parallel/src/parallel.texi

\input texinfo
@setfilename parallel.info

@documentencoding utf-8

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

@node Top
@top parallel

@chapter NAME
@anchor{NAME}

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

@chapter SYNOPSIS
@anchor{SYNOPSIS}

@strong{parallel} [options] [@emph{command} [arguments]] < list_of_arguments

@strong{parallel} [options] [@emph{command} [arguments]] ( @strong{:::} arguments |
@strong{::::} argfile(s) ) ...

@strong{parallel} --semaphore [options] @emph{command}

@strong{#!/usr/bin/parallel} --shebang [options] [@emph{command} [arguments]]

@chapter DESCRIPTION
@anchor{DESCRIPTION}

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

GNU @strong{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 @strong{parallel} as input for other programs.

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

@section Reader's guide
@anchor{Reader's guide}

Before looking at the options you may want to check out the @strong{EXAMPLE}s
after the list of options. That will give you an idea of what GNU
@strong{parallel} is capable of.

You can also watch the intro video for a quick introduction:
http://tinyogg.com/watch/TORaR/ http://tinyogg.com/watch/hfxKj/ and
http://tinyogg.com/watch/YQuXd/ or
http://www.youtube.com/playlist?list=PL284C9FF2488BC6D1

@chapter OPTIONS
@anchor{OPTIONS}

@table @asis
@item @emph{command}
@anchor{@emph{command}}

Command to execute.  If @emph{command} or the following arguments contain
replacement strings (such as @strong{@{@}}) every instance will be substituted
with the input.

If @emph{command} is given, GNU @strong{parallel} solve the same tasks as
@strong{xargs}. If @emph{command} is not given GNU @strong{parallel} will behave
similar to @strong{cat | sh}.

The @emph{command} must be an executable, a script, a composed command, or
a function. If it is a function you need to @strong{export -f} the function
first. An alias will, however, not work (see why
http://www.perlmonks.org/index.pl?node_id=484296).

@item @strong{@{@}}
@anchor{@strong{@{@}}}

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 @strong{-a}, @strong{:::}, or
@strong{::::}.

The replacement string @strong{@{@}} can be changed with @strong{-I}.

If the command line contains no replacement strings then @strong{@{@}} will be
appended to the command line.

@item @strong{@{.@}}
@anchor{@strong{@{.@}}}

Input line without extension. This replacement string will be replaced
by the input with the extension removed. If the input line contains
@strong{.} after the last @strong{/} the last @strong{.} till the end of the string will
be removed and @strong{@{.@}} will be replaced with the
remaining. E.g. @emph{foo.jpg} becomes @emph{foo}, @emph{subdir/foo.jpg} becomes
@emph{subdir/foo}, @emph{sub.dir/foo.jpg} becomes @emph{sub.dir/foo},
@emph{sub.dir/bar} remains @emph{sub.dir/bar}. If the input line does not
contain @strong{.} it will remain unchanged.

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

To understand replacement strings see @strong{@{@}}.

@item @strong{@{/@}}
@anchor{@strong{@{/@}}}

Basename of input line. This replacement string will be replaced by
the input with the directory part removed.

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

To understand replacement strings see @strong{@{@}}.

@item @strong{@{//@}}
@anchor{@strong{@{//@}}}

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

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

To understand replacement strings see @strong{@{@}}.

@item @strong{@{/.@}}
@anchor{@strong{@{/.@}}}

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 @strong{@{/@}} and @strong{@{.@}}.

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

To understand replacement strings see @strong{@{@}}.

@item @strong{@{#@}}
@anchor{@strong{@{#@}}}

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 @strong{@{#@}} can be changed with @strong{--seqreplace}.

To understand replacement strings see @strong{@{@}}.

@item @strong{@{}@emph{n}@strong{@}}
@anchor{@strong{@{}@emph{n}@strong{@}}}

Argument from input source @emph{n} or the @emph{n}'th argument. This
positional replacement string will be replaced by the input from input
source @emph{n} (when used with @strong{-a} or @strong{::::}) or with the @emph{n}'th
argument (when used with @strong{-N}).

To understand replacement strings see @strong{@{@}}.

@item @strong{@{}@emph{n}.@strong{@}}
@anchor{@strong{@{}@emph{n}.@strong{@}}}

Argument from input source @emph{n} or the @emph{n}'th argument without
extension. It is a combination of @strong{@{}@emph{n}@strong{@}} and @strong{@{.@}}.

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

To understand positional replacement strings see @strong{@{}@emph{n}@strong{@}}.

@item @strong{@{}@emph{n}/@strong{@}}
@anchor{@strong{@{}@emph{n}/@strong{@}}}

Basename of argument from input source @emph{n} or the @emph{n}'th argument.
It is a combination of @strong{@{}@emph{n}@strong{@}} and @strong{@{/@}}.

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

To understand positional replacement strings see @strong{@{}@emph{n}@strong{@}}.

@item @strong{@{}@emph{n}//@strong{@}}
@anchor{@strong{@{}@emph{n}//@strong{@}}}

Dirname of argument from input source @emph{n} or the @emph{n}'th argument.
It is a combination of @strong{@{}@emph{n}@strong{@}} and @strong{@{//@}}.

This positional replacement string will be replaced by the dir of the
input from input source @emph{n} (when used with @strong{-a} or @strong{::::}) or with
the @emph{n}'th argument (when used with @strong{-N}). See @strong{dirname}(1).

