XARGS(1)                   POSIX Programmer's Manual                  XARGS(1)



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       implementation of this interface may differ (consult the corresponding
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NAME
       xargs -- construct argument lists and invoke utility

SYNOPSIS
       xargs [[-ptx]] [[-E eofstr]] [[-I replstr|-L number|-n number]]
           [[-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
       constraints 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.

        *  An argument consisting of just the logical end-of-file string (see
           the -E 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.

        *  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> characters, unescaped <blank> characters,
       or <newline> characters. A string of zero or more non-double-quote
       (''""'') characters and non-<newline> characters can be quoted by
       enclosing them in double-quotes. A string of zero or more
       non-<apostrophe> (''\\'''') characters and non-<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 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
       family of functions in the System Interfaces volume of POSIX.1-2017)
       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
       POSIX.1-2017, 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
                 specified, 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 logical end-of-file string capability shall be
                 disabled and <underscore> characters shall be taken
                 literally.

       -I replstr
                 Insert mode: 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 arguments in place of each
                 occurrence of replstr.  At least five arguments in arguments
                 can each contain one or more instances of replstr.  Each of
                 these constructed arguments cannot grow larger than an
                 implementation-defined limit greater than or equal to 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 an
                 unescaped <blank>; a trailing unescaped <blank> signals
                 continuation to the next non-empty line, inclusive.

       -n number Invoke utility using as many standard input arguments as
                 possible, up to number (a positive decimal integer) arguments
                 maximum. Fewer arguments shall 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
                 shall execute the command; otherwise, that particular
                 invocation of utility shall be 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 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, 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; 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 constructed command line 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
                 Definitions volume of POSIX.1-2017, 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 Section 2.14, 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 POSIX.1-2017, Section 8.2, Internationalization
                 Variables 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
                 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-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 yesexpr locale
                 keyword in the LC__MESSAGES category.

       LC__MESSAGES
                 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.

       NLSPATH   Determine the location of message catalogs for the processing
                 of LC__MESSAGES.

       PATH      Determine the location of utility, as described in the Base
                 Definitions volume of POSIX.1-2017, 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
       constructed 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
       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 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 since input is parsed as lines, <blank> characters separate
       arguments, and <backslash>, <apostrophe>, and double-quote characters
       are used for quoting, if xargs is used to bundle the output of commands
       like find dir -print or 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 xargs, but in most cases it is preferable just to have find do the
       argument aggregation itself by using -exec with a ''++'' terminator
       instead of '';;''.  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. An easy (but
       inefficient) method that can be used to transform input consisting of
       one argument per line into a quoted form that xargs interprets
       correctly is to precede each non-<newline> character with a
       <backslash>.  More efficient alternatives are shown in Example 2 and
       Example 5 below.

       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 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
           ""$$00$*" does not include any <apostrophe> or <newline> characters.


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

        2. The following command invokes 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.


               printf "%s\n$@" | sed 's/[^[:alnum:]]/\\&/g' |
                   xargs -E "" -n 2 -x diff

        3. 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 arch; _a, one at a time or _b, many at a
           time. The commands assume that no filenames contain <blank>,
           <newline>, <backslash>, <apostrophe>, or double-quote characters.


               a. ls | xargs -E "" -p -L 1 ar -r arch

               b. ls | xargs -E "" -p -L 1 | xargs -E "" ar -r arch

        4. The following command invokes command1 one or more times with
           multiple arguments, stopping if an invocation of command1 has a
           non-zero exit status.


               xargs -E "" sh -c 'command1 "$@" || exit 255' sh < xargs_input

        5. On XSI-conformant systems, the following command moves all files
           from directory $$11 to directory $$22, and echoes each move command
           just before doing it. It assumes no filenames contain <newline>
           characters and that neither $$11 nor $$22 contains the sequence ""{{}}"".


               ls -A "$1" | sed -e 's/"/"\\""/g' -e 's/.*/"&"/' |
                   xargs -E "" -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 POSIX.1-2017 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 POSIX.1-2017 selected only the
       minimal features required.

       Although the 255 exit status is mostly an accident of historical
       implementations, 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
       utilities say little more (portably) than ``pass/fail'', no new
       algorithm was invented.

       Several other xargs options were removed because simple alternatives
       already exist within this volume of POSIX.1-2017. 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
       invocation 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 POSIX.1-2017 is
       4096 bytes and the minimum value allowed by this volume of POSIX.1-2017
       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 POSIX.1-2017 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
       performed, 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
       Guidelines), by saying that if eofstr is the null string, logical end-
       of-file processing is disabled.  Historical implementations of 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 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.

       The -I, -L, and -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.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Chapter _2, Shell Command Language, diff, echo, find

       The Base Definitions volume of POSIX.1-2017, Chapter _8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines

       The System Interfaces volume of POSIX.1-2017, exec

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 .

       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 .



IEEE/The Open Group                  2017                             XARGS(1)
