1XARGS(1P) POSIX Programmer's Manual XARGS(1P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 xargs — construct argument lists and invoke utility
14
16 xargs [−ptx] [−E eofstr] [−I replstr|−L number|−n number]
17 [−s size] [utility [argument...]]
18
20 The xargs utility shall construct a command line consisting of the
21 utility and argument operands specified followed by as many arguments
22 read in sequence from standard input as fit in length and number con‐
23 straints specified by the options. The xargs utility shall then invoke
24 the constructed command line and wait for its completion. This sequence
25 shall be repeated until one of the following occurs:
26 * An end-of-file condition is detected on standard input.
28 * An argument consisting of just the logical end-of-file string (see
30 the −E eofstr option) is found on standard input after double-quote
31 processing, <apostrophe> processing, and <backslash>-escape pro‐
32 cessing (see next paragraph). All arguments up to but not including
33 the argument consisting of just the logical end-of-file string
34 shall be used as arguments in constructed command lines.
35 * An invocation of a constructed command line returns an exit status
37 of 255.
38 The application shall ensure that arguments in the standard input are
40 separated by unquoted <blank> characters, unescaped <blank> characters,
41 or <newline> characters. A string of zero or more non-double-quote
42 ('"') characters and non-<newline> characters can be quoted by enclos‐
43 ing them in double-quotes. A string of zero or more non-<apostrophe>
44 ('\'') characters and non-<newline> characters can be quoted by enclos‐
45 ing them in <apostrophe> characters. Any unquoted character can be
46 escaped by preceding it with a <backslash>. The utility named by util‐
47 ity shall be executed one or more times until the end-of-file is
48 reached or the logical end-of file string is found. The results are
49 unspecified if the utility named by utility attempts to read from its
50 standard input.
51 The generated command line length shall be the sum of the size in bytes
53 of the utility name and each argument treated as strings, including a
54 null byte terminator for each of these strings. The xargs utility shall
55 limit the command line length such that when the command line is
56 invoked, the combined argument and environment lists (see the exec fam‐
57 ily of functions in the System Interfaces volume of POSIX.1‐2008) shall
58 not exceed {ARG_MAX}−2048 bytes. Within this constraint, if neither the
59 −n nor the −s option is specified, the default command line length
60 shall be at least {LINE_MAX}.
61
63 The xargs utility shall conform to the Base Definitions volume of
64 POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines.
65 The following options shall be supported:
67 −E eofstr Use eofstr as the logical end-of-file string. If −E is not
69 specified, it is unspecified whether the logical end-of-file
70 string is the <underscore> character ('_') or the end-of-file
71 string capability is disabled. When eofstr is the null
72 string, the logical end-of-file string capability shall be
73 disabled and <underscore> characters shall be taken liter‐
74 ally.
75 −I replstr
77 Insert mode: utility is executed for each logical line from
78 standard input. Arguments in the standard input shall be sep‐
79 arated only by unescaped <newline> characters, not by <blank>
80 characters. Any unquoted unescaped <blank> characters at the
81 beginning of each line shall be ignored. The resulting argu‐
82 ment shall be inserted in arguments in place of each occur‐
83 rence of replstr. At least five arguments in arguments can
84 each contain one or more instances of replstr. Each of these
85 constructed arguments cannot grow larger than an implementa‐
86 tion-defined limit greater than or equal to 255 bytes. Option
87 −x shall be forced on.
88 −L number The utility shall be executed for each non-empty number lines
90 of arguments from standard input. The last invocation of
91 utility shall be with fewer lines of arguments if fewer than
92 number remain. A line is considered to end with the first
93 <newline> unless the last character of the line is a <blank>;
94 a trailing <blank> signals continuation to the next non-empty
95 line, inclusive.
96 −n number Invoke utility using as many standard input arguments as pos‐
98 sible, up to number (a positive decimal integer) arguments
99 maximum. Fewer arguments shall be used if:
100 * The command line length accumulated exceeds the size
102 specified by the −s option (or {LINE_MAX} if there is no
103 −s option).
104 * The last iteration has fewer than number, but not zero,
106 operands remaining.
107 −p Prompt mode: the user is asked whether to execute utility at
109 each invocation. Trace mode (−t) is turned on to write the
110 command instance to be executed, followed by a prompt to
111 standard error. An affirmative response read from /dev/tty
112 shall execute the command; otherwise, that particular invoca‐
113 tion of utility shall be skipped.