To understand positional replacement strings see @strong{@{}@emph{n}@strong{@}}.

@item @strong{@{}@emph{n}/.@strong{@}}
@anchor{@strong{@{}@emph{n}/.@strong{@}}}

Basename of argument from input source @emph{n} or the @emph{n}'th argument
without extension.  It is a combination of @strong{@{}@emph{n}@strong{@}}, @strong{@{/@}}, and
@strong{@{.@}}.

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

To understand positional replacement strings see @strong{@{}@emph{n}@strong{@}}.

@item @strong{:::} @emph{arguments}
@anchor{@strong{:::} @emph{arguments}}

Use arguments from the command line as input source instead of stdin
(standard input). Unlike other options for GNU @strong{parallel} @strong{:::} is
placed after the @emph{command} and before the arguments.

The following are equivalent:

@verbatim
  (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"
@end verbatim

To avoid treating @strong{:::} as special use @strong{--arg-sep} to set the
argument separator to something else. See also @strong{--arg-sep}.

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

If multiple @strong{:::} 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.

@strong{:::} and @strong{::::} can be mixed. So these are equivalent:

@verbatim
  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
@end verbatim

@item @strong{::::} @emph{argfiles}
@anchor{@strong{::::} @emph{argfiles}}

Another way to write @strong{-a} @emph{argfile1} @strong{-a} @emph{argfile2} ...

@strong{:::} and @strong{::::} can be mixed.

See @strong{-a}, @strong{:::} and @strong{--xapply}.

@item @strong{--null}
@anchor{@strong{--null}}

@item @strong{-0}
@anchor{@strong{-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).

@item @strong{--arg-file} @emph{input-file}
@anchor{@strong{--arg-file} @emph{input-file}}

@item @strong{-a} @emph{input-file}
@anchor{@strong{-a} @emph{input-file}}

Use @emph{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 @strong{-a} are given, each @emph{input-file} will be treated as an
input source, and all combinations of input sources will be
generated. E.g. The file @strong{foo} contains @strong{1 2}, the file @strong{bar}
contains @strong{a b c}.  @strong{-a foo} @strong{-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 @strong{--xapply} and @strong{@{}@emph{n}@strong{@}}.

@item @strong{--arg-file-sep} @emph{sep-str}
@anchor{@strong{--arg-file-sep} @emph{sep-str}}

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

See also: @strong{::::}.

@item @strong{--arg-sep} @emph{sep-str}
@anchor{@strong{--arg-sep} @emph{sep-str}}

Use @emph{sep-str} instead of @strong{:::} as separator string. Useful if @strong{:::}
is used for something else by the command.

Also useful if you command uses @strong{:::} but you still want to read
arguments from stdin (standard input): Simply change @strong{--arg-sep} to a
string that is not in the command line.

See also: @strong{:::}.

@item @strong{--basefile} @emph{file}
@anchor{@strong{--basefile} @emph{file}}

@item @strong{--bf} @emph{file}
@anchor{@strong{--bf} @emph{file}}

@emph{file} will be transferred to each sshlogin before a jobs is
started. It will be removed if @strong{--cleanup} is active. The file may be
a script to run or some common base data needed for the jobs.
Multiple @strong{--bf} can be specified to transfer more basefiles. The
@emph{file} will be transferred the same way as @strong{--transfer}.

@item @strong{--basenamereplace} @emph{replace-str}
@anchor{@strong{--basenamereplace} @emph{replace-str}}

@item @strong{--bnr} @emph{replace-str}
@anchor{@strong{--bnr} @emph{replace-str}}

Use the replacement string @emph{replace-str} instead of @strong{@{/@}} for
basename of input line.

@item @strong{--basenameextensionreplace} @emph{replace-str}
@anchor{@strong{--basenameextensionreplace} @emph{replace-str}}

@item @strong{--bner} @emph{replace-str}
@anchor{@strong{--bner} @emph{replace-str}}

Use the replacement string @emph{replace-str} instead of @strong{@{/.@}} for basename of input line without extension.

@item @strong{--bg}
@anchor{@strong{--bg}}

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

See also: @strong{--fg}, @strong{man sem}

Implies @strong{--semaphore}.

@item @strong{--bibtex}
@anchor{@strong{--bibtex}}

Print the BibTeX entry for GNU @strong{parallel}.

@item @strong{--block} @emph{size}
@anchor{@strong{--block} @emph{size}}

@item @strong{--block-size} @emph{size}
@anchor{@strong{--block-size} @emph{size}}

Size of block in bytes. 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.

GNU @strong{parallel} tries to meet the block size but can be off by the
length of one record.

@emph{size} defaults to 1M.

See @strong{--pipe} for use of this.

@item @strong{--cleanup}
@anchor{@strong{--cleanup}}

Remove transferred files. @strong{--cleanup} will remove the transferred files
on the remote computer after processing is done.

@verbatim
  find log -name '*gz' | parallel \
    --sshlogin server.example.com --transfer --return {.}.bz2 \
    --cleanup "zcat {} | bzip -9 >{.}.bz2"
@end verbatim

With @strong{--transfer} 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 @strong{--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.

@strong{--cleanup} is ignored when not used with @strong{--transfer} or @strong{--return}.

