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XARGS(1P) POSIX Programmer's Manual XARGS(1P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
xargs — construct argument lists and invoke utility
xargs [−ptx] [−E eofstr] [−I replstr|−L number|−n number]
[−s size] [utility [argument...]]
The xargs utility shall construct a command line consisting of the
utility and argument operands specified followed by as many arguments
read in sequence from standard input as fit in length and number
constraints specified by the options. The xargs utility shall then
invoke the constructed command line and wait for its completion. This
sequence shall be repeated until one of the following occurs:
* An end-of-file condition is detected on standard input.
* An argument consisting of just the logical end-of-file string
(see the −E eofstr option) is found on standard input after
double-quote processing, <apostrophe> processing, and
<backslash>-escape processing (see next paragraph). All arguments
up to but not including the argument consisting of just the
logical end-of-file string shall be used as arguments in
constructed command lines.
* An invocation of a constructed command line returns an exit
status of 255.
The application shall ensure that arguments in the standard input are
separated by unquoted <blank> characters, unescaped <blank>
characters, or <newline> characters. A string of zero or more non-
double-quote ('"') characters and non-<newline> characters can be
quoted by enclosing them in double-quotes. A string of zero or more
non-<apostrophe> ('\'') characters and non-<newline> characters can
be quoted by enclosing them in <apostrophe> characters. Any unquoted
character can be escaped by preceding it with a <backslash>. The
utility named by utility shall be executed one or more times until
the end-of-file is reached or the logical end-of file string is
found. The results are unspecified if the utility named by utility
attempts to read from its standard input.
The generated command line length shall be the sum of the size in
bytes of the utility name and each argument treated as strings,
including a null byte terminator for each of these strings. The xargs
utility shall limit the command line length such that when the
command line is invoked, the combined argument and environment lists
(see the exec family of functions in the System Interfaces volume of
POSIX.1‐2008) shall not exceed {ARG_MAX}−2048 bytes. Within this
constraint, if neither the −n nor the −s option is specified, the
default command line length shall be at least {LINE_MAX}.
The xargs utility shall conform to the Base Definitions volume of
POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
−E eofstr Use eofstr as the logical end-of-file string. If −E is not
specified, it is unspecified whether the logical end-of-
file string is the <underscore> character ('_') or the end-
of-file string capability is disabled. When eofstr is the
null string, the logical end-of-file string capability
shall be disabled and <underscore> characters shall be
taken literally.
−I replstr
Insert mode: utility is executed for each logical line from
standard input. Arguments in the standard input shall be
separated only by unescaped <newline> characters, not by
<blank> characters. Any unquoted unescaped <blank>
characters at the beginning of each line shall be ignored.
The resulting argument shall be inserted in arguments in
place of each occurrence of replstr. At least five
arguments in arguments can each contain one or more
instances of replstr. Each of these constructed arguments
cannot grow larger than an implementation-defined limit
greater than or equal to 255 bytes. Option −x shall be
forced on.
−L number The utility shall be executed for each non-empty number
lines of arguments from standard input. The last invocation
of utility shall be with fewer lines of arguments if fewer
than number remain. A line is considered to end with the
first <newline> unless the last character of the line is a
<blank>; a trailing <blank> signals continuation to the
next non-empty line, inclusive.
−n number Invoke utility using as many standard input arguments as
possible, up to number (a positive decimal integer)
arguments maximum. Fewer arguments shall be used if:
* The command line length accumulated exceeds the size
specified by the −s option (or {LINE_MAX} if there is
no −s option).
* The last iteration has fewer than number, but not zero,
operands remaining.
−p Prompt mode: the user is asked whether to execute utility
at each invocation. Trace mode (−t) is turned on to write
the command instance to be executed, followed by a prompt
to standard error. An affirmative response read from
/dev/tty shall execute the command; otherwise, that
particular invocation of utility shall be skipped.
−s size Invoke utility using as many standard input arguments as
possible yielding a command line length less than size (a
positive decimal integer) bytes. Fewer arguments shall be
used if:
* The total number of arguments exceeds that specified by
the −n option.
* The total number of lines exceeds that specified by the
−L option.
* End-of-file is encountered on standard input before
size bytes are accumulated.
Values of size up to at least {LINE_MAX} bytes shall be
supported, provided that the constraints specified in the
DESCRIPTION are met. It shall not be considered an error if
a value larger than that supported by the implementation or
exceeding the constraints specified in the DESCRIPTION is
given; xargs shall use the largest value it supports within
the constraints.
−t Enable trace mode. Each generated command line shall be
written to standard error just prior to invocation.
−x Terminate if a constructed command line will not fit in the
implied or specified size (see the −s option above).
