xargs Man page

XARGS(1) General Commands Manual XARGS(1)

NAME

xargs – build and execute command lines from standard input

SYNOPSIS

xargs [-0prtx] [-E eof-str] [-e[eof-str]] [–eof[=eof-str]] [–null] [-d delimiter] [–delimiter delimiter] [-I replace-str] [-i[replace-
str]] [–replace[=replace-str]] [-l[max-lines]] [-L max-lines] [–max-lines[=max-lines]] [-n max-args] [–max-args=max-args] [-s max-
chars] [–max-chars=max-chars] [-P max-procs] [–max-procs=max-procs] [–process-slot-var=name] [–interactive] [–verbose] [–exit] [–no-run-if-empty] [–arg-file=file] [–show-limits] [–version] [–help] [command [initial-arguments]]

DESCRIPTION

This manual page documents the GNU version of xargs. xargs reads items
from the standard input, delimited by blanks (which can be protected
with double or single quotes or a backslash) or newlines, and executes
the command (default is /bin/echo) one or more times with any initial-
arguments followed by items read from standard input. Blank lines on
the standard input are ignored.

The command line for command is built up until it reaches a system-de‐
fined limit (unless the -n and -L options are used). The specified
command will be invoked as many times as necessary to use up the list
of input items. In general, there will be many fewer invocations of
command than there were items in the input. This will normally have
significant performance benefits. Some commands can usefully be exe‐
cuted in parallel too; see the -P option.

Because Unix filenames can contain blanks and newlines, this default
behaviour is often problematic; filenames containing blanks and/or new‐
lines are incorrectly processed by xargs. In these situations it is
better to use the -0 option, which prevents such problems. When using
this option you will need to ensure that the program which produces the
input for xargs also uses a null character as a separator. If that
program is GNU find for example, the -print0 option does this for you.

If any invocation of the command exits with a status of 255, xargs will
stop immediately without reading any further input. An error message
is issued on stderr when this happens.

OPTIONS

-0, –null
Input items are terminated by a null character instead of by
whitespace, and the quotes and backslash are not special (every
character is taken literally). Disables the end of file string,
which is treated like any other argument. Useful when input
items might contain white space, quote marks, or backslashes.
The GNU find -print0 option produces input suitable for this
mode.

-a file, –arg-file=file
Read items from file instead of standard input. If you use this
option, stdin remains unchanged when commands are run. Other‐
wise, stdin is redirected from /dev/null.

–delimiter=delim, -d delim
Input items are terminated by the specified character. The
specified delimiter may be a single character, a C-style charac‐
ter 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. When
processing the input, quotes and backslash are not special; ev‐
ery character in the input is taken literally. The -d option
disables any end-of-file string, which is treated like any other
argument. You can use this option 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 possi‐
ble.

-E eof-str
Set the end of file string to eof-str. If the end of file
string occurs as a line of input, the rest of the input is ig‐
nored. If neither -E nor -e is used, no end of file string is
used.

-e[eof-str], –eof[=eof-str] This option is a synonym for the -E option. Use -E instead, be‐
cause it is POSIX compliant while this option is not. If eof-
str is omitted, there is no end of file string. If neither -E
nor -e is used, no end of file string is used.

-I replace-str
Replace occurrences of replace-str in the initial-arguments with
names read from standard input. Also, unquoted blanks do not
terminate input items; instead the separator is the newline
character. Implies -x and -L 1.

-i[replace-str], –replace[=replace-str] This option is a synonym for -Ireplace-str if replace-str is
specified. If the replace-str argument is missing, the effect
is the same as -I{}. This option is deprecated; use -I instead.

-L max-lines
Use at most max-lines nonblank input lines per command line.
Trailing blanks cause an input line to be logically continued on
the next input line. Implies -x.

-l[max-lines], –max-lines[=max-lines] Synonym for the -L option. Unlike -L, the max-lines argument is
optional. If max-lines is not specified, it defaults to one.
The -l option is deprecated since the POSIX standard specifies
-L instead.

-n max-args, –max-args=max-args
Use at most max-args arguments per command line. Fewer than
max-args arguments will be used if the size (see the -s option)
is exceeded, unless the -x option is given, in which case xargs
will exit.

-P max-procs, –max-procs=max-procs
Run up to max-procs processes at a time; the default is 1. If
max-procs is 0, xargs will run as many processes as possible at
a time. Use the -n option or the -L option with -P; otherwise
chances are that only one exec will be done. While xargs is
running, you can send its process a SIGUSR1 signal to increase
the number of commands to run simultaneously, or a SIGUSR2 to
decrease the number. You cannot increase it above an implemen‐
tation-defined limit (which is shown with –show-limits). You
cannot decrease it below 1. xargs never terminates its com‐
mands; when asked to decrease, it merely waits for more than one
existing command to terminate before starting another.

Please note that it is up to the called processes to properly
manage parallel access to shared resources. For example, if
more than one of them tries to print to stdout, the ouptut will
be produced in an indeterminate order (and very likely mixed up)
unless the processes collaborate in some way to prevent this.
Using some kind of locking scheme is one way to prevent such
problems. In general, using a locking scheme will help ensure
correct output but reduce performance. If you don’t want to
tolerate the performance difference, simply arrange for each
process to produce a separate output file (or otherwise use sep‐
arate resources).

-p, –interactive
Prompt the user about whether to run each command line and read
a line from the terminal. Only run the command line if the re‐
sponse starts with `y’ or `Y’. Implies -t.