@item @strong{--colsep} @emph{regexp}
@anchor{@strong{--colsep} @emph{regexp}}

@item @strong{-C} @emph{regexp}
@anchor{@strong{-C} @emph{regexp}}

Column separator. The input will be treated as a table with @emph{regexp}
separating the columns. The n'th column can be access using
@strong{@{}@emph{n}@strong{@}} or @strong{@{}@emph{n}.@strong{@}}. E.g. @strong{@{3@}} is the 3rd column.

@strong{--colsep} implies @strong{--trim rl}.

@emph{regexp} is a Perl Regular Expression:
http://perldoc.perl.org/perlre.html

@item @strong{--delimiter} @emph{delim}
@anchor{@strong{--delimiter} @emph{delim}}

@item @strong{-d} @emph{delim}
@anchor{@strong{-d} @emph{delim}}

Input items are terminated by the specified character.  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.  This can be used when the input consists of simply
newline-separated items, although it is almost always better to design
your program to use --null where this is possible.  The specified
delimiter may be a single character, a C-style character escape such
as \n, or an octal or hexadecimal escape code.  Octal and
hexadecimal escape codes are understood as for the printf command.
Multibyte characters are not supported.

@item @strong{--dirnamereplace} @emph{replace-str}
@anchor{@strong{--dirnamereplace} @emph{replace-str}}

@item @strong{--dnr} @emph{replace-str}
@anchor{@strong{--dnr} @emph{replace-str}}

Use the replacement string @emph{replace-str} instead of @strong{@{//@}} for
dirname of input line.

@item @strong{-E} @emph{eof-str}
@anchor{@strong{-E} @emph{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 ignored.  If
neither @strong{-E} nor @strong{-e} is used, no end of file string is used.

@item @strong{--dry-run}
@anchor{@strong{--dry-run}}

Print the job to run on stdout (standard output), but do not run the
job. Use @strong{-v -v} to include the ssh/rsync wrapping if the job would
be run on a remote computer. 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.

@item @strong{--eof}[=@emph{eof-str}]
@anchor{@strong{--eof}[=@emph{eof-str}]}

@item @strong{-e}[@emph{eof-str}]
@anchor{@strong{-e}[@emph{eof-str}]}

This option is a synonym for the @strong{-E} option.  Use @strong{-E} instead,
because it is POSIX compliant for @strong{xargs} while this option is not.
If @emph{eof-str} is omitted, there is no end of file string.  If neither
@strong{-E} nor @strong{-e} is used, no end of file string is used.

@item @strong{--eta}
@anchor{@strong{--eta}}

Show the estimated number of seconds before finishing. This forces GNU
@strong{parallel} to read all jobs before starting to find the number of
jobs. GNU @strong{parallel} normally only reads the next job to run.
Implies @strong{--progress}.

@item @strong{--fg}
@anchor{@strong{--fg}}

Run command in foreground thus GNU @strong{parallel} will wait for
completion of the command before exiting.

See also: @strong{--bg}, @strong{man sem}

Implies @strong{--semaphore}.

@item @strong{--gnu}
@anchor{@strong{--gnu}}

Behave like GNU @strong{parallel}. If @strong{--tollef} and @strong{--gnu} are both set,
@strong{--gnu} takes precedence.

@item @strong{--group}
@anchor{@strong{--group}}

Group output. Output from each jobs is grouped together and is only
printed when the command is finished. stderr (standard error) first
followed by stdout (standard output). This takes some CPU time. In
rare situations GNU @strong{parallel} takes up lots of CPU time and if it is
acceptable that the outputs from different commands are mixed
together, then disabling grouping with @strong{-u} can speedup GNU
@strong{parallel} by a factor of 10.

@strong{--group} is the default. Can be reversed with @strong{-u}.

@item @strong{--help}
@anchor{@strong{--help}}

@item @strong{-h}
@anchor{@strong{-h}}

Print a summary of the options to GNU @strong{parallel} and exit.

@item @strong{--halt-on-error} <0|1|2>
@anchor{@strong{--halt-on-error} <0|1|2>}

@item @strong{--halt} <0|1|2>
@anchor{@strong{--halt} <0|1|2>}

@table @asis
@item 0
@anchor{0}

Do not halt if a job fails. Exit status will be the number of jobs
failed. This is the default.

@item 1
@anchor{1}

Do not start new jobs if a job fails, but complete the running jobs
including cleanup. The exit status will be the exit status from the
last failing job.

@item 2
@anchor{2}

Kill off all jobs immediately and exit without cleanup. The exit
status will be the exit status from the failing job.

@end table

@item @strong{--header} @emph{regexp}
@anchor{@strong{--header} @emph{regexp}}

Use upto regexp as header. For normal usage the matched header
(typically the first line: @strong{--header '\n'}) will be split using
@strong{--colsep} (which will default to '\t') and column names can be used
as replacement variables: @strong{@{column name@}}. For @strong{--pipe} the matched
header will be prepended to each output.

@strong{--header :} is an alias for @strong{--header '\n'}.

@item @strong{-I} @emph{replace-str}
@anchor{@strong{-I} @emph{replace-str}}

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

@item @strong{--replace}[=@emph{replace-str}]
@anchor{@strong{--replace}[=@emph{replace-str}]}

@item @strong{-i}[@emph{replace-str}]
@anchor{@strong{-i}[@emph{replace-str}]}

This option is a synonym for @strong{-I}@emph{replace-str} if @emph{replace-str} is
specified, and for @strong{-I}@{@} otherwise.  This option is deprecated;
use @strong{-I} instead.