The following operands shall be supported:
utility The name of the utility to be invoked, found by search path
using the PATH environment variable, described in the Base
Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables. If utility is omitted, the default shall be the
echo utility. If the utility operand names any of the
special built-in utilities in Section 2.14, Special Built-
In Utilities, the results are undefined.
argument An initial option or operand for the invocation of utility.
The standard input shall be a text file. The results are unspecified
if an end-of-file condition is detected immediately following an
escaped <newline>.
The file /dev/tty shall be used to read responses required by the −p
option.
The following environment variables shall affect the execution of
xargs:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions
volume of POSIX.1‐2008, Section 8.2, Internationalization
Variables for the precedence of internationalization
variables used to determine the values of locale
categories.)
LC_ALL If set to a non-empty string value, override the values of
all the other internationalization variables.
LC_COLLATE
Determine the locale for the behavior of ranges,
equivalence classes, and multi-character collating elements
used in the extended regular expression defined for the
yesexpr locale keyword in the LC_MESSAGES category.
LC_CTYPE Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte
as opposed to multi-byte characters in arguments and input
files) and the behavior of character classes used in the
extended regular expression defined for the yesexpr locale
keyword in the LC_MESSAGES category.
LC_MESSAGES
Determine the locale used to process affirmative responses,
and the locale used to affect the format and contents of
diagnostic messages and prompts written to standard error.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
PATH Determine the location of utility, as described in the Base
Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables.
Default.
Not used.
The standard error shall be used for diagnostic messages and the −t
and −p options. If the −t option is specified, the utility and its
constructed argument list shall be written to standard error, as it
will be invoked, prior to invocation. If −p is specified, a prompt of
the following format shall be written (in the POSIX locale):
"?..."
at the end of the line of the output from −t.
None.
None.
The following exit values shall be returned:
0 All invocations of utility returned exit status zero.
1‐125 A command line meeting the specified requirements could not
be assembled, one or more of the invocations of utility
returned a non-zero exit status, or some other error
occurred.
126 The utility specified by utility was found but could not be
invoked.
127 The utility specified by utility could not be found.
If a command line meeting the specified requirements cannot be
assembled, the utility cannot be invoked, an invocation of the
utility is terminated by a signal, or an invocation of the utility
exits with exit status 255, the xargs utility shall write a
diagnostic message and exit without processing any remaining input.
The following sections are informative.
The 255 exit status allows a utility being used by xargs to tell
xargs to terminate if it knows no further invocations using the
current data stream will succeed. Thus, utility should explicitly
exit with an appropriate value to avoid accidentally returning with
255.
Note that since input is parsed as lines, <blank> characters separate
arguments, and <backslash>, <apostrophe>, and double-quote characters
are used for quoting, if xargs is used to bundle the output of
commands like find dir −print or ls into commands to be executed,
unexpected results are likely if any filenames contain <blank>,
<newline>, or quoting characters. This can be solved by using find to
call a script that converts each file found into a quoted string that
is then piped to xargs, but in most cases it is preferable just to
have find do the argument aggregation itself by using −exec with a
'+' terminator instead of ';'. Note that the quoting rules used by
xargs are not the same as in the shell. They were not made consistent
here because existing applications depend on the current rules. An
easy (but inefficient) method that can be used to transform input
consisting of one argument per line into a quoted form that xargs
interprets correctly is to precede each non-<newline> character with
a <backslash>. More efficient alternatives are shown in Example 2
and Example 5 below.
On implementations with a large value for {ARG_MAX}, xargs may
produce command lines longer than {LINE_MAX}. For invocation of
utilities, this is not a problem. If xargs is being used to create a
text file, users should explicitly set the maximum command line
length with the −s option.
The command, env, nice, nohup, time, and xargs utilities have been
specified to use exit code 127 if an error occurs so that
applications can distinguish ``failure to find a utility'' from
``invoked utility exited with an error indication''. The value 127
was chosen because it is not commonly used for other meanings; most
utilities use small values for ``normal error conditions'' and the
values above 128 can be confused with termination due to receipt of a
signal. The value 126 was chosen in a similar manner to indicate that
the utility could be found, but not invoked. Some scripts produce
meaningful error messages differentiating the 126 and 127 cases. The
distinction between exit codes 126 and 127 is based on KornShell
practice that uses 127 when all attempts to exec the utility fail
with [ENOENT], and uses 126 when any attempt to exec the utility
fails for any other reason.
1. The following command combines the output of the parenthesized
commands (minus the <apostrophe> characters) onto one line, which
is then appended to the file log. It assumes that the expansion
of "$0$*" does not include any <apostrophe> or <newline>
characters.
