parallel/src/parallel.pod

#!/usr/bin/perl -w

=head1 NAME

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

=head1 SYNOPSIS

B<parallel> [options] [I<command> [arguments]] < list_of_arguments

B<parallel> [options] [I<command> [arguments]] B<:::> arguments

B<parallel> [options] [I<command> [arguments]] B<::::> argfile(s)

B<parallel> --semaphore [options] I<command>

B<#!/usr/bin/parallel> --shebang [options] [I<command> [arguments]]

=head1 DESCRIPTION

GNU B<parallel> is a shell tool for executing jobs concurrently locally
or using remote computers. A job is typically a single command or a
small script that has to be run for each of the lines in the
input. The typical input is a list of files, a list of hosts, a list
of users, a list of URLs, or a list of tables.

If you use B<xargs> today you will find GNU B<parallel> very easy to
use as GNU B<parallel> is written to have the same options as
B<xargs>. If you write loops in shell, you will find GNU B<parallel>
may be able to replace most of the loops and make them run faster by
running several jobs simultaneously. If you use B<ppss> or B<pexec> you
will find GNU B<parallel> will often make the command easier to read.

GNU B<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 B<parallel> as input for other
programs.

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

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

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

=head1 OPTIONS

=over 9

=item I<command>

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

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


=item B<{}>

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


=item B<{.}>

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

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


=item B<{/}> (beta testing)

Basename of input line. This is a specialized replacement string
with the directory part removed.

B<{/}> can be used the same places as B<{}>. The replacement string
B<{/}> can be changed with B<--basenamereplace>.


=item B<{/.}> (beta testing)

Basename of input line without extension. This is a specialized
replacement string with the directory and extension part removed. It
is a combination of B<{/}> and B<{.}>.

B<{/.}> can be used the same places as B<{}>. The replacement string
B<{/.}> can be changed with B<--basenameextensionreplace>.


=item B<{>I<n>B<}>

Argument from argument file I<n> or the I<n>'th argument. See B<-a>
and B<-N>.

B<{>I<n>B<}> can be used the same places as B<{}>.


=item B<{>I<n>.B<}>

Argument from argument file I<n> or the I<n>'th argument without
extension. It is a combination of B<{>I<n>B<}> and B<{.}>.

B<{>I<n>.B<}> can be used the same places as B<{>I<n>B<}>.


=item B<{>I<n>/B<}> (beta testing)

Basename of argument from argument file I<n> or the I<n>'th argument.
It is a combination of B<{>I<n>B<}> and B<{/}>.  See B<-a> and B<-N>.

B<{>I<n>/B<}> can be used the same places as B<{>I<n>B<}>.


=item B<{>I<n>/.B<}> (beta testing)

Basename of argument from argument file I<n> or the I<n>'th argument
without extension.  It is a combination of B<{>I<n>B<}>, B<{/}>, and
B<{.}>.  See B<-a> and B<-N>.

B<{>I<n>/.B<}> can be used the same places as B<{>I<n>B<}>.



=item B<:::> I<arguments>

Use arguments from the command line as input instead of from stdin
(standard input). Unlike other options for GNU B<parallel> B<:::> is
placed after the I<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 B<:::> as special use B<--arg-sep> to set the
argument separator to something else. See also B<--arg-sep>.

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

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


=item B<::::> I<argfiles>

Another way to write B<-a> I<argfile1> B<-a> I<argfile2> ...

See B<-a>.


=item B<--null>

=item B<-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 B<--arg-file> I<input-file>

=item B<-a> I<input-file>

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

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


=item B<--arg-file-sep> I<sep-str>

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

See also: B<::::>.


=item B<--arg-sep> I<sep-str>

Use I<sep-str> instead of B<:::> as separator string. Useful if B<:::>
is used for something else by the command.

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

See also: B<:::>.


=item B<--basefile> I<file>

=item B<-B> I<file>

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


=item B<--basenamereplace> I<replace-str> (beta testing)

Use the replacement string I<replace-str> instead of B<{/}> for basename of input line.


=item B<--basenameextensionreplace> I<replace-str> (beta testing)

Use the replacement string I<replace-str> instead of B<{/.}> for basename of input line without extension.


=item B<--bg> (beta testing)

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

See also: B<--fg>

Implies B<--semaphore>.


=item B<--cleanup>

Remove transferred files. B<--cleanup> will remove the transferred files
on the remote server after processing is done.

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

With B<--transfer> the file transferred to the remote server will be
removed on the remote server.  Directories created will not be removed
- even if they are empty.

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

B<--cleanup> is ignored when not used with B<--transfer> or B<--return>.