@item @strong{--joblog} @emph{logfile} (alpha testing)
@anchor{@strong{--joblog} @emph{logfile} (alpha testing)}

Logfile for executed jobs. Save a list of the executed jobs to
@emph{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,
and command run.

To convert the times into ISO-8601 strict do:

@strong{perl -a -F"\t" -ne 'chomp($F[2]=`date -d \@@$F[2] +%FT%T`); print join("\t",@@F)'}

See also @strong{--resume}.

@item @strong{--jobs} @emph{N}
@anchor{@strong{--jobs} @emph{N}}

@item @strong{-j} @emph{N}
@anchor{@strong{-j} @emph{N}}

@item @strong{--max-procs} @emph{N}
@anchor{@strong{--max-procs} @emph{N}}

@item @strong{-P} @emph{N}
@anchor{@strong{-P} @emph{N}}

Number of jobslots. Run up to N jobs in parallel.  0 means as many as
possible. Default is 100% which will run one job per CPU core.

If @strong{--semaphore} is set default is 1 thus making a mutex.

@item @strong{--jobs} @emph{+N}
@anchor{@strong{--jobs} @emph{+N}}

@item @strong{-j} @emph{+N}
@anchor{@strong{-j} @emph{+N}}

@item @strong{--max-procs} @emph{+N}
@anchor{@strong{--max-procs} @emph{+N}}

@item @strong{-P} @emph{+N}
@anchor{@strong{-P} @emph{+N}}

Add N to the number of CPU cores.  Run this many jobs in parallel.
See also @strong{--use-cpus-instead-of-cores}.

@item @strong{--jobs} @emph{-N}
@anchor{@strong{--jobs} @emph{-N}}

@item @strong{-j} @emph{-N}
@anchor{@strong{-j} @emph{-N}}

@item @strong{--max-procs} @emph{-N}
@anchor{@strong{--max-procs} @emph{-N}}

@item @strong{-P} @emph{-N}
@anchor{@strong{-P} @emph{-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
@strong{--use-cpus-instead-of-cores}.

@item @strong{--jobs} @emph{N}%
@anchor{@strong{--jobs} @emph{N}%}

@item @strong{-j} @emph{N}%
@anchor{@strong{-j} @emph{N}%}

@item @strong{--max-procs} @emph{N}%
@anchor{@strong{--max-procs} @emph{N}%}

@item @strong{-P} @emph{N}%
@anchor{@strong{-P} @emph{N}%}

Multiply N% with 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
@strong{--use-cpus-instead-of-cores}.

@item @strong{--jobs} @emph{procfile}
@anchor{@strong{--jobs} @emph{procfile}}

@item @strong{-j} @emph{procfile}
@anchor{@strong{-j} @emph{procfile}}

@item @strong{--max-procs} @emph{procfile}
@anchor{@strong{--max-procs} @emph{procfile}}

@item @strong{-P} @emph{procfile}
@anchor{@strong{-P} @emph{procfile}}

Read parameter from file. Use the content of @emph{procfile} as parameter
for @emph{-j}. E.g. @emph{procfile} could contain the string 100% or +2 or
10. If @emph{procfile} is changed when a job completes, @emph{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 @strong{parallel} is running.

@item @strong{--keep-order}
@anchor{@strong{--keep-order}}

@item @strong{-k}
@anchor{@strong{-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:

@verbatim
  parallel -j4 sleep {}\; echo {} ::: 2 1 4 3
  parallel -j4 -k sleep {}\; echo {} ::: 2 1 4 3
@end verbatim

@item @strong{-L} @emph{max-lines}
@anchor{@strong{-L} @emph{max-lines}}

When used with @strong{--pipe}: Read records of @emph{max-lines}.

When used otherwise: Use at most @emph{max-lines} nonblank input lines per
command line.  Trailing blanks cause an input line to be logically
continued on the next input line.

@strong{-L 0} means read one line, but insert 0 arguments on the command
line.

Implies @strong{-X} unless @strong{-m}, @strong{--xargs}, or @strong{--pipe} is set.

@item @strong{--max-lines}[=@emph{max-lines}]
@anchor{@strong{--max-lines}[=@emph{max-lines}]}

@item @strong{-l}[@emph{max-lines}]
@anchor{@strong{-l}[@emph{max-lines}]}

When used with @strong{--pipe}: Read records of @emph{max-lines}.

When used otherwise: Synonym for the @strong{-L} option.  Unlike @strong{-L}, the
@emph{max-lines} argument is optional.  If @emph{max-lines} is not specified,
it defaults to one.  The @strong{-l} option is deprecated since the POSIX
standard specifies @strong{-L} instead.

@strong{-l 0} is an alias for @strong{-l 1}.

Implies @strong{-X} unless @strong{-m} is set.

@item @strong{--load} @emph{max-load} (alpha testing)
@anchor{@strong{--load} @emph{max-load} (alpha testing)}

Do not start new jobs on a given computer unless the load is less than
@emph{max-load}. @emph{max-load} uses the same syntax as @strong{--jobs}, so @emph{100%}
for one per CPU is a valid setting. Only difference is 0 which is
interpreted as 0.01.

The load average is only sampled every 10 seconds using @strong{uptime} to
avoid stressing small computers. Only the first (1 minute) load is
used.

@item @strong{--controlmaster} (experimental)
@anchor{@strong{--controlmaster} (experimental)}

@item @strong{-M} (experimental)
@anchor{@strong{-M} (experimental)}

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.

