NAME

xargs - construct argument lists and invoke utility

 SYNOPSIS

xargs [-t][-p][-e[eofstr]][-E eofstr][-I replstr][-i[replstr]]
[-L number][-l[number]][-n number [-x]][-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 will 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 will be 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 will 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 will limit the command line length such that when the command line is invoked, the combined argument and environment lists (see the exec family of functions in the XSH specification) will not exceed {ARG_MAX}-2048 bytes. Within this constraint, if neither the -n nor the -s option is specified, the default command line length will be at least {LINE_MAX}.

 OPTIONS

The xargs utility supports the XBD specification, Utility Syntax Guidelines  , except that the -e, -i and -l take optional option-arguments that cannot be separate arguments.

The following options are supported:

-e[eofstr]

Use 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

Specify a logical end-of-file string to replace the default underscore. The xargs utility reads standard input until either end-of-file or the logical EOF string is encountered.

-I replstr

Insert mode: utility will be 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 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 will be executed for each non-empty number lines of arguments from standard input. The last invocation of utility will be 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

Invoke utility using as many standard input arguments as possible, up to number (a positive decimal integer) arguments maximum. Fewer arguments will 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 will execute the command; otherwise, that particular invocation of utility is skipped.

-s size

Invoke utility using as many standard input arguments as possible yielding a command line length less than size (a positive decimal integer) bytes. Fewer arguments will 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 are supported, provided that the constraints specified in the DESCRIPTION section are met. It is not considered an error if a value larger than that supported by the implementation or exceeding the constraints specified in the DESCRIPTION section is given; xargs will use the largest value it supports within the constraints.

-t

Enable trace mode. Each generated command line will 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 are supported:

utility

The name of the utility to be invoked, found by search path using the PATH environment variable, described in the XBD specification, Environment Variables  . If utility is omitted, the default is 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 must be a text file. The results are unspecified if an end-of-file condition is detected immediately following an escaped newline character.

 INPUT FILES

The file /dev/tty is used to read responses required by the -p option.

 ENVIRONMENT VARIABLES

The following environment variables affect the execution of xargs:

LANG

Provide a default value for the internationalisation variables that are unset or null. If LANG is unset or null, the corresponding value from the implementation-dependent default locale will be used. If any of the internationalisation variables contains an invalid setting, the utility will behave as if none of the variables had been defined.

LC_ALL

If set to a non-empty string value, override the values of all the other internationalisation variables.

LC_COLLATE

Determine the locale for the behaviour 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- as opposed to multi-byte characters in arguments and input files) and the behaviour 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 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 catalogues for the processing of LC_MESSAGES .

PATH

Determine the location of utility, as described in the XBD specification, Environment Variables  .

 ASYNCHRONOUS EVENTS

Default.

 STDOUT

Not used.

 STDERR

Used for diagnostic messages and the -t and -p options. If the -t option is specified, the utility and its constructed argument list will be written to standard error, as it will be invoked, prior to invocation. If -p is specified, a prompt of the following format will 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 are 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 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 will write a diagnostic message and exit without processing any remaining input.

 APPLICATION USAGE

The 255 exit status (described as -1 in Issue 3) 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 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. 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 will interpret 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 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 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 will move all files from directory $1 to directory $2, and echo each move command just before doing it:

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

2. The following command will combine the output of the parenthesised commands onto one line, which is then written to the end-of-file log:

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

3. The following command will invoke 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):

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

4. 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
   
   

5. The following will execute with successive pairs of arguments originally typed as command line arguments:

   
   echo $* | xargs -n 2 diff
   
   

 FUTURE DIRECTIONS

A version supporting the Utility Syntax Guidelines may be introduced.

The IEEE PASC 1003.2 Interpretations Committee has forwarded concerns about parts of this interface definition to the IEEE PASC Shell and Utilities Working Group which is identifying the corrections. A future revision of this specification will align with IEEE Std. 1003.2b when finalised.

 SEE ALSO

echo.