'\" et

.TH XARGS "1P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"

.SH 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.

.SH NAME

xargs

\(em construct argument lists and invoke utility

.SH SYNOPSIS

.LP

.nf

xargs \fB[\fR\(miptx\fB] [\fR\(miE \fIeofstr\fB] [\fR\(miI \fIreplstr\fR|\(miL \fInumber\fR|\(min \fInumber\fB]\fR

    \fB[\fR\(mis \fIsize\fB] [\fIutility \fB[\fIargument\fR...\fB]]\fR

.fi

.SH DESCRIPTION

The

.IR xargs

utility shall construct a command line consisting of the

.IR utility

and

.IR 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

.IR 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:

.IP " *" 4

An end-of-file condition is detected on standard input.

.IP " *" 4

An argument consisting of just the logical end-of-file string

(see the

.BR \(miE

.IR 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.

.IP " *" 4

An invocation of a constructed command line returns an exit status of

255.

.P

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 (\c

.BR '\&"' )

characters and non-\c

<newline>

characters can be quoted by enclosing them in double-quotes. A string

of zero or more non-\c

<apostrophe>

(\c

.BR '\e'' )

characters and non-\c

<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

.IR 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

.IR utility

attempts to read from its standard input.

.P

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

.IR xargs

utility shall limit the command line length such that when the command

line is invoked, the combined argument and environment lists (see the

.IR exec

family of functions in the System Interfaces volume of POSIX.1\(hy2008) shall not exceed

{ARG_MAX}\(mi2\|048

bytes. Within this constraint, if neither the

.BR \(min

nor the

.BR \(mis

option is specified, the default command line length shall be at least

{LINE_MAX}.

.SH OPTIONS

The

.IR xargs

utility shall conform to the Base Definitions volume of POSIX.1\(hy2008,

.IR "Section 12.2" ", " "Utility Syntax Guidelines".

.P

The following options shall be supported:

.IP "\fB\(miE\ \fIeofstr\fR" 10

Use

.IR eofstr

as the logical end-of-file string. If

.BR \(miE

is not specified, it is unspecified whether the logical end-of-file

string is the

<underscore>

character (\c

.BR '_' )

or the end-of-file string capability is disabled. When

.IR eofstr

is the null string, the logical end-of-file string capability shall be

disabled and

<underscore>

characters shall be taken literally.

.IP "\fB\(miI\ \fIreplstr\fR" 10

Insert mode:

.IR 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

.IR arguments

in place of each occurrence of

.IR replstr .

At least five arguments in

.IR arguments

can each contain one or more instances of

.IR replstr .

Each of these constructed arguments cannot grow larger than an

implementation-defined limit greater than or equal to 255 bytes. Option

.BR \(mix

shall be forced on.

.IP "\fB\(miL\ \fInumber\fR" 10

The

.IR utility

shall be executed for each non-empty

.IR number

lines of arguments from standard input. The last invocation of

.IR utility

shall be with fewer lines of arguments if fewer than

.IR 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.

.IP "\fB\(min\ \fInumber\fR" 10

Invoke

.IR utility

using as many standard input arguments as possible, up to

.IR number

(a positive decimal integer) arguments maximum. Fewer arguments shall

be used if:

.RS 10 

.IP " *" 4

The command line length accumulated exceeds the size specified by the

.BR \(mis

option (or

{LINE_MAX}

if there is no

.BR \(mis

option).

.IP " *" 4

The last iteration has fewer than

.IR number ,

but not zero, operands remaining.

.RE

.IP "\fB\(mip\fP" 10

Prompt mode: the user is asked whether to execute

.IR utility

at each invocation. Trace mode (\c

.BR \(mit )

is turned on to write the command instance to be executed, followed by

a prompt to standard error. An affirmative response read from

.BR /dev/tty

shall execute the command; otherwise, that particular invocation of

.IR utility

shall be skipped.