@item @strong{--xargs}
@anchor{@strong{--xargs}}

Multiple arguments. Insert as many arguments as the command line
length permits.

If @strong{@{@}} is not used the arguments will be appended to the
line.  If @strong{@{@}} is used multiple times each @strong{@{@}} will be replaced
with all the arguments.

Support for @strong{--xargs} with @strong{--sshlogin} is limited and may fail.

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

@item @strong{-m}
@anchor{@strong{-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 @strong{-j1} to avoid this.

If @strong{@{@}} is not used the arguments will be appended to the
line.  If @strong{@{@}} is used multiple times each @strong{@{@}} will be replaced
with all the arguments.

Support for @strong{-m} with @strong{--sshlogin} is limited and may fail.

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

@item @strong{--minversion} @emph{version}
@anchor{@strong{--minversion} @emph{version}}

Print the version GNU @strong{parallel} and exit.  If the current version of
GNU @strong{parallel} is less than @emph{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 @strong{parallel}.

@item @strong{--nonall}
@anchor{@strong{--nonall}}

@strong{--onall} with no arguments. Run the command on all computers given
with @strong{--sshlogin} but take no arguments. GNU @strong{parallel} will log
into @strong{--jobs} number of computers in parallel and run the job on the
computer. @strong{-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.

@item @strong{--onall}
@anchor{@strong{--onall}}

Run all the jobs on all computers given with @strong{--sshlogin}. GNU
@strong{parallel} will log into @strong{--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. @strong{-j}
adjusts how many computers to log into in parallel.

When using @strong{--group} the output will be grouped by each server, so
all the output from one server will be grouped together.

@item @strong{--output-as-files}
@anchor{@strong{--output-as-files}}

@item @strong{--outputasfiles}
@anchor{@strong{--outputasfiles}}

@item @strong{--files}
@anchor{@strong{--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.

@item @strong{--pipe} (alpha testing)
@anchor{@strong{--pipe} (alpha testing)}

@item @strong{--spreadstdin} (alpha testing)
@anchor{@strong{--spreadstdin} (alpha testing)}

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 @strong{--block}. The strings @strong{--recstart}
and @strong{--recend} tell GNU @strong{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 @strong{--recstart} is given this will be used to split at record start.

If @strong{--recend} is given this will be used to split at record end.

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

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

@strong{--files} is often used with @strong{--pipe}.

@item @strong{--progress}
@anchor{@strong{--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 @strong{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 @strong{parallel} SIGUSR2 you can toggle turning on/off
@strong{--progress} on a running GNU @strong{parallel} process.

See also: @strong{--eta}

@item @strong{--max-args}=@emph{max-args}
@anchor{@strong{--max-args}=@emph{max-args}}

@item @strong{-n} @emph{max-args}
@anchor{@strong{-n} @emph{max-args}}

Use at most @emph{max-args} arguments per command line.  Fewer than
@emph{max-args} arguments will be used if the size (see the @strong{-s} option)
is exceeded, unless the @strong{-x} option is given, in which case
GNU @strong{parallel} will exit.

@strong{-n 0} means read one argument, but insert 0 arguments on the command
line.

Implies @strong{-X} unless @strong{-m} is set.

@item @strong{--max-replace-args}=@emph{max-args}
@anchor{@strong{--max-replace-args}=@emph{max-args}}

@item @strong{-N} @emph{max-args}
@anchor{@strong{-N} @emph{max-args}}

Use at most @emph{max-args} arguments per command line. Like @strong{-n} but
also makes replacement strings @strong{@{1@}} .. @strong{@{}@emph{max-args}@strong{@}} that
represents argument 1 .. @emph{max-args}. If too few args the @strong{@{}@emph{n}@strong{@}} will
be empty.

@strong{-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:

@strong{tr ':' '\n' < /etc/passwd | parallel -N7 chown @{1@} @{6@}}

Implies @strong{-X} unless @strong{-m} or @strong{--pipe} is set.

When used with @strong{--pipe} @strong{-N} is the number of records to read. This
is much slower than @strong{--block} so avoid it if performance is
important.

@item @strong{--max-line-length-allowed}
@anchor{@strong{--max-line-length-allowed}}

Print the maximal number of characters allowed on the command line and
exit (used by GNU @strong{parallel} itself to determine the line length
on remote computers).

@item @strong{--number-of-cpus}
@anchor{@strong{--number-of-cpus}}

Print the number of physical CPUs and exit (used by GNU @strong{parallel}
itself to determine the number of physical CPUs on remote computers).

@item @strong{--number-of-cores}
@anchor{@strong{--number-of-cores}}

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

@item @strong{--nice} @emph{niceness}
@anchor{@strong{--nice} @emph{niceness}}

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

@item @strong{--interactive}
@anchor{@strong{--interactive}}

@item @strong{-p}
@anchor{@strong{-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 @strong{-t}.

@item @strong{--profile} @emph{profilename}
@anchor{@strong{--profile} @emph{profilename}}

@item @strong{-J} @emph{profilename}
@anchor{@strong{-J} @emph{profilename}}

Use profile @emph{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.

@emph{profilename} corresponds to the file ~/.parallel/@emph{profilename}.

You can give multiple profiles by repeating @strong{--profile}. If parts of
the profiles conflict, the later ones will be used.

Default: config

@item @strong{--quote}
@anchor{@strong{--quote}}

@item @strong{-q}
@anchor{@strong{-q}}

Quote @emph{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.

