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Linux 2.6 - man page for xargs (linux section 1posix)

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

       xargs - construct argument lists and invoke utility

       xargs [-t][-p]][-E eofstr][-I replstr][-L number][-n number [-x]]
	       [-s size][utility [argument...]]

       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.

	* 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).

	* 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 pre-
       ceding 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

       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 spec-
       ified, the default command line length shall be at least {LINE_MAX}.

       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 unspec-
	      ified 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 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>s at the beginning of each line shall be ignored.
	      Constructed arguments cannot grow larger than 255 bytes. Option -x shall be  forced

       -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  argu-
	      ments if fewer than number remain. A line is considered to end with the first <new-
	      line> unless the last character of the line is a <blank>; a trailing  <blank>  sig-
	      nals  continuation to the next non-empty line, inclusive. The -L and -n options are
	      mutually-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 com-
	      mand 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 con-
       straints 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 speci-
       fied in the DESCRIPTION is given; xargs shall use the largest value it supports within the

       -t     Enable  trace  mode. Each generated command line shall 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 speci-
	      fied size (see the -s option above).

       The following operands shall be supported:

	      The name of the utility to be invoked, found by search path using the PATH environ-
	      ment  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 special built-in utilities
	      in Special Built-In Utilities , the results are undefined.

	      An initial option or operand for the invocation of utility.

       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>.

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

       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, Inter-
	      nationalization 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  interna-
	      tionalization 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.

	      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  argu-
	      ments  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  cate-

	      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

	      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.


       Not used.

       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.



       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.

       If a command line meeting the specified requirements cannot be assembled, the utility can-
       not 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 mes-
       sage 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 255.

       Note that input is parsed as lines; <blank>s separate arguments. If xargs is used to  bun-
       dle 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 cur-
       rent 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 pre-
       cede 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  util-
       ity"  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.

	1. The following command combines 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

	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

	   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  direc-
	   tory $2, and echoes each move command just before doing it:

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

       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 var-
       ious 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  stan-
       dard 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

       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 IEEE Std 1003.1-2001 is 4096 bytes and the min-
       imum value allowed by this volume of IEEE Std 1003.1-2001 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}

       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 want-
       ing 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,  mak-
	  ing 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  Porta-
       bility  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 Util-
       ity  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 logi-
       cal  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.


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

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard 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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					 XARGS(P)

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