findutils/xargs/xargs.1

.TH XARGS 1 \" -*- nroff -*-
.SH NAME
xargs \- build and execute command lines from standard input
.SH SYNOPSIS
.B xargs
.nh
[\fB\-0prtx\fR]
[\fB\-E \fIeof-str\fR]
[\fB\-e\fR[\fIeof-str\fR]]
[\fB\-\-eof\fR[=\fIeof-str\fR]]
[\fB\-\-null\fR]
[\fB\-d \fIdelimiter\fR]
[\fB\-\-delimiter \fIdelimiter\fR]
[\fB\-I \fIreplace-str\fR]
[\fB\-i\fR[\fIreplace-str\fR]]
[\fB\-\-replace\fR[=\fIreplace-str\fR]]
[\fB\-l\fR[\fImax-lines\fR]]
[\fB\-L \fImax-lines\fR]
[\fB\-\-max\-lines\fR[=\fImax-lines\fR]]
[\fB\-n \fImax-args\fR]
[\fB\-\-max\-args\fR=\fImax-args\fR]
[\fB\-s \fImax-chars\fR]
[\fB\-\-max\-chars\fR=\fImax-chars\fR]
[\fB\-P \fImax-procs\fR]
[\fB\-\-max\-procs\fR=\fImax-procs\fR]
[\fB\-\-process\-slot\-var\fR=\fIname\fR]
[\fB\-\-interactive\fR]
[\fB\-\-verbose\fR]
[\fB\-\-exit\fR]
[\fB\-\-no\-run\-if\-empty\fR]
[\fB\-\-arg\-file\fR=\fIfile\fR]
[\fB\-\-show\-limits\fR]
[\fB\-\-version\fR]
[\fB\-\-help\fR]
[\fIcommand\fR [\fIinitial-arguments\fR]]
.hy
.SH DESCRIPTION
This manual page
documents the GNU version of
.BR xargs .
.B 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
.I command
(default is
.IR /bin/echo )
one or more times with any
.I initial-arguments
followed by items read from standard input.  Blank lines on the
standard input are ignored.
.P
The command line for
.I command
is built up until it reaches a system-defined limit (unless the
.B \-n
and
.B \-L
options are used).  The specified
.I 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
.I command
than there were items in the input.  This will normally have
significant performance benefits.  Some commands can usefully be
executed in parallel too; see the
.B \-P
option.
.P
Because Unix filenames can contain blanks and newlines, this default
behaviour is often problematic; filenames containing blanks
and/or newlines are incorrectly processed by
.BR xargs .
In these situations it is better to use the
.B \-0
option, which
prevents such problems.   When using this option you will need to
ensure that the program which produces the input for
.B xargs
also uses a null character as a separator.  If that program is
GNU
.B find
for example, the
.B \-print0
option does this for you.
.P
If any invocation of the command exits with a status of 255,
.B xargs
will stop immediately without reading any further input.  An error
message is issued on stderr when this happens.
.SH OPTIONS
.TP
.PD
.B \-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.

.TP
.PD
.BI "\-a " file ", \-\-arg\-file=" file
Read items from
.I file
instead of standard input.  If you use this option, stdin remains
unchanged when commands are run.  Otherwise, stdin is redirected
from
.IR /dev/null .

.TP
.PD
.BI "\-\-delimiter=" delim ", \-d" " delim"
Input items are terminated by the specified character.  The specified
delimiter may be a single character, a C-style character escape such
as
.BR \en ,
or an octal or hexadecimal escape code.  Octal and hexadecimal
escape codes are understood as for the
.B printf
command.   Multibyte characters are not supported.
When processing the input, quotes and backslash are not special; every
character in the input is taken literally.  The
.B \-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
.B \-\-null
where this is possible.

.TP
.BI \-E " eof-str"
Set the end of file string to \fIeof-str\fR.  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.
.TP
.PD
.BR \-e "[\fIeof-str\fR], " "\-\-eof" [\fI=eof-str\fR]
This option is a synonym for the
.B \-E
option.  Use
.B \-E
instead,
because it is POSIX compliant while this option is not.  If
\fIeof-str\fR 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.
.TP
.BI \-I " replace-str"
Replace occurrences of \fIreplace-str\fR 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
.B \-x
and
.B \-L
1.
.TP
.PD
.BR \-i "[\fIreplace-str\fR], " "\-\-replace" [\fI=replace-str\fR]
This option is a synonym for
.BI \-I replace-str
if
.I replace-str
is specified.  If the
.I replace-str
argument is missing, the effect is the same as
.BR \-I {}.
This option is deprecated; use
.B \-I
instead.
.TP
.BI \-L " max-lines"
Use at most \fImax-lines\fR nonblank input lines per command line.
Trailing blanks cause an input line to be logically continued on the
next input line.  Implies
.BR \-x .
.TP
.PD
.BR \-l "[\fImax-lines\fR], " \-\-max-lines "[=\fImax-lines\fR]"
Synonym for the
.B \-L
option.  Unlike
.BR \-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.
.TP
.PD
.BI \-n " max-args\fB, \fI" "\-\-max\-args" \fR=\fImax-args
Use at most \fImax-args\fR 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
.B xargs will exit.
.TP
.PD
.BI \-P " max-procs\fR, \fI" \-\-max\-procs "\fR=\fImax-procs"
Run up to
.I max-procs
processes at a time; the default is 1.  If
.I max-procs
is 0,
.B xargs
will run as many processes as
possible at a time.  Use the
.B \-n
option or the
.B \-L
option with
.BR \-P ;
otherwise chances are that only one exec will be done.
While
.B 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 decrease it below 1.
.B xargs
never terminates its commands; when asked to decrease, it merely
waits for more than one existing command to terminate before starting
another.