=item B<--colsep> I<regexp>

=item B<-C> I<regexp>

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

B<--colsep> implies B<--trim rl>.

I<regexp> is a Perl Regular Expression:
http://perldoc.perl.org/perlre.html


=item B<--command>

=item B<-c> (Use B<--command> as B<-c> may be removed in later versions)

Line is a command.  The input line contains more than one argument or
the input line needs to be evaluated by the shell. This is the default
if I<command> is not set. Can be reversed with B<--file>.

Most people will never need this because GNU B<parallel> normally
selects the correct B<--file> or B<--command>.


=item B<--delimiter> I<delim>

=item B<-d> I<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 B<-E> I<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 B<-E> nor B<-e> is used, no end of file string is used.


=item B<--dry-run>

Print the job to run on standard output, but do not run the job. Use
B<-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 B<--eof>[=I<eof-str>]

=item B<-e>[I<eof-str>]

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


=item B<--eta>

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


=item B<--fg> (beta testing)

Run command in foreground thus GNU B<parallel> will wait for
completion of the command before exiting.

See also: B<--bg>

Implies B<--semaphore>.


=item B<--file>

=item B<-f> (Use B<--file> as B<-f> may be removed in later versions)

Line is a filename.  The input line contains a filename that will be
quoted so it is not evaluated by the shell. This is the default if
I<command> is set. Can be reversed with B<--command>.

Most people will never need this because GNU B<parallel> normally
selects the correct B<--file> or B<--command>.


=item B<--group>

=item B<-g>

Group output.  Output from each jobs is grouped together and is only
printed when the command is finished. STDERR first followed by STDOUT.
B<-g> is the default. Can be reversed with B<-u>.

=item B<--help>

=item B<-h>

Print a summary of the options to GNU B<parallel> and exit.


=item B<--halt-on-error> <0|1|2>

=item B<-H> <0|1|2>

=over 3

=item 0

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

=item 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

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

=back


=item B<-I> I<replace-str>

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


=item B<--replace>[=I<replace-str>]

=item B<-i>[I<replace-str>]

This option is a synonym for B<-I>I<replace-str> if I<replace-str> is
specified, and for B<-I>{} otherwise.  This option is deprecated;
use B<-I> instead.


=item B<--jobs> I<N>

=item B<-j> I<N>

=item B<--max-procs> I<N>

=item B<-P> I<N>

Run up to N jobs in parallel.  0 means as many as possible. Default is
9.

If B<--semaphore> is set default is 1 thus making a mutex.


=item B<--jobs> I<+N>

=item B<-j> I<+N>

=item B<--max-procs> I<+N>

=item B<-P> I<+N>

Add N to the number of CPU cores.  Run this many jobs in parallel. For
compute intensive jobs B<-j> +0 is useful as it will run
number-of-cpu-cores jobs simultaneously. See also
B<--use-cpus-instead-of-cores>.


=item B<--jobs> I<-N>

=item B<-j> I<-N>

=item B<--max-procs> I<-N>

=item B<-P> I<-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
B<--use-cpus-instead-of-cores>.


=item B<--jobs> I<N>%

=item B<-j> I<N>%

=item B<--max-procs> I<N>%

=item B<-P> I<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
B<--use-cpus-instead-of-cores>.


=item B<--jobs> I<procfile>

=item B<-j> I<procfile>

=item B<--max-procs> I<procfile>

=item B<-P> I<procfile>

Read parameter from file. Use the content of I<procfile> as parameter
for I<-j>. E.g. I<procfile> could contain the string 100% or +2 or
10. If I<procfile> is changed when a job completes, I<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 B<parallel> is running.


=item B<--keeporder>

=item B<-k>

Keep sequence of output same as the order of input. If jobs 1 2 3 4
end in the sequence 3 1 4 2 the output will still be 1 2 3 4.


=item B<-L> I<max-lines>

Use at most I<max-lines> nonblank input lines per command line.
Trailing blanks cause an input line to be logically continued on the
next input line.

Implies B<-X> unless B<-m> is set.


=item B<--max-lines>[=I<max-lines>]

=item B<-l>[I<max-lines>]

Synonym for the B<-L> option.  Unlike B<-L>, the I<max-lines> argument
is optional.  If I<max-lines> is not specified, it defaults to one.
The B<-l> option is deprecated since the POSIX standard specifies
B<-L> instead.

Implies B<-X> unless B<-m> is set.


=item B<--load> I<max-load> (experimental)

Do not start new jobs on a given machine unless the load is less than
I<max-load>. I<max-load> uses the same syntax as B<--jobs>, so I<100%>
is a valid setting.

