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xargs.1p (21536B)

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.TH XARGS "1P" 2017 "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-ptx\fB] [\fR-E \fIeofstr\fB] [\fR-I \fIreplstr\fR|-L \fInumber\fR|-n \fInumber\fB]\fR
    \fB[\fR-s \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 \-E
.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\(aq' )
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\(hy2017) shall not exceed
{ARG_MAX}\-2\|048
bytes. Within this constraint, if neither the
.BR \-n
nor the
.BR \-s
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\(hy2017,
.IR "Section 12.2" ", " "Utility Syntax Guidelines".
.P
The following options shall be supported:
.IP "\fB\-E\ \fIeofstr\fR" 10
Use
.IR eofstr
as the logical end-of-file string. If
.BR \-E
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\-I\ \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 \-x
shall be forced on.
.IP "\fB\-L\ \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 an unescaped
<blank>;
a trailing unescaped
<blank>
signals continuation to the next non-empty line, inclusive.
.IP "\fB\-n\ \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 \-s
option (or
{LINE_MAX}
if there is no
.BR \-s
option).
.IP " *" 4
The last iteration has fewer than
.IR number ,
but not zero, operands remaining.
.RE
.IP "\fB\-p\fP" 10
Prompt mode: the user is asked whether to execute
.IR utility
at each invocation. Trace mode (\c
.BR \-t )
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\-s\ \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 \-n
option.
.IP " *" 4
The total number of lines exceeds that specified by the
.BR \-L
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\-t\fP" 10
Enable trace mode. Each generated command line shall be written to
standard error just prior to invocation.
.IP "\fB\-x\fP" 10
Terminate if a constructed command line will not fit in the
implied or specified size (see the
.BR \-s
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\(hy2017,
.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 \-p
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\(hy2017,
.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\(hy2017,
.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 \-t
and
.BR \-p
options. If the
.BR \-t
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 \-p
is specified, a prompt of the following format shall be written (in the
POSIX locale):
.sp
.RS 4
.nf
"?..."
.fi
.P
.RE
.P
at the end of the line of the output from
.BR \-t .
.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 \-print
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 \-exec
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 \-s
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
(logname; date; printf "\(aq%s\(aq\en$0 $*") | xargs -E "" >>log
.fi
.P
.RE
.RE
.IP " 2." 4
The following command invokes
.IR diff
with successive pairs of arguments originally typed as command line
arguments. It assumes there are no embedded
<newline>
characters in the elements of the original argument list.
.RS 4 
.sp
.RS 4
.nf
printf "%s\en$@" | sed \(aqs/[\(ha[:alnum:]]/\e\e&/g\(aq |
    xargs -E "" -n 2 -x diff
.fi
.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
a. ls | xargs -E "" -p -L 1 ar -r arch
.P
b. ls | xargs -E "" -p -L 1 | xargs -E "" ar -r arch
.fi
.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
xargs -E "" sh -c \(aqcommand1 "$@" || exit 255\(aq sh < xargs_input
.fi
.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
ls -A "$1" | sed -e \(aqs/"/"\e\e""/g\(aq -e \(aqs/.*/"&"/\(aq |
    xargs -E "" -I {} -t mv "$1"/{} "$2"/{}
.fi
.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 \-n
.IR number .
The SVID lists
.IR xargs
as a software development extension. This volume of POSIX.1\(hy2017 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 \-exec
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\(hy2017 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\(hy2017. For example, the
.BR \-i
.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 \-i
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\(hy2017 is 4\|096 bytes and the minimum
value allowed by this volume of POSIX.1\(hy2017 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\(hy2017 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 \-e
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 \-e
option was specified. In that case, a simple
.IR sed
script could be used to duplicate the
.BR \-e
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 \-E
.IR eofstr
option was adopted from the X/Open Portability Guide. Users should
note that the description of the
.BR \-E
option matches historical documentation of the
.BR \-e
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 \-I ,
.BR \-L ,
and
.BR \-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.
.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\(hy2017,
.IR "Chapter 8" ", " "Environment Variables",
.IR "Section 12.2" ", " "Utility Syntax Guidelines"
.P
The System Interfaces volume of POSIX.1\(hy2017,
.IR "\fIexec\fR\^"
.\"
.SH 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 .
.PP
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 .