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Chet Ramey
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<HTML>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<!-- Created on January, 11 2013 by texi2html 1.64 -->
<!-- Created on March, 8 2013 by texi2html 1.64 -->
<!--
Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author)
Karl Berry <karl@freefriends.org>
@@ -33,11 +33,11 @@ Send bugs and suggestions to <texi2html@mathematik.uni-kl.de>
<H1>Bash Reference Manual</H1></P><P>
This text is a brief description of the features that are present in
the Bash shell (version 4.2, 8 January 2013 ).
the Bash shell (version 4.2, 2 March 2013 ).
The Bash home page is <A HREF="http://www.gnu.org/software/bash/">http://www.gnu.org/software/bash/</A>.
</P><P>
This is Edition 4.2, last updated 8 January 2013 ,
This is Edition 4.2, last updated 2 March 2013 ,
of <CITE>The GNU Bash Reference Manual</CITE>,
for <CODE>Bash</CODE>, Version 4.2.
</P><P>
@@ -1622,43 +1622,59 @@ The return status of a coprocess is the exit status of <VAR>command</VAR>.
<!--docid::SEC24::-->
<P>
There are ways to run commands in parallel that are not built into Bash.
GNU Parallel is a tool to do just that.
</P><P>
GNU Parallel, as its name suggests, can be used to build and run commands
in parallel. You may run the same command with different arguments, whether
they are filenames, usernames, hostnames, or lines read from files.
they are filenames, usernames, hostnames, or lines read from files. GNU
Parallel provides shorthand references to many of the most common operations
(input lines, various portions of the input line, different ways to specify
the input source, and so on). Parallel can replace <CODE>xargs</CODE> or feed
commands from its input sources to several different instances of Bash.
</P><P>
For a complete description, refer to the GNU Parallel documentation. A few
examples should provide a brief introduction to its use.
</P><P>
For example, it is easy to prefix each line in a text file with a specified
string:
<TABLE><tr><td>&nbsp;</td><td class=example><pre>cat file | parallel -k echo prefix_string
</pre></td></tr></table>The <SAMP>`-k'</SAMP> option is required to preserve the lines' order.
For example, it is easy to replace <CODE>xargs</CODE> to gzip all html files in the
current directory and its subdirectories:
<TABLE><tr><td>&nbsp;</td><td class=example><pre>find . -type f -name '*.html' -print | parallel gzip
</pre></td></tr></table>If you need to protect special characters such as newlines in file names,
use find's <SAMP>`-print0'</SAMP> option and parallel's <SAMP>`-0'</SAMP> option.
</P><P>
Similarly, you can append a specified string to each line in a text file:
<TABLE><tr><td>&nbsp;</td><td class=example><pre>cat file | parallel -k echo {} append_string
</pre></td></tr></table></P><P>
You can use Parallel to move files from the current directory when the
number of files is too large to process with one <CODE>mv</CODE> invocation:
<TABLE><tr><td>&nbsp;</td><td class=example><pre>ls | parallel mv {} destdir
</pre></td></tr></table></P><P>
As you can see, the {} is replaced with each line read from standard input.
While using <CODE>ls</CODE> will work in most instances, it is not sufficient to
deal with all filenames.
If you need to accommodate special characters in filenames, you can use
</P><P>
<TABLE><tr><td>&nbsp;</td><td class=example><pre>find . -depth 1 \! -name '.*' -print0 | parallel -0 mv {} destdir
</pre></td></tr></table></P><P>
as alluded to above.
</P><P>
This will run as many <CODE>mv</CODE> commands as there are files in the current
directory. You can emulate a parallel <CODE>xargs</CODE> by adding the <SAMP>`-X'</SAMP>
option:
<TABLE><tr><td>&nbsp;</td><td class=example><pre>ls | parallel -X mv {} destdir
directory.