@item @strong{--no-run-if-empty}
@anchor{@strong{--no-run-if-empty}}

@item @strong{-r}
@anchor{@strong{-r}}

If the stdin (standard input) only contains whitespace, do not run the command.

If used with @strong{--pipe} this is slow.

@item @strong{--recstart} @emph{startstring}
@anchor{@strong{--recstart} @emph{startstring}}

@item @strong{--recend} @emph{endstring}
@anchor{@strong{--recend} @emph{endstring}}

If @strong{--recstart} is given @emph{startstring} will be used to split at record start.

If @strong{--recend} is given @emph{endstring} will be used to split at record end.

If both @strong{--recstart} and @strong{--recend} are given the combined string
@emph{endstring}@emph{startstring} will have to match to find a split
position. This is useful if either @emph{startstring} or @emph{endstring}
match in the middle of a record.

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

@strong{--recstart} and @strong{--recend} are used with @strong{--pipe}.

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

@item @strong{--regexp}
@anchor{@strong{--regexp}}

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

@item @strong{--remove-rec-sep}
@anchor{@strong{--remove-rec-sep}}

@item @strong{--removerecsep}
@anchor{@strong{--removerecsep}}

@item @strong{--rrs}
@anchor{@strong{--rrs}}

Remove the text matched by @strong{--recstart} and @strong{--recend} before piping
it to the command.

Only used with @strong{--pipe}.

@item @strong{--resume}
@anchor{@strong{--resume}}

Resumes from the last unfinished job. By reading @strong{--joblog} GNU
@strong{parallel} will figure out the last unfinished job and continue from
there. As GNU @strong{parallel} only looks at the sequence numbers in
@strong{--joblog} then the input, the command, and @strong{--joblog} all have to
remain unchanged; otherwise GNU @strong{parallel} may run wrong commands.

See also: @strong{--joblog}.

@item @strong{--retries} @emph{n}
@anchor{@strong{--retries} @emph{n}}

If a job fails, retry it on another computer. Do this @emph{n} times. If
there are fewer than @emph{n} computers in @strong{--sshlogin} GNU @strong{parallel} will
re-use the computers. This is useful if some jobs fail for no apparent
reason (such as network failure).

@item @strong{--return} @emph{filename}
@anchor{@strong{--return} @emph{filename}}

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

@verbatim
  echo foo/bar.txt | parallel \
    --sshlogin server.example.com --return {.}.out touch {.}.out
@end verbatim

This will transfer the file @emph{$HOME/foo/bar.out} from the computer
@emph{server.example.com} to the file @emph{foo/bar.out} after running
@strong{touch foo/bar.out} on @emph{server.example.com}.

@verbatim
  echo /tmp/foo/bar.txt | parallel \
    --sshlogin server.example.com --return {.}.out touch {.}.out
@end verbatim

This will transfer the file @emph{/tmp/foo/bar.out} from the computer
@emph{server.example.com} to the file @emph{/tmp/foo/bar.out} after running
@strong{touch /tmp/foo/bar.out} on @emph{server.example.com}.

Multiple files can be transferred by repeating the options multiple
times:

@verbatim
  echo /tmp/foo/bar.txt | \
    parallel --sshlogin server.example.com \
    --return {.}.out --return {.}.out2 touch {.}.out {.}.out2
@end verbatim

@strong{--return} is often used with @strong{--transfer} and @strong{--cleanup}.

@strong{--return} is ignored when used with @strong{--sshlogin :} or when not used
with @strong{--sshlogin}.

@item @strong{--max-chars}=@emph{max-chars}
@anchor{@strong{--max-chars}=@emph{max-chars}}

@item @strong{-s} @emph{max-chars}
@anchor{@strong{-s} @emph{max-chars}}

Use at most @emph{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 @strong{-X} unless @strong{-m} is set.

@item @strong{--show-limits}
@anchor{@strong{--show-limits}}

Display the limits on the command-line length which are imposed by the
operating system and the @strong{-s} option.  Pipe the input from /dev/null
(and perhaps specify --no-run-if-empty) if you don't want GNU @strong{parallel}
to do anything.

@item @strong{--semaphore}
@anchor{@strong{--semaphore}}

Work as a counting semaphore. @strong{--semaphore} will cause GNU
@strong{parallel} to start @emph{command} in the background. When the number of
simultaneous jobs is reached, GNU @strong{parallel} will wait for one of
these to complete before starting another command.

@strong{--semaphore} implies @strong{--bg} unless @strong{--fg} is specified.

@strong{--semaphore} implies @strong{--semaphorename `tty`} unless
@strong{--semaphorename} is specified.

Used with @strong{--fg}, @strong{--wait}, and @strong{--semaphorename}.

The command @strong{sem} is an alias for @strong{parallel --semaphore}.

See also: @strong{man sem}

@item @strong{--semaphorename} @emph{name}
@anchor{@strong{--semaphorename} @emph{name}}

@item @strong{--id} @emph{name}
@anchor{@strong{--id} @emph{name}}

Use @strong{name} as the name of the semaphore. Default is the name of the
controlling tty (output from @strong{tty}).

The default normally works as expected when used interactively, but
when used in a script @emph{name} should be set. @emph{$$} or @emph{my_task_name}
are often a good value.

The semaphore is stored in ~/.parallel/semaphores/

Implies @strong{--semaphore}.

