

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

EXAMPLE: Processing a big file using more cores
       To process a big file or some output you can use --pipe to split up the
       data into blocks and pipe the blocks into the processing program.

       If the program is gzip -9 you can do:

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

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

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

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

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

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

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

EXAMPLE: Grouping input lines
       When processing with --pipe you may have lines grouped by a value. Here
       is my.csv:

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

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

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

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

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

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

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

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

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

EXAMPLE: Working as mutex and counting semaphore
       The command sem is an alias for parallel --semaphore.

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

       Run 10 jobs concurrently in the background:

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

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

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

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

         #!/usr/bin/sem --shebang-wrap -u --id $0 --fg /bin/bash

         echo This will run
         sleep 5
         echo exclusively

       Here perl:

         #!/usr/bin/sem --shebang-wrap -u --id $0 --fg /usr/bin/perl

         print "This will run ";
         sleep 5;
         print "exclusively\n";

       Here python:

         #!/usr/local/bin/sem --shebang-wrap -u --id $0 --fg /usr/bin/python

         import time
         print "This will run ";
         time.sleep(5)
         print "exclusively";

EXAMPLE: Start editor with filenames from stdin (standard input)
       You can use GNU parallel to start interactive programs like emacs or
       vi:

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

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

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

       The command:

         parallel sudo echo ::: This is a bad idea

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

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

       or:

         sudo parallel echo ::: This is a good idea

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

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

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

       To submit your jobs to the queue:

         echo my_command my_arg >> jobqueue

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

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

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

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

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

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

       GNU Parallel discovers if jobfile or ~/.parallel/sshloginfile changes.

       There is a a small issue when using GNU parallel as queue system/batch
       manager: You have to submit JobSlot number of jobs before they will
       start, and after that you can submit one at a time, and job will start
       immediately if free slots are available.  Output from the running or
       completed jobs are held back and will only be printed when JobSlots
       more jobs has been started (unless you use --ungroup or --line-buffer,
       in which case the output from the jobs are printed immediately).  E.g.
       if you have 10 jobslots then the output from the first completed job
       will only be printed when job 11 has started, and the output of second
       completed job will only be printed when job 12 has started.

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

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

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

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

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

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

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

EXAMPLE: Locate the missing package
       If you have downloaded source and tried compiling it, you may have
       seen:

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

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

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

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

       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 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 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_HOME
                Dir where GNU parallel stores config files, semaphores, and
                caches information between invocations. Default:
                $HOME/.parallel.

       $PARALLEL_PID
                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.  Remember to quote the $, so it gets evaluated by
                the correct shell.

                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_SHELL
                Use this shell for the commands run by GNU Parallel:

                o $PARALLEL_SHELL. If undefined use:

                o The shell that started GNU Parallel. If that cannot be
                  determined:

                o $SHELL. If undefined use:

                o /bin/sh

       $PARALLEL_SSH
                GNU parallel defaults to using ssh 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_SEQ
                $PARALLEL_SEQ will be set to the sequence number of the job
                running. Remember to quote the $, so it gets evaluated by the
                correct shell.

                Example:

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

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

       $TMPDIR  Directory for temporary files. See: --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 \ in the middle is needed because 'myssh' and
                'user@server' must be one argument.

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.

       Example: Profile for running a command on every sshlogin in
       ~/.ssh/sshlogins and prepend the output with the sshlogin:

         echo --tag -S .. --nonall > ~/.parallel/n
         parallel -Jn 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 are 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 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 remove 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

       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. An alias will only work if you use
       env_parallel.

REPORTING BUGS
       Report bugs to <bug-parallel@gnu.org> or
       https://savannah.gnu.org/bugs/?func=additem&group=parallel

       See a perfect bug report on
       https://lists.gnu.org/archive/html/bug-parallel/2015-01/msg00000.html

       Your bug report should always include:

       o 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
         these.

       o The complete output of parallel --version. If you are not running the
         latest released version (see http://ftp.gnu.org/gnu/parallel/) you
         should specify why you believe the problem is not fixed in that
         version.

       o A minimal, complete, and verifiable example (See description on
         http://stackoverflow.com/help/mcve).

         It should be a complete example that others can run that 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, and sleep can
         reproduce most errors. If your example requires large files, see if
         you can make them by something like seq 1000000 > file or yes | head
         -n 10000000 > file.

         If your example requires remote execution, see if you can use
         localhost - maybe using another login.

         If you have access to a different system, test if the MCVE shows the
         problem on that system.

       o The output of your example. If your problem is not easily reproduced
         by others, the output might help them figure out the problem.

       o Whether you have watched the intro videos
         (http://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:).

       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:
       http://sourceforge.net/projects/virtualboximage/files/
       http://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 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 less work for you if you can reproduce
       the error on a VirtualBox.

AUTHOR
       When using GNU parallel for a publication please cite:

       O. Tange (2011): GNU Parallel - The Command-Line Power Tool, ;login:
       The USENIX Magazine, February 2011:42-47.

       This helps funding further development; and it won't cost you a cent.
       If you pay 10000 EUR you should feel free to use GNU Parallel without
       citing.

       Copyright (C) 2007-10-18 Ole Tange, http://ole.tange.dk

       Copyright (C) 2008,2009,2010 Ole Tange, http://ole.tange.dk

       Copyright (C) 2010,2011,2012,2013,2014,2015,2016,2017 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
       Copyright (C) 2007,2008,2009,2010,2011,2012,2013,2014,2015,2016 Free
       Software Foundation, Inc.

       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:

                o Your fair dealing or fair use rights, or other applicable
                  copyright exceptions and limitations;

                o The author's moral rights;

                o 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. For remote usage it also uses
       rsync with ssh.


ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       +---------------+------------------+
       |ATTRIBUTE TYPE | ATTRIBUTE VALUE  |
       +---------------+------------------+
       |Availability   | shell/parallel   |
       +---------------+------------------+
       |Stability      | Uncommitted      |
       +---------------+------------------+
SEE ALSO
       ssh(1), ssh-agent(1), sshpass(1), ssh-copy-id(1), rsync(1), find(1),
       xargs(1), dirname(1), make(1), pexec(1), ppss(1), xjobs(1), prll(1),
       dxargs(1), mdm(1)



NOTES
       This software was built from source available at
       https://github.com/oracle/solaris-userland.  The original community
       source was downloaded from
       https://ftp.gnu.org/gnu/parallel/parallel-20170722.tar.bz2

       Further information about this software can be found on the open source
       community website at https://www.gnu.org/software/parallel.



20170622                          2017-07-21                       PARALLEL(1)