# xargs

## 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.
    


## 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 **"$0** $*" 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 **$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



           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.
    


* * *