XARGS(1P) POSIX Programmer's Manual XARGS(1P)
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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 con-
straints 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 pro-
cessing (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 enclos-
ing them in double-quotes. A string of zero or more non-<apostrophe>
('\'') characters and non-<newline> characters can be quoted by enclos-
ing them in <apostrophe> characters. Any unquoted character can be
escaped by preceding it with a <backslash>. The utility named by util-
ity 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 fam-
ily 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}.
OPTIONS
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 liter-
ally.
-I replstr
Insert mode: utility is executed for each logical line from
standard input. Arguments in the standard input shall be sep-
arated 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 argu-
ment shall be inserted in arguments in place of each occur-
rence 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 implementa-
tion-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 pos-
sible, 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 invoca-
tion of utility shall be skipped.
-s size Invoke utility using as many standard input arguments as pos-
sible yielding a command line length less than size (a posi-
tive 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 sup-
ported, 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 writ-
ten 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.1-2008, 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 Utili-
ties, 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 vari-
ables that are unset or null. (See the Base Definitions vol-
ume of POSIX.1-2008, Section 8.2, Internationalization Vari-
ables 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.
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 con-
structed 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 assem-
bled, 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 exist-
ing 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 pre-
cede 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 val-
ues 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 differ-
entiating 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 com-
mands (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 $*") | xargs -E "" >>log
2. The following command invokes diff with successive pairs of argu-
ments originally typed as command line arguments. It assumes there
are no embedded <newline> characters in the elements of the origi-
nal argument list.
printf "%s\n$@" | 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>, <new-
line>, <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 mul-
tiple 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.1-2008 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 mini-
mal features required.
Although the 255 exit status is mostly an accident of historical imple-
mentations, 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 utili-
ties 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 invo-
cation 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 per-
formed, 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 Guide-
lines), by saying that if eofstr is the null string, logical end-of-
file processing is disabled. Historical implementations of xargs actu-
ally 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.1-2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1-2008, exec
COPYRIGHT
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 Electri-
cal 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.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 XARGS(1P)
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