Tribblix: manual page: xargs.1

XARGS(1) User Commands XARGS(1)

NAME

xargs - construct argument lists and invoke utility

SYNOPSIS

xargs [-t] [-0] [-p] [-e[eofstr]] [-E eofstr]
[-I replstr] [-i[replstr]] [-L number] [-l[number]]
[-n number [-x]] [-P maxprocs] [-s size]
[utility [argument...]]

DESCRIPTION

The xargs utility constructs 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 then invokes the
constructed command line and waits for its completion. This sequence
is repeated until an end-of-file condition is detected on standard
input or an invocation of a constructed command line returns an exit
status of 255.

Arguments in the standard input must be separated by unquoted blank
characters, or unescaped blank characters or newline characters. A
string of zero or more non-double-quote (") and non-newline
characters can be quoted by enclosing them in double-quotes. A string
of zero or more non-apostrophe (') and non-newline characters can be
quoted by enclosing them in apostrophes. Any unquoted character can
be escaped by preceding it with a backslash (\). The utility are
executed one or more times until the end-of-file is reached. The
results are unspecified if the utility named by utility attempts to
read from its standard input.

The generated command line length is 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
limits the command line length such that when the command line is
invoked, the combined argument and environment lists can not exceed
{ARG_MAX}-2048 bytes. Within this constraint, if neither the -n nor
the -s option is specified, the default command line length is at
least {LINE_MAX}.

OPTIONS

The following options are supported:

-e[eofstr]
Uses eofstr as the logical end-of-file string.
Underscore (_) is assumed for the logical EOF string
if neither -e nor -E is used. When the eofstr option-
argument is omitted, the logical EOF string capability
is disabled and underscores are taken literally. The
xargs utility reads standard input until either end-
of-file or the logical EOF string is encountered.

-E eofstr
Specifies a logical end-of-file string to replace the
default underscore. xargs reads standard input until
either end-of-file or the logical EOF string is
encountered. When eofstr is a null string, the logical
end-of-file string capability is disabled and
underscore characters are 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 argument s for each
occurrence of replstr. A maximum of five arguments in
arguments can each contain one or more instances of
replstr. Any blank characters at the beginning of each
line are ignored. Constructed arguments cannot grow
larger than 255 bytes. Option -x is forced on. The -I
and -i options are mutually exclusive; the last one
specified takes effect.

-i[replstr]
This option is equivalent to -I replstr. The string {}
is assumed for replstr if the option-argument is
omitted.

-L number
The utility is executed for each non-empty number
lines of arguments from standard input. The last
invocation of utility is with fewer lines of arguments
if fewer than number remain. A line is considered to
end with the first newline character unless the last
character of the line is a blank character; a trailing
blank character signals continuation to the next non-
empty line, inclusive. The -L, -l, and -n options are
mutually exclusive; the last one specified takes
effect.

-l[number]
(The letter ell.) This option is equivalent to -L
number. If number is omitted, 1 is assumed. Option -x
is forced on.

-n number
Invokes utility using as many standard input arguments
as possible, up to number (a positive decimal integer)
arguments maximum. Fewer arguments are used if:

o The command line length accumulated exceeds
the size specified by the -s option (or
{LINE_MAX} if there is no -s option), or

o 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 (specific to the user's locale) read from
/dev/tty executes the command; otherwise, that
particular invocation of utility is skipped.

-P maxprocs
Invokes utility using at most maxprocs (a positive
decimal integer) parallel child processes. If maxprocs
is zero, then the system will set a large upper bound
to try and run as many processes as possible.

-s size
Invokes utility using as many standard input arguments
as possible yielding a command line length less than
size (a positive decimal integer) bytes. Fewer
arguments are used if:

o The total number of arguments exceeds that
specified by the -n option, or

o The total number of lines exceeds that
specified by the -L option, or

o End of file is encountered on standard
input before size bytes are accumulated.
Values of size up to at least {LINE_MAX} bytes are
supported, provided that the constraints specified in
DESCRIPTION are met. It is not considered an error if
a value larger than that supported by the
implementation or exceeding the constraints specified
in DESCRIPTION is specified. xargs uses the largest
value it supports within the constraints.

-t
Enables trace mode. Each generated command line is written to
standard error just prior to invocation.

-x
Terminates if a command line containing number arguments (see
the -n option above) or number lines (see the -L option above)
does not fit in the implied or specified size (see the -s
option above).

-0
Null separator mode. Instead of using white space or new lines
to delimit arguments, zero bytes are used. This is suitable
for use with the -print0 argument to find(1).

OPERANDS

The following operands are supported:

utility
The name of the utility to be invoked, found by search
path using the PATH environment variable. (ee
environ(7).) If utility is omitted, the default is the
echo(1) utility. If the utility operand names any of the
special built-in utilities in shell_builtins(1), the
results are undefined.

argument
An initial option or operand for the invocation of
utility.

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 succeeds. Thus, utility should explicitly exit
with an appropriate value to avoid accidentally returning with 255.

Notice that input is parsed as lines. Blank characters 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 characters or newline
characters. 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. Notice 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 can
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 xargs utility returns exit status 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.

EXAMPLES

Example 1: Using the xargs command

The following example moves all files from directory $1 to directory
$2, and echo each move command just before doing it:

example% ls $1 | xargs -I {} -t mv $1/{} $2/{}

The following command combines the output of the parenthesised
commands onto one line, which is then written to the end of file log:

example% (logname; date; printf "%s\n" "$0 $*") | xargs >>log

The following command invokes diff with successive pairs of arguments
originally typed as command line arguments (assuming there are no
embedded blank characters in the elements of the original argument
list):

example% printf "%s\n" "$*" | xargs -n 2 -x diff

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:

example% ls | xargs -p -L 1 ar -r arch
ls | xargs -p -L 1 | xargs ar -r arch

The following executes with successive pairs of arguments originally
typed as command line arguments:

example% echo $* | xargs -n 2 diff

ENVIRONMENT VARIABLES

See environ(7) for descriptions of the following environment
variables that affect the execution of xargs: LANG, LC_ALL,
LC_COLLATE, LC_CTYPE, LC_MESSAGES, and NLSPATH.

PATH
Determine the location of utility.

Affirmative responses are processed using the extended regular
expression defined for the yesexpr keyword in the LC_MESSAGES
category of the user's locale. The locale specified in the LC_COLLATE
category defines the behavior of ranges, equivalence classes, and
multi-character collating elements used in the expression defined for
yesexpr. The locale specified in LC_CTYPE determines the locale for
interpretation of sequences of bytes of text data a characters, the
behavior of character classes used in the expression defined for the
yesexpr. See locale(7).

EXIT STATUS

The following exit values are returned:

0
All invocations of utility returned exit status 0.

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.

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 writes a diagnostic
message and exit without processing any remaining input.

ATTRIBUTES