–process-slot-var=name
Set the environment variable name to a unique value in each run‐
ning child process. Values are reused once child processes ex‐
it. This can be used in a rudimentary load distribution scheme,
for example.

-r, –no-run-if-empty
If the standard input does not contain any nonblanks, do not run
the command. Normally, the command is run once even if there is
no input. This option is a GNU extension.

-s max-chars, –max-chars=max-chars
Use at most max-chars characters per command line, including the
command and initial-arguments and the terminating nulls at the
ends of the argument strings. The largest allowed value is sys‐
tem-dependent, and is calculated as the argument length limit
for exec, less the size of your environment, less 2048 bytes of
headroom. If this value is more than 128KiB, 128Kib is used as
the default value; otherwise, the default value is the maximum.
1KiB is 1024 bytes. xargs automatically adapts to tighter con‐
straints.

–show-limits
Display the limits on the command-line length which are imposed
by the operating system, xargs’ choice of buffer size and the -s
option. Pipe the input from /dev/null (and perhaps specify
–no-run-if-empty) if you don’t want xargs to do anything.

-t, –verbose
Print the command line on the standard error output before exe‐
cuting it.

-x, –exit
Exit if the size (see the -s option) is exceeded.

–help Print a summary of the options to xargs and exit.

–version
Print the version number of xargs and exit.

EXAMPLES
find /tmp -name core -type f -print | xargs /bin/rm -f

Find files named core in or below the directory /tmp and delete them.
Note that this will work incorrectly if there are any filenames con‐
taining newlines or spaces.

find /tmp -name core -type f -print0 | xargs -0 /bin/rm -f

Find files named core in or below the directory /tmp and delete them,
processing filenames in such a way that file or directory names con‐
taining spaces or newlines are correctly handled.

find /tmp -depth -name core -type f -delete

Find files named core in or below the directory /tmp and delete them,
but more efficiently than in the previous example (because we avoid the
need to use fork(2) and exec(2) to launch rm and we don’t need the ex‐
tra xargs process).

cut -d: -f1 < /etc/passwd | sort | xargs echo Generates a compact listing of all the users on the system. xargs sh -c 'emacs "$@" < /dev/tty' emacs Launches the minimum number of copies of Emacs needed, one after the other, to edit the files listed on xargs' standard input. This example achieves the same effect as BSD's -o option, but in a more flexible and portable way. EXIT STATUS xargs exits with the following status: 0 if it succeeds 123 if any invocation of the command exited with status 1-125 124 if the command exited with status 255 125 if the command is killed by a signal 126 if the command cannot be run 127 if the command is not found 1 if some other error occurred. Exit codes greater than 128 are used by the shell to indicate that a program died due to a fatal signal. STANDARDS CONFORMANCE As of GNU xargs version 4.2.9, the default behaviour of xargs is not to have a logical end-of-file marker. POSIX (IEEE Std 1003.1, 2004 Edi‐ tion) allows this. The -l and -i options appear in the 1997 version of the POSIX standard, but do not appear in the 2004 version of the standard. Therefore you should use -L and -I instead, respectively. The POSIX standard allows implementations to have a limit on the size of arguments to the exec functions. This limit could be as low as 4096 bytes including the size of the environment. For scripts to be porta‐ ble, they must not rely on a larger value. However, I know of no im‐ plementation whose actual limit is that small. The --show-limits op‐ tion can be used to discover the actual limits in force on the current system.

SEE ALSO

find(1), locate(1), locatedb(5), updatedb(1), fork(2), execvp(3),
kill(1), signal(7),

The full documentation for xargs is maintained as a Texinfo manual.
If the info and xargs programs are properly installed at your site, the
command info xargs should give you access to the complete manual.

BUGS

The -L option is incompatible with the -I option, but perhaps should
not be.

It is not possible for xargs to be used securely, since there will al‐
ways be a time gap between the production of the list of input files
and their use in the commands that xargs issues. If other users have
access to the system, they can manipulate the filesystem during this
time window to force the action of the commands xargs runs to apply to
files that you didn’t intend. For a more detailed discussion of this
and related problems, please refer to the “Security Considerations”
chapter in the findutils Texinfo documentation. The -execdir option of
find can often be used as a more secure alternative.

When you use the -I option, each line read from the input is buffered
internally. This means that there is an upper limit on the length of
input line that xargs will accept when used with the -I option. To
work around this limitation, you can use the -s option to increase the
amount of buffer space that xargs uses, and you can also use an extra
invocation of xargs to ensure that very long lines do not occur. For
example:

somecommand | xargs -s 50000 echo | xargs -I ‘{}’ -s 100000 rm ‘{}’

Here, the first invocation of xargs has no input line length limit be‐
cause it doesn’t use the -i option. The second invocation of xargs
does have such a limit, but we have ensured that the it never encoun‐
ters a line which is longer than it can handle. This is not an ideal
solution. Instead, the -i option should not impose a line length lim‐
it, which is why this discussion appears in the BUGS section. The
problem doesn’t occur with the output of find(1) because it emits just
one filename per line.

The best way to report a bug is to use the form at http://savan‐
nah.gnu.org/bugs/?group=findutils. The reason for this is that you
will then be able to track progress in fixing the problem. Other com‐
ments about xargs(1) and about the findutils package in general can be
sent to the bug-findutils mailing list. To join the list, send email
to bug-findutils-request@gnu.org.

XARGS(1)