.IP "\fB\(mis\ \fIsize\fR" 10

Invoke

.IR utility

using as many standard input arguments as possible yielding a command

line length less than

.IR size

(a positive decimal integer) bytes. Fewer arguments shall be used if:

.RS 10 

.IP " *" 4

The total number of arguments exceeds that specified by the

.BR \(min

option.

.IP " *" 4

The total number of lines exceeds that specified by the

.BR \(miL

option.

.IP " *" 4

End-of-file is encountered on standard input before

.IR size

bytes are accumulated.

.P

Values of

.IR 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;

.IR xargs

shall use the largest value it supports within the constraints.

.RE

.IP "\fB\(mit\fP" 10

Enable trace mode. Each generated command line shall be written to

standard error just prior to invocation.

.IP "\fB\(mix\fP" 10

Terminate if a constructed command line will not fit in the

implied or specified size (see the

.BR \(mis

option above).

.SH OPERANDS

The following operands shall be supported:

.IP "\fIutility\fR" 10

The name of the utility to be invoked, found by search path using the

.IR PATH

environment variable, described in the Base Definitions volume of POSIX.1\(hy2008,

.IR "Chapter 8" ", " "Environment Variables".

If

.IR utility

is omitted, the default shall be the

.IR echo

utility. If the

.IR utility

operand names any of the special built-in utilities in

.IR "Section 2.14" ", " "Special Built-In Utilities",

the results are undefined.

.IP "\fIargument\fR" 10

An initial option or operand for the invocation of

.IR utility .

.SH 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>.

.SH "INPUT FILES"

The file

.BR /dev/tty

shall be used to read responses required by the

.BR \(mip

option.

.SH "ENVIRONMENT VARIABLES"

The following environment variables shall affect the execution of

.IR xargs :

.IP "\fILANG\fP" 10

Provide a default value for the internationalization variables that are

unset or null. (See the Base Definitions volume of POSIX.1\(hy2008,

.IR "Section 8.2" ", " "Internationalization Variables"

for the precedence of internationalization variables used to determine

the values of locale categories.)

.IP "\fILC_ALL\fP" 10

If set to a non-empty string value, override the values of all the

other internationalization variables.

.IP "\fILC_COLLATE\fP" 10

.br

Determine the locale for the behavior of ranges, equivalence classes,

and multi-character collating elements used in the extended regular

expression defined for the

.BR yesexpr

locale keyword in the

.IR LC_MESSAGES

category.

.IP "\fILC_CTYPE\fP" 10

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

.BR yesexpr

locale keyword in the

.IR LC_MESSAGES

category.

.IP "\fILC_MESSAGES\fP" 10

.br

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.

.IP "\fINLSPATH\fP" 10

Determine the location of message catalogs for the processing of

.IR LC_MESSAGES .

.IP "\fIPATH\fP" 10

Determine the location of

.IR utility ,

as described in the Base Definitions volume of POSIX.1\(hy2008,

.IR "Chapter 8" ", " "Environment Variables".

.SH "ASYNCHRONOUS EVENTS"

Default.

.SH STDOUT

Not used.

.SH STDERR

The standard error shall be used for diagnostic messages and the

.BR \(mit

and

.BR \(mip

options. If the

.BR \(mit

option is specified, the

.IR utility

and its constructed argument list shall be written to standard error,

as it will be invoked, prior to invocation. If

.BR \(mip

is specified, a prompt of the following format shall be written (in the

POSIX locale):

.sp

.RS 4

.nf

\fB

"?..."

.fi \fR

.P

.RE

.P

at the end of the line of the output from

.BR \(mit .

.SH "OUTPUT FILES"

None.

.SH "EXTENDED DESCRIPTION"

None.

.SH "EXIT STATUS"

The following exit values shall be returned:

.IP "\0\0\0\00" 8

All invocations of

.IR utility

returned exit status zero.

.IP "1\(hy125" 8

A command line meeting the specified requirements could not be

assembled, one or more of the invocations of

.IR utility

returned a non-zero exit status, or some other error occurred.

