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XARGS(1P) POSIX Programmer's Manual XARGS(1P)
PROLOG
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.
NAME
xargs -- construct argument lists and invoke utility
SYNOPSIS
xargs [-ptx] [-E eofstr] [-I replstr|-L number|-n number]
[-s size] [utility [argument...]]
DESCRIPTION
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.1-2017) 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}.
OPTIONS
The xargs utility shall conform to the Base Definitions volume of
POSIX.1-2017, 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 an unescaped <blank>; a
trailing unescaped <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).
OPERANDS
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.1-2017, 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.
STDIN
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>.
INPUT FILES
The file /dev/tty shall be used to read responses required by the
-p option.
ENVIRONMENT VARIABLES
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.1-2017, 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.1-2017, Chapter 8,
Environment Variables.
ASYNCHRONOUS EVENTS
Default.
STDOUT
Not used.
STDERR
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.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
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.
CONSEQUENCES OF ERRORS
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.
APPLICATION USAGE
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.
EXAMPLES
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"/{}
RATIONALE
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.1-2017
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.1-2017 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.1-2017.
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.1-2017 is 4096 bytes and the minimum
value allowed by this volume of POSIX.1-2017 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.1-2017 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.
FUTURE DIRECTIONS
None.
SEE ALSO
Chapter 2, Shell Command Language, diff, echo, find
The Base Definitions volume of POSIX.1-2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1-2017, exec
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/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 2017 XARGS(1P)