XARGS(P) POSIX Programmer's Manual XARGS(P)NAMExargs - construct argument lists and invoke utility
SYNOPSISxargs [-t][-p]][-E eofstr][-I replstr][-L number][-n number [-x]]
[-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.
* The logical end-of-file string (see the -E eofstr option) is found
on standard input after double-quote processing, apostrophe process‐
ing, and backslash escape processing (see next paragraph).
* 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>s, unescaped <blank>s, or <newline>s. A
string of zero or more non-double-quote ( ' )' characters and non-
<newline>s can be quoted by enclosing them in double-quotes. A string
of zero or more non-apostrophe ( '" ) characters and non- <newline>s
can be quoted by enclosing them in apostrophes. 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 fam‐
ily of functions in the System Interfaces volume of
IEEE Std 1003.1-2001) 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
IEEE Std 1003.1-2001, 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 spec‐
ified, 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 log‐
ical end-of-file string capability shall be disabled and under‐
score characters shall be taken literally.
-I replstr
Insert mode: utility is executed for each line from standard
input, taking the entire line as a single argument, inserting it
in arguments for each occurrence of replstr. A maximum of five
arguments in arguments can each contain one or more instances of
replstr. Any <blank>s at the beginning of each line shall be
ignored. Constructed arguments cannot grow larger than 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, inclu‐
sive. The -L and -n options are mutually-exclusive; the last one
specified shall take effect.
-n number
Invoke utility using as many standard input arguments as possi‐
ble, up to number (a positive decimal integer) arguments maxi‐
mum. Fewer arguments shall be used if:
* The command line length accumulated exceeds the size speci‐
fied by the -s option (or {LINE_MAX} if there is no -s
option).
* The last iteration has fewer than number, but not zero, oper‐
ands remaining.
-p Prompt mode: the user is asked whether to execute utility at
each invocation. Trace mode ( -t) is turned on to write the com‐
mand 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 possi‐
ble 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, pro‐
vided 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 command line containing number arguments (see the
-n option above) or number lines (see the -L option above)
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 Defi‐
nitions volume of IEEE Std 1003.1-2001, 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 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
IEEE Std 1003.1-2001, 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 key‐
word 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 regu‐
lar expression defined for the yesexpr locale keyword in the
LC_MESSAGES category.
LC_MESSAGES
Determine the locale for the processing of affirmative responses
and that should be used to affect the format and contents of
diagnostic messages 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 Def‐
initions volume of IEEE Std 1003.1-2001, 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 input is parsed as lines; <blank>s separate arguments. If
xargs is used to bundle output of commands like find dir -print or ls
into commands to be executed, unexpected results are likely if any
filenames contain any <blank>s or <newline>s. This can be fixed by
using find to call a script that converts each file found into a quoted
string that is then piped to xargs. 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 and the
shell syntax is not fully compatible with it. An easy rule that can be
used to transform any string into a quoted form that xargs interprets
correctly is to precede each character in the string with a backslash.
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 con‐
fused 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 parenthesised com‐
mands onto one line, which is then written to the end-of-file log:
(logname; date; printf "%s\n" "$0 $*") | xargs >>log
2. The following command invokes diff with successive pairs of argu‐
ments originally typed as command line arguments (assuming there
are no embedded <blank>s in the elements of the original argument
list):
printf "%s\n" "$*" | xargs-n 2 -x diff
3. In the following commands, the user is asked which files in the
current directory are to be archived. The files are archived into
arch; a, one at a time, or b, many at a time.
a. ls | xargs-p -L 1 ar -r arch
b. ls | xargs-p -L 1 | xargs ar -r arch
4. The following executes with successive pairs of arguments origi‐
nally typed as command line arguments:
echo $* | xargs-n 2 diff
5. On XSI-conformant systems, the following moves all files from
directory $1 to directory $2, and echoes each move command just
before doing it:
ls $1 | xargs-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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 selected only
the minimal 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 withdrawn because simple alternatives
already exist within this volume of IEEE Std 1003.1-2001. 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
IEEE Std 1003.1-2001 is 4096 bytes and the minimum value allowed by
this volume of IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 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.
FUTURE DIRECTIONS
None.
SEE ALSO
Shell Command Language , echo , find , the System Interfaces volume of
IEEE Std 1003.1-2001, exec
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 XARGS(P)