See also: @strong{man sem}

@item @strong{--semaphoretimeout} @emph{secs} (not implemented)
@anchor{@strong{--semaphoretimeout} @emph{secs} (not implemented)}

If the semaphore is not released within secs seconds, take it anyway.

Implies @strong{--semaphore}.

See also: @strong{man sem}

@item @strong{--seqreplace} @emph{replace-str}
@anchor{@strong{--seqreplace} @emph{replace-str}}

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

@item @strong{--shellquote}
@anchor{@strong{--shellquote}}

Does not run the command but quotes it. Useful for making quoted
composed commands for GNU @strong{parallel}.

@item @strong{--skip-first-line}
@anchor{@strong{--skip-first-line}}

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

@item @strong{-S} @emph{[ncpu/]sshlogin[,[ncpu/]sshlogin[,...]]}
@anchor{@strong{-S} @emph{[ncpu/]sshlogin[@comma{}[ncpu/]sshlogin[@comma{}...]]}}

@item @strong{--sshlogin} @emph{[ncpu/]sshlogin[,[ncpu/]sshlogin[,...]]}
@anchor{@strong{--sshlogin} @emph{[ncpu/]sshlogin[@comma{}[ncpu/]sshlogin[@comma{}...]]}}

Distribute jobs to remote computers. The jobs will be run on a list of
remote computers.  GNU @strong{parallel} will determine the number of CPU
cores on the remote computers and run the number of jobs as specified by
@strong{-j}.  If the number @emph{ncpu} is given GNU @strong{parallel} will use this
number for number of CPU cores on the host. Normally @emph{ncpu} will not
be needed.

An @emph{sshlogin} is of the form:

@verbatim
  [sshcommand [options]][username@]hostname
@end verbatim

The sshlogin must not require a password.

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 or repeat
the options multiple times.

For examples: see @strong{--sshloginfile}.

The remote host must have GNU @strong{parallel} installed.

@strong{--sshlogin} is known to cause problems with @strong{-m} and @strong{-X}.

@strong{--sshlogin} is often used with @strong{--transfer}, @strong{--return},
@strong{--cleanup}, and @strong{--trc}.

@item @strong{--sshloginfile} @emph{filename}
@anchor{@strong{--sshloginfile} @emph{filename}}

@item @strong{--slf} @emph{filename}
@anchor{@strong{--slf} @emph{filename}}

File with sshlogins. The file consists of sshlogins on separate
lines. Empty lines and lines starting with '#' are ignored. Example:

@verbatim
  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.example.net
  # Use a different ssh program with 6 cores
  6//usr/local/bin/myssh -p 2222 -l myusername hexacpu.example.net
  # Assume 16 cores on the local computer
  16/:
@end verbatim

When using a different ssh program the last argument must be the hostname.

Multiple @strong{--sshloginfile} are allowed.

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).

@item @strong{--noswap}
@anchor{@strong{--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.

@item @strong{--silent}
@anchor{@strong{--silent}}

Silent.  The job to be run will not be printed. This is the default.
Can be reversed with @strong{-v}.

@item @strong{--tty}
@anchor{@strong{--tty}}

Open terminal tty. If GNU @strong{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. @strong{-j1}), not buffer the output (i.e. @strong{-u}),
and it will open a tty for the job. When the job is done, the next job
will get the tty.

@item @strong{--tag}
@anchor{@strong{--tag}}

Tag lines with arguments. Each output line will be prepended with the
arguments and TAB (\t). When combined with @strong{--onall} or @strong{--nonall}
the lines will be prepended with the sshlogin instead.

@strong{--tag} is ignored when using @strong{-u}.

@item @strong{--tagstring} @emph{str}
@anchor{@strong{--tagstring} @emph{str}}

Tag lines with a string. Each output line will be prepended with
@emph{str} and TAB (\t). @emph{str} can contain replacement strings such as
@{@}.

@strong{--tagstring} is ignored when using @strong{-u}, @strong{--onall}, and @strong{--nonall}.

@item @strong{--tmpdir} @emph{dirname}
@anchor{@strong{--tmpdir} @emph{dirname}}

Directory for temporary files. GNU @strong{parallel} normally buffers output
into temporary files in /tmp. By setting @strong{--tmpdir} you can use a
different dir for the files. Setting @strong{--tmpdir} is equivalent to
setting $TMPDIR.

@item @strong{--timeout} @emph{sec}
@anchor{@strong{--timeout} @emph{sec}}

Time out for command. If the command runs for longer than @emph{sec}
seconds it will get killed with SIGTERM, followed by SIGTERM 200 ms
later, followed by SIGKILL 200 ms later.

@item @strong{--tollef}
@anchor{@strong{--tollef}}

Make GNU @strong{parallel} behave more like Tollef's parallel command. It
activates @strong{-u}, @strong{-q}, and @strong{--arg-sep --}. It also causes @strong{-l} to
change meaning to @strong{--load}.

Not giving '--' is unsupported.

@strong{Do not use --tollef unless you know what you are doing}.

To override use @strong{--gnu}.

@item @strong{--verbose}
@anchor{@strong{--verbose}}

@item @strong{-t}
@anchor{@strong{-t}}

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

See also @strong{-v} and @strong{-p}.

@item @strong{--transfer}
@anchor{@strong{--transfer}}

Transfer files to remote computers. @strong{--transfer} is used with
@strong{--sshlogin} when the arguments are files and should be transferred to
the remote computers. The files will be transferred using @strong{rsync} and
will be put relative to the default login dir. E.g.