The load average is only sampled every 10 seconds to avoid stressing
small machines.


=item B<--controlmaster> (experimental)

=item B<-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 B<--xargs>

=item B<-m>

Multiple. Insert as many arguments as the command line length
permits. If B<{}> is not used the arguments will be appended to the
line.  If B<{}> is used multiple times each B<{}> will be replaced
with all the arguments.

Support for B<-m> with B<--sshlogin> is limited and may fail.

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


=item B<--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 B<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 B<parallel> SIGUSR2 you can toggle turning on/off
B<--progress> on a running GNU B<parallel> process.


=item B<--max-args>=I<max-args>

=item B<-n> I<max-args>

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

Implies B<-X> unless B<-m> is set.


=item B<--max-replace-args>=I<max-args>

=item B<-N> I<max-args>

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

This will set the owner of the homedir to the user:

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

Implies B<-X> unless B<-m> is set.


=item B<--max-line-length-allowed>

Print the maximal number characters allowed on the command line and
exit (used by GNU B<parallel> itself to determine the line length
on remote computers).


=item B<--number-of-cpus>

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


=item B<--number-of-cores>

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


=item B<--nice> I<niceness> (beta testing)

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


=item B<--interactive>

=item B<-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 B<-t>.


=item B<--profile> I<profilename>

=item B<-J> I<profilename>

Use profile I<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 machine and a different profile for running jobs
on remote machines. See the section PROFILE FILES for examples.

I<profilename> corresponds to the file ~/.parallel/I<profilename>.

Default: config

=item B<--quote>

=item B<-q>

Quote I<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 B<--no-run-if-empty>

=item B<-r>

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


=item B<--retries> I<n> (beta testing)

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


=item B<--return> I<filename>

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

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

This will transfer the file I<$HOME/foo/bar.out> from the server
I<server.example.com> to the file I<foo/bar.out> after running
B<touch foo/bar.out> on I<server.example.com>.

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

This will transfer the file I</tmp/foo/bar.out> from the server
I<server.example.com> to the file I</tmp/foo/bar.out> after running
B<touch /tmp/foo/bar.out> on I<server.example.com>.

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

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

B<--return> is often used with B<--transfer> and B<--cleanup>.

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


=item B<--max-chars>=I<max-chars>

=item B<-s> I<max-chars>

Use at most I<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 B<-X> unless B<-m> is set.


=item B<--show-limits>

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


=item B<--semaphore>

Work as a counting semaphore. B<--semaphore> will cause GNU
B<parallel> to start I<command> in the background. When the number of
simultaneous jobs is reached, GNU B<parallel> will wait for one of
these to complete before starting another command.

B<--semaphore> implies B<--bg> unless B<--fg> is specified.

B<--semaphore> implies B<--semaphorename `tty`> unless
B<--semaphorename> is specified.

Used with B<--fg>, B<--wait>, and B<--semaphorename>.

The command B<sem> is an alias for B<parallel --semaphore>.


=item B<--semaphorename> I<name>

=item B<--id> I<name>

The name of the semaphore to use. The semaphore can be shared between
multiple processes.

Implies B<--semaphore>.


=item B<--semaphoretimeout> I<secs> (not implemented)

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

Implies B<--semaphore>.


=item B<--skip-first-line>

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


=item B<-S> I<[ncpu/]sshlogin[,[ncpu/]sshlogin[,...]]>

=item B<--sshlogin> I<[ncpu/]sshlogin[,[ncpu/]sshlogin[,...]]>

Distribute jobs to remote servers. The jobs will be run on a list of
remote servers.  GNU B<parallel> will determine the number of CPU
cores on the remote servers and run the number of jobs as specified by
B<-j>.  If the number I<ncpu> is given GNU B<parallel> will use this
number for number of CPU cores on the host. Normally I<ncpu> will not
be needed.

An I<sshlogin> is of the form:

  [sshcommand [options]][username@]hostname

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

To specify more sshlogins separate the sshlogins by comma or repeat
the options multiple times.

For examples: see B<--sshloginfile>.

The remote host must have GNU B<parallel> installed.

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


=item B<--sshloginfile> I<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.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/:

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

The sshloginfile '..' is special, it read sshlogins from
~/.parallel/sshloginfile


=item B<--silent>

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


=item B<--tmpdir> I<dirname>

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


=item B<--verbose>

=item B<-t>

Print the job to be run on standard error.

See also B<-v> and B<-p>.


=item B<--transfer>

Transfer files to remote servers. B<--transfer> is used with
B<--sshlogin> when the arguments are files and should be transferred to
the remote servers. The files will be transferred using B<rsync> and
will be put relative to the default login dir. E.g.

