Man Pages

xargs(1p) - phpMan xargs(1p) - phpMan

Command: man perldoc info search(apropos)  


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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may dif-
       fer (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 [-t][-p]][-E eofstr][-I replstr][-L number][-n number [-x]]
               [-s size][utility [argument...]]

DESCRIPTION
       The xargs utility shall construct a command line consisting of the utility and argument operands specified fol-
       lowed by as many arguments read in sequence from standard input as fit in length and number constraints  speci-
       fied  by the options. The xargs utility shall then invoke the constructed command line and wait for its comple-
       tion. This sequence shall be repeated until one of the following occurs:

        * An end-of-file condition is detected on standard input.


        * The logical end-of-file string (see the -E eofstr option) is found on standard input after double-quote pro-
          cessing, apostrophe processing, and backslash escape processing (see next paragraph).


        * 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>s, unescaped
       <blank>s, or <newline>s. A string of zero or more non-double-quote ( ' )' characters and non- <newline>s can be
       quoted  by  enclosing them in double-quotes. A string of zero or more non-apostrophe ( '" ) characters and non-
       <newline>s can be quoted by enclosing them in apostrophes. 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  IEEE Std 1003.1-2001)  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 IEEE Std 1003.1-2001, 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 log-
              ical end-of-file string is the underscore character ( '_' ) or the end-of-file string capability is dis-
              abled.  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 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 argu-
              ments can each contain one or more instances of replstr. Any <blank>s at  the  beginning  of  each  line
              shall be ignored. Constructed arguments cannot grow larger than 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  trail-
              ing  <blank> signals continuation to the next non-empty line, inclusive. The -L and -n options are mutu-
              ally-exclusive; the last one specified shall take effect.

       -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 invoca-
              tion.

       -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 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 IEEE Std 1003.1-2001, Chapter 8, Environment Variables.   If
              utility is omitted, the default shall be the echo utility.  If the utility operand names any of the spe-
              cial 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 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 IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for  the  prece-
              dence 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 ele-
              ments  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  exam-
              ple,  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 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 catalogs for the processing of LC_MESSAGES .

       PATH   Determine the location of utility, as described in the Base Definitions volume of  IEEE Std 1003.1-2001,
              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 speci-
       fied, 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  appropri-
       ate value to avoid accidentally returning with 255.

       Note  that input is parsed as lines; <blank>s 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>s or <newline>s. 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 interprets 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 command combines the output of the parenthesised commands onto one line, which is then writ-
           ten to the end-of-file log:


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


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


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


        3. In  the  following commands, 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


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


           echo $* | xargs -n 2 diff


        5. On XSI-conformant systems, the following moves all files from directory $1 to directory $2, and echoes each
           move command just before doing it:


           ls $1 | xargs -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 IEEE Std 1003.1-2001 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
       IEEE Std 1003.1-2001 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  withdrawn because simple alternatives already exist within this volume of
       IEEE Std 1003.1-2001. 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 mod-
       ify 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 IEEE Std 1003.1-2001 is 4096 bytes and the
       minimum value allowed by this volume of IEEE Std 1003.1-2001 is 2048 bytes; therefore, the 2048  bytes  differ-
       ence  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 IEEE Std 1003.1-2001 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  dis-
       able  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.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Shell Command Language, echo, find, the System Interfaces volume of IEEE Std 1003.1-2001, exec

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  6, Copyright (C) 2001-2003 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 .



IEEE/The Open Group                  2003                            XARGS(1P)