@verbatim
  echo foo/bar.txt | parallel \
    --sshlogin server.example.com --transfer wc
@end verbatim

This will transfer the file @emph{foo/bar.txt} to the computer
@emph{server.example.com} to the file @emph{$HOME/foo/bar.txt} before running
@strong{wc foo/bar.txt} on @emph{server.example.com}.

@verbatim
  echo /tmp/foo/bar.txt | parallel \
    --sshlogin server.example.com --transfer wc
@end verbatim

This will transfer the file @emph{foo/bar.txt} to the computer
@emph{server.example.com} to the file @emph{/tmp/foo/bar.txt} before running
@strong{wc /tmp/foo/bar.txt} on @emph{server.example.com}.

@strong{--transfer} is often used with @strong{--return} and @strong{--cleanup}.

@strong{--transfer} is ignored when used with @strong{--sshlogin :} or when not used with @strong{--sshlogin}.

@item @strong{--trc} @emph{filename}
@anchor{@strong{--trc} @emph{filename}}

Transfer, Return, Cleanup. Short hand for:

@strong{--transfer} @strong{--return} @emph{filename} @strong{--cleanup}

@item @strong{--trim} <n|l|r|lr|rl>
@anchor{@strong{--trim} <n|l|r|lr|rl>}

Trim white space in input.

@table @asis
@item n
@anchor{n}

No trim. Input is not modified. This is the default.

@item l
@anchor{l}

Left trim. Remove white space from start of input. E.g. " a bc " -> "a bc ".

@item r
@anchor{r}

Right trim. Remove white space from end of input. E.g. " a bc " -> " a bc".

@item lr
@anchor{lr}

@item rl
@anchor{rl}

Both trim. Remove white space from both start and end of input. E.g. "
a bc " -> "a bc". This is the default if @strong{--colsep} is used.

@end table

@item @strong{--ungroup}
@anchor{@strong{--ungroup}}

@item @strong{-u}
@anchor{@strong{-u}}

Ungroup output.  Output is printed as soon as possible and by passes
GNU @strong{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:

@strong{parallel -j0 'sleep @{@};echo -n start@{@};sleep @{@};echo @{@}end' ::: 1 2 3 4}

@strong{parallel -u -j0 'sleep @{@};echo -n start@{@};sleep @{@};echo @{@}end' ::: 1 2 3 4}

It also disables @strong{--tag}. GNU @strong{parallel} runs faster with @strong{-u}. Can
be reversed with @strong{--group}.

@item @strong{--extensionreplace} @emph{replace-str}
@anchor{@strong{--extensionreplace} @emph{replace-str}}

@item @strong{--er} @emph{replace-str}
@anchor{@strong{--er} @emph{replace-str}}

Use the replacement string @emph{replace-str} instead of @{.@} for input line without extension.

@item @strong{--use-cpus-instead-of-cores}
@anchor{@strong{--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 @strong{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.

@item @strong{-v}
@anchor{@strong{-v}}

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

Use @strong{-v} @strong{-v} to print the wrapping ssh command when running remotely.

@item @strong{--version}
@anchor{@strong{--version}}

@item @strong{-V}
@anchor{@strong{-V}}

Print the version GNU @strong{parallel} and exit.

@item @strong{--workdir} @emph{mydir}
@anchor{@strong{--workdir} @emph{mydir}}

@item @strong{--wd} @emph{mydir}
@anchor{@strong{--wd} @emph{mydir}}

Files transferred using @strong{--transfer} and @strong{--return} will be relative
to @emph{mydir} on remote computers, and the command will be executed in
the dir @emph{mydir}.

The special @emph{mydir} value @strong{...} will create working dirs under
@strong{~/.parallel/tmp/} on the remote computers. If @strong{--cleanup} is given
these dirs will be removed.

The special @emph{mydir} value @strong{.} uses the current working dir.  If the
current working dir is beneath your home dir, the value @strong{.} 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.

@item @strong{--wait}
@anchor{@strong{--wait}}

Wait for all commands to complete.

Implies @strong{--semaphore}.

See also: @strong{man sem}

@item @strong{-X}
@anchor{@strong{-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 @strong{-j1}
to avoid this.

If @strong{@{@}} is not used the arguments will be appended to the line.  If
@strong{@{@}} is used as part of a word (like @emph{pic@{@}.jpg}) then the whole
word will be repeated. If @strong{@{@}} is used multiple times each @strong{@{@}} will
be replaced with the arguments.

Normally @strong{-X} will do the right thing, whereas @strong{-m} can give
unexpected results if @strong{@{@}} is used as part of a word.

Support for @strong{-X} with @strong{--sshlogin} is limited and may fail.

See also @strong{-m}.

@item @strong{--exit}
@anchor{@strong{--exit}}

@item @strong{-x}
@anchor{@strong{-x}}

Exit if the size (see the @strong{-s} option) is exceeded.

@item @strong{--xapply}
@anchor{@strong{--xapply}}

Read multiple input sources like @strong{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 @strong{@{1@}}
.. @strong{@{}@emph{n}@strong{@}}, so @strong{@{1@}} will be a line from the first input source, and
@strong{@{6@}} will refer to the line with the same line number from the 6th
input source.

Compare these two:

@verbatim
  parallel echo {1} {2} ::: 1 2 3 ::: a b c
  parallel --xapply e