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

This will transfer the file I<foo/bar.txt> to the server
I<server.example.com> to the file I<$HOME/foo/bar.txt> before running
B<wc foo/bar.txt> on I<server.example.com>.

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

This will transfer the file I<foo/bar.txt> to the server
I<server.example.com> to the file I</tmp/foo/bar.txt> before running
B<wc /tmp/foo/bar.txt> on I<server.example.com>.

B<--transfer> is often used with B<--return> and B<--cleanup>.

B<--transfer> is ignored when used with B<--sshlogin :> or when not used with B<--sshlogin>.


=item B<--trc> I<filename>

Transfer, Return, Cleanup. Short hand for:

B<--transfer> B<--return> I<filename> B<--cleanup>


=item B<--trim> <n|l|r|lr|rl>

Trim white space in input.

=over 4

=item n

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

=item l

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

=item r

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

=item lr

=item rl

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

=back


=item B<--ungroup>

=item B<-u>

Ungroup output.  Output is printed as soon as possible. This may cause
output from different commands to be mixed. GNU B<parallel> runs
faster with B<-u>. Can be reversed with B<-g>.


=item B<--extensionreplace> I<replace-str>

=item B<-U> I<replace-str>

Use the replacement string I<replace-str> instead of {.} for input line without extension.


=item B<--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 B<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 B<-v>

Verbose.  Print the job to be run on standard output. Can be reversed
with B<--silent>. See also B<-t>.

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


=item B<--version>

=item B<-V>

Print the version GNU B<parallel> and exit.


=item B<--workdir> I<mydir> (beta testing)

=item B<-W> I<mydir> (beta testing)

Files transferred using B<--transfer> and B<--return> will be relative
to I<mydir> on remote machines, and the command will be executed in
that dir. The special workdir B<...> will create a workdir in
B<~/.parallel/tmp/> on the remote machines and will be removed if
using B<--cleanup>.


=item B<--wait> (beta testing)

Wait for all commands to complete.

Implies B<--semaphore>.


=item B<-X>

Multiple arguments with context replace. Insert as many arguments as
the command line length permits. If B<{}> is not used the arguments
will be appended to the line.  If B<{}> is used as part of a word
(like I<pic{}.jpg>) then the whole word will be repeated. If B<{}> is
used multiple times each B<{}> will be replaced with the arguments.

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

Support for B<-X> with B<--sshlogin> is limited and may fail.

See also B<-m>.


=item B<--exit>

=item B<-x>

Exit if the size (see the B<-s> option) is exceeded.


=item B<--shebang>

=item B<--hashbang>

=item B<-Y>

GNU B<Parallel> can be called as a shebang (#!) command as the first line of a script. Like this:

  #!/usr/bin/parallel -Yr traceroute

  foss.org.my
  debian.org
  freenetproject.org

For this to work B<--shebang> or B<-Y> must be set as the first option.


=back

=head1 EXAMPLE: Working as xargs -n1. Argument appending

GNU B<parallel> can work similar to B<xargs -n1>.

To compress all html files using B<gzip> run:

B<find . -name '*.html' | parallel gzip>

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

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

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


=head1 EXAMPLE: Reading arguments from command line

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

B<parallel gzip ::: *.html>

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

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


=head1 EXAMPLE: Inserting multiple arguments

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

B<bash: /bin/mv: Argument list too long>

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

B<ls | parallel mv {} destdir>

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

B<ls | parallel -m mv {} destdir>


=head1 EXAMPLE: Context replace

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

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

You could also do:

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

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

You could also run:

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

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


=head1 EXAMPLE: Compute intensive jobs and substitution

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

B<convert -geometry 120 foo.jpg thumb_foo.jpg>

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

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

To do it recursively use B<find>:

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

Notice how the argument has to start with B<{}> as B<{}> will include path
(e.g. running B<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 B<{.}> to avoid the extra .jpg in the file name. This command will
make files like ./foo/bar_thumb.jpg:

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


=head1 EXAMPLE: Substitution and redirection

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

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

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

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

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

=head1 EXAMPLE: Composed commands

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

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

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

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

Even small shell scripts can be run by GNU B<parallel>:

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

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

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


=head1 EXAMPLE: Removing file extension when processing files

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

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

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

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

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

Convert all WAV files to MP3 using LAME:

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

Put all converted in the same directory:

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

=head1 EXAMPLE: Removing two file extensions when processing files and
calling GNU Parallel from itself

