xargs(1p) — Linux manual page


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

PROLOG         top

       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.

NAME         top

       xargs — construct argument lists and invoke utility

SYNOPSIS         top

       xargs [−ptx] [−E eofstr] [−I replstr|−L number|−n number]
           [−s size] [utility [argument...]]

DESCRIPTION         top

       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.1‐2008) 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         top

       The xargs utility shall conform to the Base Definitions volume of
       POSIX.1‐2008, 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 a
                 <blank>; a trailing <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         top

       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.1‐2008, 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         top

       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         top

       The file /dev/tty shall be used to read responses required by the −p


       The following environment variables shall affect the execution of

       LANG      Provide a default value for the internationalization
                 variables that are unset or null. (See the Base Definitions
                 volume of POSIX.1‐2008, Section 8.2, Internationalization
                 Variables for the precedence of internationalization
                 variables used to determine the values of locale

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

                 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.

                 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.1‐2008, Chapter 8, Environment



STDOUT         top

       Not used.

STDERR         top

       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         top




EXIT STATUS         top

       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

         126   The utility specified by utility was found but could not be

         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 shall write a
       diagnostic message and exit without processing any remaining input.

       The following sections are informative.


       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

       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         top

        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 "$0$*" does not include any <apostrophe> or <newline>

               (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 $1 to directory $2, and echoes each move command
           just before doing it. It assumes no filenames contain <newline>
           characters and that neither $1 nor $2 contains the sequence "{}".

               ls −A "$1" | sed −e 's/"/"\\""/g' −e 's/.*/"&"/' |
                   xargs −E "" −I {} −t mv "$1"/{} "$2"/{}

RATIONALE         top

       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.1‐2008 does not share the
       view that it is used only for development, and therefore it is not

       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.1‐2008
       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.1‐2008. 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.1‐2008 is 4096 bytes and the minimum value allowed by this
       volume of POSIX.1‐2008 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.1‐2008 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

       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.



SEE ALSO         top

       Chapter 2, Shell Command Language, diff(1p), echo(1p), find(1p)

       The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines

       The System Interfaces volume of POSIX.1‐2008, exec(1p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                           XARGS(1P)