.IP "\0\0126" 8

The utility specified by

.IR utility

was found but could not be invoked.

.IP "\0\0127" 8

The utility specified by

.IR utility

could not be found.

.SH "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

.IR xargs

utility shall write a diagnostic message and exit without processing

any remaining input.

.LP

.IR "The following sections are informative."

.SH "APPLICATION USAGE"

The 255 exit status allows a utility being used by

.IR xargs

to tell

.IR xargs

to terminate if it knows no further invocations using the current data

stream will succeed. Thus,

.IR utility

should explicitly

.IR exit

with an appropriate value to avoid accidentally returning with 255.

.P

Note that since input is parsed as lines,

<blank>

characters separate arguments, and

<backslash>,

<apostrophe>,

and double-quote characters are used for quoting, if

.IR xargs

is used to bundle the output of commands like

.IR find

.IR dir

.BR \(miprint

or

.IR 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

.IR xargs ,

but in most cases it is preferable just to have

.IR find

do the argument aggregation itself by using

.BR \(miexec

with a

.BR '+' 

terminator instead of

.BR ';' .

Note that the quoting rules used by

.IR 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

.IR xargs

interprets correctly is to precede each non-\c

<newline>

character with a

<backslash>.

More efficient alternatives are shown in Example 2 and Example 5 below.

.P

On implementations with a large value for

{ARG_MAX},

.IR xargs

may produce command lines longer than

{LINE_MAX}.

For invocation of utilities, this is not a problem. If

.IR xargs

is being used to create a text file, users should explicitly set the

maximum command line length with the

.BR \(mis

option.

.P

The

.IR command ,

.IR env ,

.IR nice ,

.IR nohup ,

.IR time ,

and

.IR 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

.IR exec

the utility fail with

.BR [ENOENT] ,

and uses 126 when any attempt to

.IR exec

the utility fails for any other reason.

.SH EXAMPLES

.IP " 1." 4

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

.BR \(dq$0 $*\(dq 

does not include any

<apostrophe>

or

<newline>

characters.

.RS 4 

.sp

.RS 4

.nf

\fB

(logname; date; printf "'%s'\en$0 $*") | xargs \(miE "" >>log

.fi \fR

.P

.RE

.RE

.IP " 2." 4

The following command invokes

.IR 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.

.RS 4 

.sp

.RS 4

.nf

\fB

printf "%s\en$@" | sed 's/[^[:alnum:]]/\e\e&/g' |

    xargs \(miE "" \(min 2 \(mix diff

.fi \fR

.P

.RE

.RE

.IP " 3." 4

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

.BR arch ;

.IR a ,

one at a time or

.IR b ,

many at a time. The commands assume that no filenames contain

<blank>,

<newline>,

<backslash>,

<apostrophe>,

or double-quote characters.

.RS 4 

.sp

.RS 4

.nf

\fB

a. ls | xargs \(miE "" \(mip \(miL 1 ar \(mir arch

.P

b. ls | xargs \(miE "" \(mip \(miL 1 | xargs \(miE "" ar \(mir arch

.fi \fR

.P

.RE

.RE

.IP " 4." 4

The following command invokes

.IR command1

one or more times with multiple arguments, stopping if an invocation of

.IR command1

has a non-zero exit status.

.RS 4 

.sp

.RS 4

.nf

\fB

xargs \(miE "" sh \(mic 'command1 "$@" || exit 255' sh < xargs_input

.fi \fR

.P

.RE

.RE

.IP " 5." 4

On XSI-conformant systems, the following command moves all files

from directory

.BR $1

to directory

.BR $2 ,

and echoes each move command just before doing it. It assumes no

filenames contain

<newline>

characters and that neither

.BR $1

nor

.BR $2

contains the sequence

.BR \(dq{}\(dq .