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

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

=head1 EXAMPLE: Download 10 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 01-10.  This will
generate the past 30 days as YYYYMMDD:

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

Based on this we can let GNU B<parallel> generate 10 B<wget>s per day:

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

=head1 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:

B<cat list | parallel do_something | process_output>

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

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

and while-loops like this:

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

can be written like this:

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


=head1 EXAMPLE: Group output lines

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

Compare the output of:

B<parallel traceroute ::: foss.org.my debian.org freenetproject.org>

to the output of:

B<parallel -u traceroute ::: foss.org.my debian.org freenetproject.org>


=head1 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. B<-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:

B<cat textfile | parallel -k echo {} append_string>

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

Another example is B<traceroute>:

B<parallel traceroute ::: foss.org.my debian.org freenetproject.org>

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

To keep the order the same as input run:

B<parallel -k traceroute ::: foss.org.my debian.org freenetproject.org>

This will make sure the traceroute to foss.org.my 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 B<curl>:

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

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

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

=head1 EXAMPLE: Parallel grep

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

B<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 B<grep>.


=head1 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 (B<ssh-agent> may be
handy).

To run B<echo> on B<server.example.com>:

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

To run commands on more than one remote computer run:

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

Or:

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

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

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

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

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

Then run:

  seq 1 10 | parallel --sshloginfile mycomputers echo

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

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

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

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

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


=head1 EXAMPLE: Transferring of files

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

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

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

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

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

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

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

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

If you want run on several servers add the servers to I<--sshlogin>
either using ',' or multiple I<--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 I<--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 I<--transfer>, I<--return> and I<--cleanup> are used together. They can be
shortened to I<--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 I<mycomputers> containing the list of computers it becomes:

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

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

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

=head1 EXAMPLE: Distributing work to local and remote computers

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

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

=head1 EXAMPLE: Use multiple inputs in one command

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

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

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

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

B<parallel -a <(seq 6) -a <(seq 6 -1 1) echo>

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

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

Alternative version:

B<find . -type f | sort | parallel convert {} \$PARALLEL_SEQ.png>


=head1 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:

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

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

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


=head1 EXAMPLE: Working as cat | sh. Resource inexpensive jobs and evaluation

GNU B<parallel> can work similar to B<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://status-server/status.cgi?ip=10.0.0.1
  ping -c 1 10.0.0.2
  wget http://status-server/status.cgi?ip=10.0.0.2
  ...
  ping -c 1 10.0.0.255
  wget http://status-server/status.cgi?ip=10.0.0.255

To run 100 processes simultaneously do:

B<parallel -j 100 < jobs_to_run>

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


=head1 EXAMPLE: Working as mutex and counting semaphore

The command B<sem> is an alias for B<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 B<sem> will wait for one of these to complete before
starting another command. B<sem --wait> will wait for all jobs to
complete.

Run 10 jobs concurrently in the background:

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

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

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

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

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

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


=head1 EXAMPLE: Start editor with filenames from stdin (standard input)

You can use GNU Parallel to start interactive programs like emacs or vi:

B<cat filelist | parallel -uXj1 emacs>

B<cat filelist | parallel -uXj1 vi>

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


=head1 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:

B<echo >>B<jobqueue>; B<tail -f jobqueue | parallel>

To submit your jobs to the queue:

B<echo my_command my_arg >>>B< jobqueue>

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

B<echo >>B<jobqueue>; B<tail -f jobqueue | parallel -S ..>


=head1 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 B<inotifywait> is
called on other platforms file a bug report):

B<inotifywait -q -m -r -e CLOSE_WRITE --format %w%f my_dir | parallel
-u echo>

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

The B<-u> is needed because of a small bug in GNU B<parallel>. If that
proves to be a problem, file a bug report.

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

B<inotifywait -q -m -r -e CLOSE_WRITE --format %w%f my_dir | parallel -S ..
-u echo>


=head1 QUOTING

GNU B<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.

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 B<parallel> itself, so the evaluation of the $ must be done by
the sub shell started by GNU B<parallel>:

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

If the variable is set before GNU B<parallel> starts you can do this:

B<VAR=this_is_set_before_starting>

B<echo test | parallel echo {} $VAR>

Prints: B<test this_is_set_before_starting>

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

B<VAR="two  spaces  between  each  word">

B<echo test | parallel echo {} \'"$VAR"\'>

Prints: B<test two  spaces  between  each  word>

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

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

Prints: B<test this_is_set_after_starting>

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

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

Prints: B<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 B<parallel>:

B<seq 1 10 | parallel echo $$>

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

B<seq 1 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 B<parallel> and the sub shell:

B<echo test | parallel echo {} \\\$VAR>

Prints: B<test $VAR>

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

B<echo test | parallel -q echo {} \$VAR>

Prints: B<test $VAR>

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

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

It needs to be quoted like this:

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

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

B<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 B<-q> this WILL NOT WORK:

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

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

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

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

Or for substituting output:

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

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


=head1 LIST RUNNING JOBS

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

B<killall -USR1 parallel>

GNU B<parallel> will then print the currently running jobs on STDERR.