114 −s size Invoke utility using as many standard input arguments as pos‐
116 sible yielding a command line length less than size (a posi‐
117 tive decimal integer) bytes. Fewer arguments shall be used
118 if:
119 * The total number of arguments exceeds that specified by
121 the −n option.
122 * The total number of lines exceeds that specified by the
124 −L option.
125 * End-of-file is encountered on standard input before size
127 bytes are accumulated.
128 Values of size up to at least {LINE_MAX} bytes shall be sup‐
130 ported, provided that the constraints specified in the
131 DESCRIPTION are met. It shall not be considered an error if a
132 value larger than that supported by the implementation or
133 exceeding the constraints specified in the DESCRIPTION is
134 given; xargs shall use the largest value it supports within
135 the constraints.
136 −t Enable trace mode. Each generated command line shall be writ‐
138 ten to standard error just prior to invocation.
139 −x Terminate if a constructed command line will not fit in the
141 implied or specified size (see the −s option above).
142
144 The following operands shall be supported:
145 utility The name of the utility to be invoked, found by search path
147 using the PATH environment variable, described in the Base
148 Definitions volume of POSIX.1‐2008, Chapter 8, Environment
149 Variables. If utility is omitted, the default shall be the
150 echo utility. If the utility operand names any of the special
151 built-in utilities in Section 2.14, Special Built-In Utili‐
152 ties, the results are undefined.
153 argument An initial option or operand for the invocation of utility.
155
157 The standard input shall be a text file. The results are unspecified if
158 an end-of-file condition is detected immediately following an escaped
159 <newline>.
160
162 The file /dev/tty shall be used to read responses required by the −p
163 option.
164
166 The following environment variables shall affect the execution of
167 xargs:
168 LANG Provide a default value for the internationalization vari‐
170 ables that are unset or null. (See the Base Definitions vol‐
171 ume of POSIX.1‐2008, Section 8.2, Internationalization Vari‐
172 ables for the precedence of internationalization variables
173 used to determine the values of locale categories.)
174 LC_ALL If set to a non-empty string value, override the values of
176 all the other internationalization variables.
177 LC_COLLATE
179 Determine the locale for the behavior of ranges, equivalence
180 classes, and multi-character collating elements used in the
181 extended regular expression defined for the yesexpr locale
182 keyword in the LC_MESSAGES category.
183 LC_CTYPE Determine the locale for the interpretation of sequences of
185 bytes of text data as characters (for example, single-byte as
186 opposed to multi-byte characters in arguments and input
187 files) and the behavior of character classes used in the
188 extended regular expression defined for the yesexpr locale
189 keyword in the LC_MESSAGES category.
190 LC_MESSAGES
192 Determine the locale used to process affirmative responses,
193 and the locale used to affect the format and contents of
194 diagnostic messages and prompts written to standard error.
195 NLSPATH Determine the location of message catalogs for the processing
197 of LC_MESSAGES.
198 PATH Determine the location of utility, as described in the Base
200 Definitions volume of POSIX.1‐2008, Chapter 8, Environment
201 Variables.
202
204 Default.
205
207 Not used.
208
210 The standard error shall be used for diagnostic messages and the −t and
211 −p options. If the −t option is specified, the utility and its con‐
212 structed argument list shall be written to standard error, as it will
213 be invoked, prior to invocation. If −p is specified, a prompt of the
214 following format shall be written (in the POSIX locale):
215 "?..."
217 at the end of the line of the output from −t.
219
221 None.
222
224 None.
225
227 The following exit values shall be returned:
228 0 All invocations of utility returned exit status zero.
230 1‐125 A command line meeting the specified requirements could not be
232 assembled, one or more of the invocations of utility returned a
233 non-zero exit status, or some other error occurred.
234 126 The utility specified by utility was found but could not be
236 invoked.
237 127 The utility specified by utility could not be found.
239
241 If a command line meeting the specified requirements cannot be assem‐
242 bled, the utility cannot be invoked, an invocation of the utility is
243 terminated by a signal, or an invocation of the utility exits with exit
244 status 255, the xargs utility shall write a diagnostic message and exit
245 without processing any remaining input.
246 The following sections are informative.
248
250 The 255 exit status allows a utility being used by xargs to tell xargs
251 to terminate if it knows no further invocations using the current data
252 stream will succeed. Thus, utility should explicitly exit with an
253 appropriate value to avoid accidentally returning with 255.