.RS 4 

.sp

.RS 4

.nf

\fB

ls \(miA "$1" | sed \(mie 's/"/"\e\e""/g' \(mie 's/.*/"&"/' |

    xargs \(miE "" \(miI {} \(mit mv "$1"/{} "$2"/{}

.fi \fR

.P

.RE

.RE

.SH RATIONALE

The

.IR xargs

utility was usually found only in System V-based systems; BSD systems

included an

.IR apply

utility that provided functionality similar to

.IR xargs

.BR \(min

.IR number .

The SVID lists

.IR xargs

as a software development extension. This volume of POSIX.1\(hy2008 does not share the view that

it is used only for development, and therefore it is not optional.

.P

The classic application of the

.IR xargs

utility is in conjunction with the

.IR find

utility to reduce the number of processes launched by a simplistic use

of the

.IR find

.BR \(miexec

combination. The

.IR 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\(hy2008 selected only the

minimal features required.

.P

Although the 255 exit status is mostly an accident of historical

implementations, it allows a utility being used by

.IR xargs

to tell

.IR 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\(hy125 range when

.IR xargs

exits. There is no statement of how the various non-zero utility exit

status codes are accumulated by

.IR 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.

.P

Several other

.IR xargs

options were removed because simple alternatives already exist within

\&this volume of POSIX.1\(hy2008. For example, the

.BR \(mii

.IR replstr

option can be just as efficiently performed using a shell

.BR for

loop. Since

.IR xargs

calls an

.IR exec

function with each input line, the

.BR \(mii

option does not usually exploit the grouping capabilities of

.IR xargs .

.P

The requirement that

.IR xargs

never produces command lines such that invocation of

.IR utility

is within 2\|048 bytes of hitting the POSIX

.IR 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\(hy2008 is 4\|096 bytes and the minimum

value allowed by this volume of POSIX.1\(hy2008 is 2\|048 bytes; therefore,

the 2\|048 bytes difference seems reasonable. Note, however, that

.IR xargs

may never be able to invoke a utility if the environment passed in to

.IR xargs

comes close to using

{ARG_MAX}

bytes.

.P

The version of

.IR xargs

required by this volume of POSIX.1\(hy2008 is required to wait for the completion of the invoked

command before invoking another command. This was done because

historical scripts using

.IR 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

.IR xargs

terminates normally.

.P

The

.BR \(mie

option was omitted from the ISO\ POSIX\(hy2:\|1993 standard in the belief that the

.IR 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

.BR \(mie

option was specified. In that case, a simple

.IR sed

script could be used to duplicate the

.BR \(mie

functionality. Further investigation revealed that:

.IP " *" 4

The logical end-of-file string was checked for after quote and escape

processing, making a

.IR sed

script that provided equivalent functionality much more difficult to

write.

.IP " *" 4

The default was to perform logical end-of-file processing with an

<underscore>

as the logical end-of-file string.

.P

To correct this misunderstanding, the

.BR \(miE

.IR eofstr

option was adopted from the X/Open Portability Guide. Users should

note that the description of the

.BR \(miE

option matches historical documentation of the

.BR \(mie

option (which was not adopted because it did not support the Utility

Syntax Guidelines), by

saying that if

.IR eofstr

is the null string, logical end-of-file processing is disabled.

Historical implementations of

.IR 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

.IR string

argument could be generated using single or double-quotes (\c

.BR '\^' 

or

.BR \(dq\^\(dq ).

Since this behavior was not documented historically, it is considered

to be a bug.

.P

The

.BR \(miI ,

.BR \(miL ,

and

.BR \(min

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.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

.IR "Chapter 2" ", " "Shell Command Language",

.IR "\fIdiff\fR\^",

.IR "\fIecho\fR\^",

.IR "\fIfind\fR\^"

.P

The Base Definitions volume of POSIX.1\(hy2008,

.IR "Chapter 8" ", " "Environment Variables",

.IR "Section 12.2" ", " "Utility Syntax Guidelines"

.P

The System Interfaces volume of POSIX.1\(hy2008,

.IR "\fIexec\fR\^"

.SH 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

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 .