.B 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 separate resources).
.TP
.PD
.B \-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 response starts
with `y' or `Y'.  Implies
.BR -t .
.TP
.PD
.BR \-\-process\-slot\-var "=\fIname\fR"
Set the environment variable
.I name
to a unique value in each running child process.  Values are reused
once child processes exit.  This can be used in a rudimentary load
distribution scheme, for example.
.TP
.PD
.B \-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.
.TP
.PD
.BI -s " max-chars\fR, \fI" \-\-max\-chars "=\fImax-chars\fR"
Use at most \fImax-chars\fR 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, 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.
.B xargs
automatically adapts to tighter constraints.
.TP
.B "\-\-show\\-limits"
Display the limits on the command-line length which are imposed by the
operating system,
.BR xargs '
choice of buffer size and the
.B \-s
option.  Pipe the input from
.I /dev/null
(and perhaps specify
.BR --no-run-if-empty )
if you don't want
.B xargs
to do anything.
.TP
.B \-t, \-\-verbose
Print the command line on the standard error output before executing
it.
.TP
.PD
.B \-x, \-\-exit
Exit if the size (see the
.B \-s
option) is exceeded.
.TP
.B "\-\-help"
Print a summary of the options to
.B xargs
and exit.
.TP
.B "\-\-version"
Print the version number of
.B xargs
and exit.
.SH "EXAMPLES"
.nf
.B find /tmp \-name core \-type f \-print | xargs /bin/rm \-f

.fi
Find files named
.B core
in or below the directory
.B /tmp
and delete them.  Note that this will work incorrectly if there are
any filenames containing newlines or spaces.
.P
.B find /tmp \-name core \-type f \-print0 | xargs \-0 /bin/rm \-f

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

.P
.B find /tmp \-depth \-name core \-type f \-delete

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

.P
.nf
.B cut \-d: \-f1 < /etc/passwd | sort | xargs echo

.fi
Generates a compact listing of all the users on the system.

.P
.nf
.B xargs sh -c 'emacs \(dq$@\(dq < /dev/tty' emacs

.fi
Launches the minimum number of copies of Emacs needed, one after the
other, to edit the files listed on
.BR xargs '
standard input.  This example achieves the same effect as BSD's
.B -o
option, but in a more flexible and portable way.



.SH "EXIT STATUS"
.B xargs
exits with the following status:
.nf
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.
.fi
.P
Exit codes greater than 128 are used by the shell to indicate that
a program died due to a fatal signal.
.SH "STANDARDS CONFORMANCE"
As of GNU xargs version 4.2.9, the default behaviour of
.B xargs
is not to have a logical end-of-file marker.  POSIX (IEEE Std 1003.1,
2004 Edition) allows this.
.P
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.
.P
The POSIX standard allows implementations to have a limit on the size
of arguments to the
.B exec
functions.  This limit could be as low as 4096 bytes including the size of the
environment.  For scripts to be portable, they must not rely on a
larger value.  However, I know of no implementation whose actual limit
is that small.  The
.B \-\-show\-limits
option can be used to discover the actual limits in force on the
current system.


.SH "SEE ALSO"
\fBfind\fP(1), \fBlocate\fP(1), \fBlocatedb\fP(5), \fBupdatedb\fP(1),
\fBfork\fP(2), \fBexecvp\fP(3),
\fBkill\fP(1), \fBsignal\fP(7),

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

.SH "BUGS"
The
.B \-L
option is incompatible with the
.B \-I
option, but perhaps should not be.
.P
It is not possible for
.B xargs
to be used securely, since there will always be a time gap between the
production of the list of input files and their use in the commands
that
.B 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
.B 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
.B \-execdir
option of
.B find
can often be used as a more secure alternative.

When you use the
.B \-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
.B xargs
will accept when used with the
.B \-I
option.  To work around this
limitation, you can use the
.B \-s
option to increase the amount of
buffer space that
.B xargs
uses, and you can also use an extra invocation of
.B xargs
to ensure that very long lines do not occur.
For example:
.P
.B somecommand | xargs \-s 50000 echo | xargs \-I '{}' \-s 100000 rm '{}'
.P
Here, the first invocation of
.B xargs
has no input line length limit
because it doesn't use the
.B \-i
option.  The second invocation of
.B xargs
does have such a limit, but we have ensured that the it never encounters
a line which is longer than it can handle.   This is not an ideal
solution.  Instead, the
.B \-i
option should not impose a line length
limit, which is why this discussion appears in the BUGS section.
The problem doesn't occur with the output of
.BR find (1)
because it emits just one filename per line.
.P
The best way to report a bug is to use the form at
http://savannah.gnu.org/bugs/?group=findutils.
The reason for this is that you will then be able to track progress in
fixing the problem.   Other comments about \fBxargs\fP(1) and about
the findutils package in general can be sent to the
.I bug\-findutils
mailing list.  To join the list, send email to
.IR bug\-findutils\-request@gnu.org .