254 Note that since input is parsed as lines, <blank> characters separate
256 arguments, and <backslash>, <apostrophe>, and double-quote characters
257 are used for quoting, if xargs is used to bundle the output of commands
258 like find dir −print or ls into commands to be executed, unexpected
259 results are likely if any filenames contain <blank>, <newline>, or
260 quoting characters. This can be solved by using find to call a script
261 that converts each file found into a quoted string that is then piped
262 to xargs, but in most cases it is preferable just to have find do the
263 argument aggregation itself by using −exec with a '+' terminator
264 instead of ';'. Note that the quoting rules used by xargs are not the
265 same as in the shell. They were not made consistent here because exist‐
266 ing applications depend on the current rules. An easy (but inefficient)
267 method that can be used to transform input consisting of one argument
268 per line into a quoted form that xargs interprets correctly is to pre‐
269 cede each non-<newline> character with a <backslash>. More efficient
270 alternatives are shown in Example 2 and Example 5 below.
271 On implementations with a large value for {ARG_MAX}, xargs may produce
273 command lines longer than {LINE_MAX}. For invocation of utilities,
274 this is not a problem. If xargs is being used to create a text file,
275 users should explicitly set the maximum command line length with the −s
276 option.
277 The command, env, nice, nohup, time, and xargs utilities have been
279 specified to use exit code 127 if an error occurs so that applications
280 can distinguish ``failure to find a utility'' from ``invoked utility
281 exited with an error indication''. The value 127 was chosen because it
282 is not commonly used for other meanings; most utilities use small val‐
283 ues for ``normal error conditions'' and the values above 128 can be
284 confused with termination due to receipt of a signal. The value 126 was
285 chosen in a similar manner to indicate that the utility could be found,
286 but not invoked. Some scripts produce meaningful error messages differ‐
287 entiating the 126 and 127 cases. The distinction between exit codes 126
288 and 127 is based on KornShell practice that uses 127 when all attempts
289 to exec the utility fail with [ENOENT], and uses 126 when any attempt
290 to exec the utility fails for any other reason.
291
293 1. The following command combines the output of the parenthesized com‐
294 mands (minus the <apostrophe> characters) onto one line, which is
295 then appended to the file log. It assumes that the expansion of
296 "$0$*" does not include any <apostrophe> or <newline> characters.
297 (logname; date; printf "'%s'\n$0 $*") | xargs −E "" >>log
299 2. The following command invokes diff with successive pairs of argu‐
301 ments originally typed as command line arguments. It assumes there
302 are no embedded <newline> characters in the elements of the origi‐
303 nal argument list.
304 printf "%s\n$@" | sed 's/[^[:alnum:]]/\\&/g' |
306 xargs −E "" −n 2 −x diff
307 3. In the following commands, the user is asked which files in the
309 current directory (excluding dotfiles) are to be archived. The
310 files are archived into arch; a, one at a time or b, many at a
311 time. The commands assume that no filenames contain <blank>, <new‐
312 line>, <backslash>, <apostrophe>, or double-quote characters.
313 a. ls | xargs −E "" −p −L 1 ar −r arch
315 b. ls | xargs −E "" −p −L 1 | xargs −E "" ar −r arch
317 4. The following command invokes command1 one or more times with mul‐
319 tiple arguments, stopping if an invocation of command1 has a non-
320 zero exit status.
321 xargs −E "" sh −c 'command1 "$@" || exit 255' sh < xargs_input
323 5. On XSI-conformant systems, the following command moves all files
325 from directory $1 to directory $2, and echoes each move command
326 just before doing it. It assumes no filenames contain <newline>
327 characters and that neither $1 nor $2 contains the sequence "{}".
328 ls −A "$1" | sed −e 's/"/"\\""/g' −e 's/.*/"&"/' |
330 xargs −E "" −I {} −t mv "$1"/{} "$2"/{}
331
333 The xargs utility was usually found only in System V-based systems; BSD
334 systems included an apply utility that provided functionality similar
335 to xargs −n number. The SVID lists xargs as a software development
336 extension. This volume of POSIX.1‐2008 does not share the view that it
337 is used only for development, and therefore it is not optional.
338 The classic application of the xargs utility is in conjunction with the
340 find utility to reduce the number of processes launched by a simplistic
341 use of the find −exec combination. The xargs utility is also used to
342 enforce an upper limit on memory required to launch a process. With
343 this basis in mind, this volume of POSIX.1‐2008 selected only the mini‐
344 mal features required.
