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
12 xargs — construct argument lists and invoke utility
13
15 xargs [-ptx] [-E eofstr] [-I replstr|-L number|-n number]
16 [-s size] [utility [argument...]]
17
19 The xargs utility shall construct a command line consisting of the
20 utility and argument operands specified followed by as many arguments
21 read in sequence from standard input as fit in length and number con‐
22 straints specified by the options. The xargs utility shall then invoke
23 the constructed command line and wait for its completion. This sequence
24 shall be repeated until one of the following occurs:
25 * An end-of-file condition is detected on standard input.
27 * An argument consisting of just the logical end-of-file string (see
29 the -E eofstr option) is found on standard input after double-quote
30 processing, <apostrophe> processing, and <backslash>-escape pro‐
31 cessing (see next paragraph). All arguments up to but not including
32 the argument consisting of just the logical end-of-file string
33 shall be used as arguments in constructed command lines.
34 * An invocation of a constructed command line returns an exit status
36 of 255.
37 The application shall ensure that arguments in the standard input are
39 separated by unquoted <blank> characters, unescaped <blank> characters,
40 or <newline> characters. A string of zero or more non-double-quote
41 ('"') characters and non-<newline> characters can be quoted by enclos‐
42 ing them in double-quotes. A string of zero or more non-<apostrophe>
43 ('\'') characters and non-<newline> characters can be quoted by enclos‐
44 ing them in <apostrophe> characters. Any unquoted character can be
45 escaped by preceding it with a <backslash>. The utility named by util‐
46 ity shall be executed one or more times until the end-of-file is
47 reached or the logical end-of file string is found. The results are
48 unspecified if the utility named by utility attempts to read from its
49 standard input.
50 The generated command line length shall be the sum of the size in bytes
52 of the utility name and each argument treated as strings, including a
53 null byte terminator for each of these strings. The xargs utility shall
54 limit the command line length such that when the command line is
55 invoked, the combined argument and environment lists (see the exec fam‐
56 ily of functions in the System Interfaces volume of POSIX.1‐2017) shall
57 not exceed {ARG_MAX}-2048 bytes. Within this constraint, if neither the
58 -n nor the -s option is specified, the default command line length
59 shall be at least {LINE_MAX}.
60
62 The xargs utility shall conform to the Base Definitions volume of
63 POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.
64 The following options shall be supported:
66 -E eofstr Use eofstr as the logical end-of-file string. If -E is not
68 specified, it is unspecified whether the logical end-of-file
69 string is the <underscore> character ('_') or the end-of-file
70 string capability is disabled. When eofstr is the null
71 string, the logical end-of-file string capability shall be
72 disabled and <underscore> characters shall be taken liter‐
73 ally.
74 -I replstr
76 Insert mode: utility is executed for each logical line from
77 standard input. Arguments in the standard input shall be sep‐
78 arated only by unescaped <newline> characters, not by <blank>
79 characters. Any unquoted unescaped <blank> characters at the
80 beginning of each line shall be ignored. The resulting argu‐
81 ment shall be inserted in arguments in place of each occur‐
82 rence of replstr. At least five arguments in arguments can
83 each contain one or more instances of replstr. Each of these
84 constructed arguments cannot grow larger than an implementa‐
85 tion-defined limit greater than or equal to 255 bytes. Option
86 -x shall be forced on.
87 -L number The utility shall be executed for each non-empty number lines
89 of arguments from standard input. The last invocation of
90 utility shall be with fewer lines of arguments if fewer than
91 number remain. A line is considered to end with the first
92 <newline> unless the last character of the line is an
93 unescaped <blank>; a trailing unescaped <blank> signals con‐
94 tinuation to the next non-empty line, inclusive.
95 -n number Invoke utility using as many standard input arguments as pos‐
97 sible, up to number (a positive decimal integer) arguments
98 maximum. Fewer arguments shall be used if:
99 * The command line length accumulated exceeds the size
101 specified by the -s option (or {LINE_MAX} if there is no
102 -s option).
103 * The last iteration has fewer than number, but not zero,
105 operands remaining.
106 -p Prompt mode: the user is asked whether to execute utility at
108 each invocation. Trace mode (-t) is turned on to write the
109 command instance to be executed, followed by a prompt to
110 standard error. An affirmative response read from /dev/tty
111 shall execute the command; otherwise, that particular invoca‐
112 tion of utility shall be skipped.
113 -s size Invoke utility using as many standard input arguments as pos‐
115 sible yielding a command line length less than size (a posi‐
116 tive decimal integer) bytes. Fewer arguments shall be used
117 if:
118 * The total number of arguments exceeds that specified by
120 the -n option.