You can emulate a parallel <CODE>xargs</CODE> by adding the <SAMP>`-X'</SAMP> option:
<TABLE><tr><td>&nbsp;</td><td class=example><pre>find . -depth 1 \! -name '.*' -print0 | parallel -0 -X mv {} destdir
</pre></td></tr></table></P><P>
GNU Parallel can replace certain common idioms that operate on lines read
from a file (in this case, filenames):
<TABLE><tr><td>&nbsp;</td><td class=example><pre> for x in $(cat list); do
do-something1 $x config-$x
do-something2 &#60; $x
done | process-output
from a file (in this case, filenames listed one per line):
<TABLE><tr><td>&nbsp;</td><td class=example><pre> while read -r x; do
do-something1 "$x" "config-$x"
do-something2 &#60; "$x"
done &#60; file | process-output
</pre></td></tr></table></P><P>
with a more compact syntax reminiscent of lambdas:
@@ -1669,18 +1685,40 @@ Parallel provides a built-in mechanism to remove filename extensions, which
lends itself to batch file transformations or renaming:
<TABLE><tr><td>&nbsp;</td><td class=example><pre>ls *.gz | parallel -j+0 "zcat {} | bzip2 >{.}.bz2 &&#38; rm {}"
</pre></td></tr></table>This will recompress all files in the current directory with names ending
in .gz using bzip2, running one job per CPU (-j+0) in parallel.
in .gz using bzip2, running one job per CPU (-j+0) in parallel.
(We use <CODE>ls</CODE> for brevity here; using <CODE>find</CODE> as above is more
robust in the face of filenames containing unexpected characters.)
Parallel can take arguments from the command line; the above can also be
written as
</P><P>
<TABLE><tr><td>&nbsp;</td><td class=example><pre>parallel "zcat {} | bzip2 >{.}.bz2 &&#38; rm {}" ::: *.gz
</pre></td></tr></table></P><P>
If a command generates output, you may want to preserve the input order in
the output. For instance, the following command
<TABLE><tr><td>&nbsp;</td><td class=example><pre>{ echo foss.org.my ; echo debian.org; echo freenetproject.org; } | parallel traceroute
</pre></td></tr></table>will display as output the traceroute invocation that finishes first. Using
the <SAMP>`-k'</SAMP> option, as we saw above
</pre></td></tr></table>will display as output the traceroute invocation that finishes first.
Adding the <SAMP>`-k'</SAMP> option
<TABLE><tr><td>&nbsp;</td><td class=example><pre>{ echo foss.org.my ; echo debian.org; echo freenetproject.org; } | parallel -k traceroute
</pre></td></tr></table>will ensure that the output of <CODE>traceroute foss.org.my</CODE> is displayed first.
</P><P>
Finally, Parallel can be used to run a sequence of shell commands in parallel,
similar to <SAMP>`cat file | bash'</SAMP>.
It is not uncommon to take a list of filenames, create a series of shell
commands to operate on them, and feed that list of commnds to a shell.
Parallel can speed this up. Assuming that <TT>`file'</TT> contains a list of
shell commands, one per line,
</P><P>
<TABLE><tr><td>&nbsp;</td><td class=example><pre>parallel -j 10 &#60; file
</pre></td></tr></table></P><P>
will evaluate the commands using the shell (since no explicit command is
supplied as an argument), in blocks of ten shell jobs at a time.
</P><P>
<A NAME="Shell Functions"></A>
<HR SIZE="6">
<A NAME="SEC25"></A>
@@ -6223,7 +6261,7 @@ interrupt; previous versions continue with the next command in the list.
<P>
<DT><CODE>compat41</CODE>
<DD>If set, Bash, when in posix mode, treats a single quote in a double-quoted
<DD>If set, Bash, when in POSIX mode, treats a single quote in a double-quoted
parameter expansion as a special character. The single quotes must match
(an even number) and the characters between the single quotes are considered
quoted. This is the behavior of POSIX mode through version 4.1.
@@ -17202,7 +17240,7 @@ to permit their use in free software.
<TD VALIGN="MIDDLE" ALIGN="LEFT">[<A HREF="bashref.html#SEC_About"> ? </A>]</TD>
</TR></TABLE>
<H1>About this document</H1>
This document was generated by <I>Chet Ramey</I> on <I>January, 11 2013</I>
This document was generated by <I>Chet Ramey</I> on <I>March, 8 2013</I>
using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html
"><I>texi2html</I></A>
<P></P>
@@ -17364,7 +17402,7 @@ the following structure:
<BR>
<FONT SIZE="-1">
This document was generated
by <I>Chet Ramey</I> on <I>January, 11 2013</I>
by <I>Chet Ramey</I> on <I>March, 8 2013</I>
using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html
"><I>texi2html</I></A>