=head1 COMPLETE RUNNING JOBS BUT DO NOT START NEW JOBS

If you regret starting a lot of jobs you can simply break GNU B<parallel>,
but if you want to make sure you do not have halfcompleted jobs you
should send the signal B<SIGTERM> to GNU B<parallel>:

B<killall -TERM parallel>

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


=head1 ENVIRONMENT VARIABLES

=over 9

=item $PARALLEL_PID

The environment variable $PARALLEL_PID is set by GNU B<parallel> and
is visible to the jobs started from GNU B<parallel>. This makes it
possible for the jobs to communicate directly to GNU B<parallel>.
Remember to quote the $, so it gets evaluated by the correct
shell.

B<Example:> If each of the jobs tests a solution and one of jobs finds
the solution the job can tell GNU B<parallel> not to start more jobs
by: B<kill -TERM $PARALLEL_PID>. This only works on the local
computer.


=item $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.

B<Example:>

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


=item $TMPDIR

Directory for temporary files. See: B<--tmpdir>.


=item $PARALLEL

The environment variable $PARALLEL will be used as default options for
GNU B<parallel>. If the variable contains special shell characters
(e.g. $, *, or space) then these need to be to be escaped with \.

B<Example:>

B<cat list | parallel -j1 -k -v ls>

can be written as:

B<cat list | PARALLEL="-kvj1" parallel ls>

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

can be written as:

B<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.

=back

=head1 DEFAULT PROFILE (CONFIG FILE)

The file ~/.parallel/config (formerly known as .parallelrc) will be
read if it exists.  Lines starting with '#' will be ignored. It can be
formatted like the environment variable $PARALLEL, but it is often
easier to simply put each option on its own line.

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

=head1 PROFILE FILES

If B<--profile> set, GNU B<parallel> will read the profile from that file instead of 
~/.parallel/config.

Example: Profile for running every command with B<-j+0> and B<nice>

  echo -j+0 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 B<nice> on the
remote machines:

  echo -S .. nice > ~/.parallel/dist
  parallel -J dist --trc {.}.bz2 bzip2 -9 ::: *


=head1 EXIT STATUS

If B<--halt-on-error> 0 or not specified:

=over 6

=item 0

All jobs ran without error.

=item 1-253

Some of the jobs failed. The exit status gives the number of failed jobs

=item 254

More than 253 jobs failed.

=item 255

Other error.

=back

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


=head1 DIFFERENCES BETWEEN GNU Parallel AND ALTERNATIVES

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

=head2 SUMMARY TABLE

The following features are in some of the comparable tools:

Inputs
 I1. Arguments can be read from stdin
 I2. Arguments can be read from a file
 I3. Arguments can be read from multiple files
 I4. Arguments can be read from command line
 I5. Arguments can be read from a table
 I6. Arguments can be read from the same file using #! (shebang)
 I7. Line oriented input as default (Quoting of special chars not needed)

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

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

Execution
 E1. Running jobs in parallel
 E2. List running jobs
 E3. Finish running jobs, but do not start new jobs
 E4. Number of running jobs can depend on number of cpus
 E5. Finish running jobs, but do not start new jobs after first failure
 E6. Number of running jobs can be adjusted while running

Remote execution
 R1. Jobs can be run on remote computers
 R2. Basefiles can be transferred
 R3. Argument files can be transferred
 R4. Result files can be transferred
 R5. Cleanup of transferred files
 R6. No config files needed
 R7. Do not run more than SSHD's MaxStartup can handle
 R8. Configurable SSH command
 R9. Retry if connection breaks occationally

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

Legend
 - = no
 x = not applicable
 ID = yes

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

parallel:
I1 I2 I3 I4 I5 I6 I7
M1 M2 M3 M4 M5 M6
O1 O2 O3 O4 O5 O6
E1 E2 E3 E4 E5 E6
R1 R2 R3 R4 R5 R6 R7 R8 R9
S1 S2

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

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

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

ppss:
I1 I2 -  -  -  -  I7
M1 -  M3 -  -  M6
O1 -  -  x  -  -
E1 E2 ?E3 E4 - -
R1 R2 R3 R4 -  -  ?R7 ? ?
-  -

pexec:
I1 I2 -  I4 I5 -  -
M1 -  M3 -  -  M6
O1 O2 O3 -  O5 O6
E1 -  -  E4 -  E6
R1 -  -  -  -  R6 -  -  -
S1 -

xjobs: TODO - Please file a bug-report if you know what features xjobs
supports (See REPORTING BUGS).