(logname; date; printf "'%s'\n$0 $*") | xargs −E "" >>log
2. The following command invokes diff with successive pairs of
arguments originally typed as command line arguments. It assumes
there are no embedded <newline> characters in the elements of the
original argument list.
printf "%s\n$@" | sed 's/[^[:alnum:]]/\\&/g' |
xargs −E "" −n 2 −x diff
3. In the following commands, the user is asked which files in the
current directory (excluding dotfiles) are to be archived. The
files are archived into arch; a, one at a time or b, many at a
time. The commands assume that no filenames contain <blank>,
<newline>, <backslash>, <apostrophe>, or double-quote characters.
a. ls | xargs −E "" −p −L 1 ar −r arch
b. ls | xargs −E "" −p −L 1 | xargs −E "" ar −r arch
4. The following command invokes command1 one or more times with
multiple arguments, stopping if an invocation of command1 has a
non-zero exit status.
xargs −E "" sh −c 'command1 "$@" || exit 255' sh < xargs_input
5. On XSI-conformant systems, the following command moves all files
from directory $1 to directory $2, and echoes each move command
just before doing it. It assumes no filenames contain <newline>
characters and that neither $1 nor $2 contains the sequence "{}".
ls −A "$1" | sed −e 's/"/"\\""/g' −e 's/.*/"&"/' |
xargs −E "" −I {} −t mv "$1"/{} "$2"/{}
The xargs utility was usually found only in System V-based systems;
BSD systems included an apply utility that provided functionality
similar to xargs −n number. The SVID lists xargs as a software
development extension. This volume of POSIX.1‐2008 does not share the
view that it is used only for development, and therefore it is not
optional.
The classic application of the xargs utility is in conjunction with
the find utility to reduce the number of processes launched by a
simplistic use of the find −exec combination. The xargs utility is
also used to enforce an upper limit on memory required to launch a
process. With this basis in mind, this volume of POSIX.1‐2008
selected only the minimal features required.
Although the 255 exit status is mostly an accident of historical
implementations, it allows a utility being used by xargs to tell
xargs to terminate if it knows no further invocations using the
current data stream shall succeed. Any non-zero exit status from a
utility falls into the 1‐125 range when xargs exits. There is no
statement of how the various non-zero utility exit status codes are
accumulated by xargs. The value could be the addition of all codes,
their highest value, the last one received, or a single value such as
1. Since no algorithm is arguably better than the others, and since
many of the standard utilities say little more (portably) than
``pass/fail'', no new algorithm was invented.
Several other xargs options were removed because simple alternatives
already exist within this volume of POSIX.1‐2008. For example, the −i
replstr option can be just as efficiently performed using a shell for
loop. Since xargs calls an exec function with each input line, the −i
option does not usually exploit the grouping capabilities of xargs.
The requirement that xargs never produces command lines such that
invocation of utility is within 2048 bytes of hitting the POSIX exec
{ARG_MAX} limitations is intended to guarantee that the invoked
utility has room to modify its environment variables and command line
arguments and still be able to invoke another utility. Note that the
minimum {ARG_MAX} allowed by the System Interfaces volume of
POSIX.1‐2008 is 4096 bytes and the minimum value allowed by this
volume of POSIX.1‐2008 is 2048 bytes; therefore, the 2048 bytes
difference seems reasonable. Note, however, that xargs may never be
able to invoke a utility if the environment passed in to xargs comes
close to using {ARG_MAX} bytes.
The version of xargs required by this volume of POSIX.1‐2008 is
required to wait for the completion of the invoked command before
invoking another command. This was done because historical scripts
using xargs assumed sequential execution. Implementations wanting to
provide parallel operation of the invoked utilities are encouraged to
add an option enabling parallel invocation, but should still wait for
termination of all of the children before xargs terminates normally.
The −e option was omitted from the ISO POSIX‐2:1993 standard in the
belief that the eofstr option-argument was recognized only when it
was on a line by itself and before quote and escape processing were
performed, and that the logical end-of-file processing was only
enabled if a −e option was specified. In that case, a simple sed
script could be used to duplicate the −e functionality. Further
investigation revealed that:
* The logical end-of-file string was checked for after quote and
escape processing, making a sed script that provided equivalent
functionality much more difficult to write.
* The default was to perform logical end-of-file processing with an
<underscore> as the logical end-of-file string.
To correct this misunderstanding, the −E eofstr option was adopted
from the X/Open Portability Guide. Users should note that the
description of the −E option matches historical documentation of the
−e option (which was not adopted because it did not support the
Utility Syntax Guidelines), by saying that if eofstr is the null
string, logical end-of-file processing is disabled. Historical
implementations of xargs actually did not disable logical end-of-file
processing; they treated a null argument found in the input as a
logical end-of-file string. (A null string argument could be
generated using single or double-quotes ('' or ""). Since this
behavior was not documented historically, it is considered to be a
bug.
The −I, −L, and −n options are mutually-exclusive. Some
implementations use the last one specified if more than one is given
on a command line; other implementations treat combinations of the
options in different ways.
None.
Chapter 2, Shell Command Language, diff(1p), echo(1p), find(1p)
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2008, exec(1p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 XARGS(1P)
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