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Chet Ramey
2021-06-01 11:30:37 -04:00
parent 2047e28584
commit 6e11792d93
7 changed files with 2018 additions and 2012 deletions
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doc/bashref.texi
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@@ -1487,117 +1487,9 @@ Parallel provides shorthand references to many of the most common operations
the input source, and so on). Parallel can replace @code{xargs} or feed
commands from its input sources to several different instances of Bash.
For a complete description, refer to the GNU Parallel documentation. A few
examples should provide a brief introduction to its use.
For example, it is easy to replace @code{xargs} to gzip all html files in the
current directory and its subdirectories:
@example
find . -type f -name '*.html' -print | parallel gzip
@end example
@noindent
If you need to protect special characters such as newlines in file names,
use find's @option{-print0} option and parallel's @option{-0} option.
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} invocation:
@example
printf '%s\n' * | parallel mv @{@} destdir
@end example
As you can see, the @{@} is replaced with each line read from standard input.
While using @code{ls} will work in most instances, it is not sufficient to
deal with all filenames. @code{printf} is a shell builtin, and therefore is
not subject to the kernel's limit on the number of arguments to a program,
so you can use @samp{*} (but see below about the @code{dotglob} shell option).
If you need to accommodate special characters in filenames, you can use
@example
printf '%s\0' * | parallel -0 mv @{@} destdir
@end example
@noindent
as alluded to above.
This will run as many @code{mv} commands as there are files in the current
directory.
You can emulate a parallel @code{xargs} by adding the @option{-X} option:
@example
printf '%s\0' * | parallel -0 -X mv @{@} destdir
@end example
(You may have to modify the pattern if you have the @code{dotglob} option
enabled.)
GNU Parallel can replace certain common idioms that operate on lines read
from a file (in this case, filenames listed one per line):
@example
while IFS= read -r x; do
do-something1 "$x" "config-$x"
do-something2 < "$x"
done < file | process-output
@end example
@noindent
with a more compact syntax reminiscent of lambdas:
@example
cat list | parallel "do-something1 @{@} config-@{@} ; do-something2 < @{@}" |
process-output
@end example
Parallel provides a built-in mechanism to remove filename extensions, which
lends itself to batch file transformations or renaming:
@example
ls *.gz | parallel -j+0 "zcat @{@} | bzip2 >@{.@}.bz2 && rm @{@}"
@end example
@noindent
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.
(We use @code{ls} for brevity here; using @code{find} 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
@example
parallel "zcat @{@} | bzip2 >@{.@}.bz2 && rm @{@}" ::: *.gz
@end example
If a command generates output, you may want to preserve the input order in
the output. For instance, the following command
@example
@{
echo foss.org.my ;
echo debian.org ;
echo freenetproject.org ;
@} | parallel traceroute
@end example
@noindent
will display as output the traceroute invocation that finishes first.
Adding the @option{-k} option
@example
@{
echo foss.org.my ;
echo debian.org ;
echo freenetproject.org ;
@} | parallel -k traceroute
@end example
@noindent
will ensure that the output of @code{traceroute foss.org.my} is displayed first.
Finally, Parallel can be used to run a sequence of shell commands in parallel,
similar to @samp{cat file | bash}.
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 commands to a shell.
Parallel can speed this up. Assuming that @file{file} contains a list of
shell commands, one per line,
@example
parallel -j 10 < file
@end example
@noindent
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.
For a complete description, refer to the GNU Parallel documentation, which
is available at
@url{https://www.gnu.org/software/parallel/parallel_tutorial.html}.
@node Shell Functions
@section Shell Functions
@@ -2769,9 +2661,12 @@ without regard to the case of alphabetic characters.
When a pattern is used for filename expansion, the character @samp{.}
at the start of a filename or immediately following a slash
must be matched explicitly, unless the shell option @code{dotglob} is set.
The filenames @samp{.} and @samp{..} must always be matched explicitly,
The filenames @samp{.} and @samp{..} must be matched
by a pattern beginning with @samp{.} (for example, @samp{.?}),
or a pattern beginning with @samp{.} must be one of the patterns in an
extended pattern matching expression (see below),
even if @code{dotglob} is set.
In other cases, the @samp{.} character is not treated specially.
When not matching filenames, the @samp{.} character is not treated specially.
When matching a filename, the slash character must always be
matched explicitly by a slash in the pattern, but in other matching
@@ -4096,10 +3991,20 @@ Otherwise, the expression is false.
@end enumerate
@item 4 arguments
The following conditions are applied in the order listed.
@enumerate
@item
If the first argument is @samp{!}, the result is the negation of
the three-argument expression composed of the remaining arguments.
@item
If the first argument is exactly @samp{(} and the fourth argument is
exactly @samp{)}, the result is the two-argument test of the second
and third arguments.
@item
Otherwise, the expression is parsed and evaluated according to
precedence using the rules listed above.
@end enumerate
@item 5 or more arguments
The expression is parsed and evaluated according to precedence
@@ -4601,7 +4506,7 @@ each builtin with an indication of whether or not it is enabled.
The @option{-f} option means to load the new builtin command @var{name}
from shared object @var{filename}, on systems that support dynamic loading.
Bash will use the value of the @enable{BASH_LOADABLES_PATH} variable as a
Bash will use the value of the @env{BASH_LOADABLES_PATH} variable as a
colon-separated list of directories in which to search for @var{filename}.
The default is system-dependent.
The @option{-d} option will delete a builtin loaded with @option{-f}.