prll: TODO - Please file a bug-report if you know what features prll
supports (See REPORTING BUGS).

dxargs: TODO - Please file a bug-report if you know what features dxargs
supports (See REPORTING BUGS).

mdm/middelman: TODO - Please file a bug-report if you know what
features mdm/middelman supports (See REPORTING BUGS).

xapply: TODO - Please file a bug-report if you know what features xapply
supports (See REPORTING BUGS).

paexec: TODO - Please file a bug-report if you know what features paexec
supports (See REPORTING BUGS).

ClusterSSH: TODO - Please file a bug-report if you know what features ClusterSSH
supports (See REPORTING BUGS).


=head2 DIFFERENCES BETWEEN xargs AND GNU Parallel

B<xargs> offer some of the same possibilites as GNU B<parallel>.

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

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

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

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

B<cat | xargs -d "\n" -n1 I<command>>

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

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

  parallel perl -e '\$a=\"1{}\"x10000000\;print\ \$a,\"\\n\"' '>' {} ::: a b c d e f
  ls -l a b c d e f
  parallel -kP4 -n1 grep 1 > out.par ::: a b c d e f
  echo a b c d e f | xargs -P4 -n1 grep 1 > out.xargs-unbuf
  echo a b c d e f | xargs -P4 -n1 grep --line-buffered 1 > out.xargs-linebuf
  echo a b c d e f | xargs -n1 grep --line-buffered 1 > out.xargs-serial
  ls -l out*
  md5sum out*

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

B<xargs> has no support for running jobs on remote computers.

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

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

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

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

becomes

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

and

B<ls | parallel "echo {}; ls {}|wc">

becomes

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


=head2 DIFFERENCES BETWEEN find -exec AND GNU Parallel

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

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


=head2 DIFFERENCES BETWEEN make -j AND GNU Parallel

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

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

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


=head2 DIFFERENCES BETWEEN ppss AND GNU Parallel

B<ppss> is also a tool for running jobs in parallel.

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

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

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

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

=head3 EXAMPLES FROM ppss MANUAL

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

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

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

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

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

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

B<3> parallel -a list-of-urls.txt wget -q

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

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

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

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

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

B<6> ./ppss stop -C config.cfg

B<6> killall -TERM parallel

B<7> ./ppss pause -C config.cfg

B<7> Press: CTRL-Z or killall -SIGTSTP parallel

B<8> ./ppss continue -C config.cfg

B<8> Enter: fg or killall -SIGCONT parallel

B<9> ./ppss.sh status -C config.cfg

B<9> killall -SIGUSR2 parallel


=head2 DIFFERENCES BETWEEN pexec AND GNU Parallel

B<pexec> is also a tool for running jobs in parallel.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

B<8> Combining GNU B<parallel> and GNU B<sem>.

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

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

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

=head2 DIFFERENCES BETWEEN xjobs AND GNU Parallel

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

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

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

B<1> ls -1 *.zip | xjobs unzip

B<1> ls *.zip | parallel unzip

B<2> ls -1 *.zip | xjobs -n unzip

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

B<3> find . -name '*.bak' | xjobs gzip

B<3> find . -name '*.bak' | parallel gzip

B<4> ls -1 *.jar | sed 's/\(.*\)/\1 > \1.idx/' | xjobs jar tf

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

B<5> xjobs -s script

B<5> cat script | parallel

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

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


=head2 DIFFERENCES BETWEEN prll AND GNU Parallel

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

B<prll> encourages using BASH aliases and BASH functions instead of
scripts. GNU B<parallel> can use the aliases and functions that are
defined at login (using: B<parallel bash -ci myalias>) but it will
never support running aliases and functions that are defined defined
later (see why
http://www.perlmonks.org/index.pl?node_id=484296). However, scripts or
composed commands work just fine.

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

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

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

parallel mogrify -flip ::: *.jpg


=head2 DIFFERENCES BETWEEN dxargs AND GNU Parallel

B<dxargs> is also a tool for running jobs in parallel.

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


=head2 DIFFERENCES BETWEEN mdm/middleman AND GNU Parallel

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

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

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

B<cat files | parallel -j+0 cmd>

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

=head2 DIFFERENCES BETWEEN xapply AND GNU Parallel

B<xapply> can run jobs in parallel on the local computer.