345 Although the 255 exit status is mostly an accident of historical imple‐
347 mentations, it allows a utility being used by xargs to tell xargs to
348 terminate if it knows no further invocations using the current data
349 stream shall succeed. Any non-zero exit status from a utility falls
350 into the 1‐125 range when xargs exits. There is no statement of how the
351 various non-zero utility exit status codes are accumulated by xargs.
352 The value could be the addition of all codes, their highest value, the
353 last one received, or a single value such as 1. Since no algorithm is
354 arguably better than the others, and since many of the standard utili‐
355 ties say little more (portably) than ``pass/fail'', no new algorithm
356 was invented.
357 Several other xargs options were removed because simple alternatives
359 already exist within this volume of POSIX.1‐2008. For example, the −i
360 replstr option can be just as efficiently performed using a shell for
361 loop. Since xargs calls an exec function with each input line, the −i
362 option does not usually exploit the grouping capabilities of xargs.
363 The requirement that xargs never produces command lines such that invo‐
365 cation of utility is within 2048 bytes of hitting the POSIX exec
366 {ARG_MAX} limitations is intended to guarantee that the invoked utility
367 has room to modify its environment variables and command line arguments
368 and still be able to invoke another utility. Note that the minimum
369 {ARG_MAX} allowed by the System Interfaces volume of POSIX.1‐2008 is
370 4096 bytes and the minimum value allowed by this volume of POSIX.1‐2008
371 is 2048 bytes; therefore, the 2048 bytes difference seems reasonable.
372 Note, however, that xargs may never be able to invoke a utility if the
373 environment passed in to xargs comes close to using {ARG_MAX} bytes.
374 The version of xargs required by this volume of POSIX.1‐2008 is
376 required to wait for the completion of the invoked command before
377 invoking another command. This was done because historical scripts
378 using xargs assumed sequential execution. Implementations wanting to
379 provide parallel operation of the invoked utilities are encouraged to
380 add an option enabling parallel invocation, but should still wait for
381 termination of all of the children before xargs terminates normally.
382 The −e option was omitted from the ISO POSIX‐2:1993 standard in the
384 belief that the eofstr option-argument was recognized only when it was
385 on a line by itself and before quote and escape processing were per‐
386 formed, and that the logical end-of-file processing was only enabled if
387 a −e option was specified. In that case, a simple sed script could be
388 used to duplicate the −e functionality. Further investigation revealed
389 that:
390 * The logical end-of-file string was checked for after quote and
392 escape processing, making a sed script that provided equivalent
393 functionality much more difficult to write.
394 * The default was to perform logical end-of-file processing with an
396 <underscore> as the logical end-of-file string.
397 To correct this misunderstanding, the −E eofstr option was adopted from
399 the X/Open Portability Guide. Users should note that the description of
400 the −E option matches historical documentation of the −e option (which
401 was not adopted because it did not support the Utility Syntax Guide‐
402 lines), by saying that if eofstr is the null string, logical end-of-
403 file processing is disabled. Historical implementations of xargs actu‐
404 ally did not disable logical end-of-file processing; they treated a
405 null argument found in the input as a logical end-of-file string. (A
406 null string argument could be generated using single or double-quotes
407 ('' or ""). Since this behavior was not documented historically, it is
408 considered to be a bug.
409 The −I, −L, and −n options are mutually-exclusive. Some implementations
411 use the last one specified if more than one is given on a command line;
412 other implementations treat combinations of the options in different
413 ways.
414
416 None.
417
419 Chapter 2, Shell Command Language, diff, echo, find
420 The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
422 Variables, Section 12.2, Utility Syntax Guidelines
423 The System Interfaces volume of POSIX.1‐2008, exec
425
427 Portions of this text are reprinted and reproduced in electronic form
428 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
429 -- Portable Operating System Interface (POSIX), The Open Group Base
430 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
431 cal and Electronics Engineers, Inc and The Open Group. (This is
432 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
433 event of any discrepancy between this version and the original IEEE and
434 The Open Group Standard, the original IEEE and The Open Group Standard
435 is the referee document. The original Standard can be obtained online
436 at http://www.unix.org/online.html .
437 Any typographical or formatting errors that appear in this page are
439 most likely to have been introduced during the conversion of the source
440 files to man page format. To report such errors, see https://www.ker‐
441 nel.org/doc/man-pages/reporting_bugs.html .
442IEEE/The Open Group 2013 XARGS(1P)