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

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

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

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

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

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

B<3> parallel diff {1} {2} :::: manifest1 checklist1

B<4> xapply 'indent' *.c

B<4> parallel indent ::: *.c

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

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

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

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

B<6> find */ -... | parallel -s 1024 -Xuj1 vi

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

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

B<7> find ... |parallel -n5 -uj1 vi

B<8> xapply -fn "" /etc/passwd

B<8> parallel -k echo < /etc/passwd

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

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

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

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

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

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


=head2 DIFFERENCES BETWEEN paexec AND GNU Parallel

B<paexec> can run jobs in parallel on both the local and remote computers.

B<paexec> requires commands to print a blank line as the last
output. This means you will have to write a wrapper for most programs.

B<paexec> has a job dependency facility so a job can depend on another
job to be executed successfully. Sort of a poor-man's B<make>.

Here are the examples from B<paexec>'s example catalog with the equivalent
using GNU B<parallel>:

=over 1

=item 1_div_X_run:

  ../../paexec -s -l -c "`pwd`/1_div_X_cmd" -n +1 <<EOF [...]
  parallel echo {} '|' `pwd`/1_div_X_cmd <<EOF [...]

=item all_substr_run:

  ../../paexec -lp -c "`pwd`/all_substr_cmd" -n +3 <<EOF [...]
  parallel echo {} '|' `pwd`/all_substr_cmd <<EOF [...]

=item cc_wrapper_run:

  ../../paexec -c "env CC=gcc CFLAGS=-O2 `pwd`/cc_wrapper_cmd" \
             -n 'host1 host2' \
             -t '/usr/bin/ssh -x' <<EOF [...]
  parallel echo {} '|' "env CC=gcc CFLAGS=-O2 `pwd`/cc_wrapper_cmd" \
             -S host1,host2 <<EOF [...]
  # This is not exactly the same, but avoids the wrapper
  parallel gcc -O2 -c -o {.}.o {} \
             -S host1,host2 <<EOF [...]

=item toupper_run:

  ../../paexec -lp -c "`pwd`/toupper_cmd" -n +10 <<EOF [...]
  parallel echo {} '|' ./toupper_cmd <<EOF [...]
  # Without the wrapper:
  parallel echo {} '| awk {print\ toupper\(\$0\)}' <<EOF [...]

=back

=head2 DIFFERENCES BETWEEN ClusterSSH AND GNU Parallel

ClusterSSH solves a different problem than GNU B<parallel>.

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

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

GNU B<parallel> can be used as a poor-man's version of ClusterSSH:

B<cat hostlist | parallel ssh {} do_stuff>


=head1 BUGS

=head2 Quoting of newline

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

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

However, this will work:

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

=head2 Startup speed

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

When using multiple computers GNU B<parallel> opens B<ssh> connections
to them to figure out how many connections can be used reliably
simultaneously (Namely SSHD's MaxStartup). This test is done for each
host in serial, so if your --sshloginfile contains many hosts it may
be slow.

=head2 --nice limits command length

The current implementation of B<--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.


=head1 REPORTING BUGS

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


=head1 AUTHOR

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 Ole Tange, http://ole.tange.dk and Free Software
Foundation, Inc.

Parts of the manual concerning B<xargs> compatibility is inspired by
the manual of B<xargs> from GNU findutils 4.4.2.



=head1 LICENSE

Copyright (C) 2007,2008,2009,2010 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/>.

=head2 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.

=head2 Documentation license II

You are free:

=over 9

=item B<to Share>

to copy, distribute and transmit the work

=item B<to Remix>

to adapt the work

=back

Under the following conditions:

=over 9

=item B<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).

=item B<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.

=back

With the understanding that:

=over 9

=item B<Waiver>

Any of the above conditions can be waived if you get permission from
the copyright holder.

=item B<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.

=item B<Other Rights>

In no way are any of the following rights affected by the license:

=over 2

=item *

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

=item *

The author's moral rights;

=item *

Rights other persons may have either in the work itself or in
how the work is used, such as publicity or privacy rights.

=back

=back

=over 9

=item B<Notice>

For any reuse or distribution, you must make clear to others the
license terms of this work.

=back

A copy of the full license is included in the file as cc-by-sa.txt.

=head1 DEPENDENCIES

GNU B<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.


=head1 SEE ALSO

B<find>(1), B<xargs>(1), B<make>(1), B<pexec>(1), B<ppss>(1),
B<xjobs>(1), B<prll>(1), B<dxargs>(1), B<mdm>(1)

=cut