121 * The total number of lines exceeds that specified by the
123 -L option.
124 * End-of-file is encountered on standard input before size
126 bytes are accumulated.
127 Values of size up to at least {LINE_MAX} bytes shall be sup‐
129 ported, provided that the constraints specified in the
130 DESCRIPTION are met. It shall not be considered an error if a
131 value larger than that supported by the implementation or
132 exceeding the constraints specified in the DESCRIPTION is
133 given; xargs shall use the largest value it supports within
134 the constraints.
135 -t Enable trace mode. Each generated command line shall be writ‐
137 ten to standard error just prior to invocation.
138 -x Terminate if a constructed command line will not fit in the
140 implied or specified size (see the -s option above).
141
143 The following operands shall be supported:
144 utility The name of the utility to be invoked, found by search path
146 using the PATH environment variable, described in the Base
147 Definitions volume of POSIX.1‐2017, Chapter 8, Environment
148 Variables. If utility is omitted, the default shall be the
149 echo utility. If the utility operand names any of the special
150 built-in utilities in Section 2.14, Special Built-In Utili‐
151 ties, the results are undefined.
152 argument An initial option or operand for the invocation of utility.
154
156 The standard input shall be a text file. The results are unspecified if
157 an end-of-file condition is detected immediately following an escaped
158 <newline>.
159
161 The file /dev/tty shall be used to read responses required by the -p
162 option.
163
165 The following environment variables shall affect the execution of
166 xargs:
167 LANG Provide a default value for the internationalization vari‐
169 ables that are unset or null. (See the Base Definitions vol‐
170 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
171 ables for the precedence of internationalization variables
172 used to determine the values of locale categories.)
173 LC_ALL If set to a non-empty string value, override the values of
175 all the other internationalization variables.
176 LC_COLLATE
178 Determine the locale for the behavior of ranges, equivalence
179 classes, and multi-character collating elements used in the
180 extended regular expression defined for the yesexpr locale
181 keyword in the LC_MESSAGES category.
182 LC_CTYPE Determine the locale for the interpretation of sequences of
184 bytes of text data as characters (for example, single-byte as
185 opposed to multi-byte characters in arguments and input
186 files) and the behavior of character classes used in the
187 extended regular expression defined for the yesexpr locale
188 keyword in the LC_MESSAGES category.
189 LC_MESSAGES
191 Determine the locale used to process affirmative responses,
192 and the locale used to affect the format and contents of
193 diagnostic messages and prompts written to standard error.
194 NLSPATH Determine the location of message catalogs for the processing
196 of LC_MESSAGES.
197 PATH Determine the location of utility, as described in the Base
199 Definitions volume of POSIX.1‐2017, Chapter 8, Environment
200 Variables.
201
203 Default.
204
206 Not used.
207
209 The standard error shall be used for diagnostic messages and the -t and
210 -p options. If the -t option is specified, the utility and its con‐
211 structed argument list shall be written to standard error, as it will
212 be invoked, prior to invocation. If -p is specified, a prompt of the
213 following format shall be written (in the POSIX locale):
214 "?..."
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 $*") | xargs -E "" >>log
300 2. The following command invokes diff with successive pairs of argu‐
302 ments originally typed as command line arguments. It assumes there
303 are no embedded <newline> characters in the elements of the origi‐
304 nal argument list.
305 printf "%s\n$@" | sed 's/[^[:alnum:]]/\\&/g' |
308 xargs -E "" -n 2 -x diff
309 3. In the following commands, the user is asked which files in the
311 current directory (excluding dotfiles) are to be archived. The
312 files are archived into arch; a, one at a time or b, many at a
313 time. The commands assume that no filenames contain <blank>, <new‐
314 line>, <backslash>, <apostrophe>, or double-quote characters.
315 a. ls | xargs -E "" -p -L 1 ar -r arch
318 b. ls | xargs -E "" -p -L 1 | xargs -E "" ar -r arch
320 4. The following command invokes command1 one or more times with mul‐
322 tiple arguments, stopping if an invocation of command1 has a non-
323 zero exit status.
324 xargs -E "" sh -c 'command1 "$@" || exit 255' sh < xargs_input
327 5. On XSI-conformant systems, the following command moves all files
329 from directory $1 to directory $2, and echoes each move command
330 just before doing it. It assumes no filenames contain <newline>
331 characters and that neither $1 nor $2 contains the sequence "{}".
332 ls -A "$1" | sed -e 's/"/"\\""/g' -e 's/.*/"&"/' |
335 xargs -E "" -I {} -t mv "$1"/{} "$2"/{}
336
338 The xargs utility was usually found only in System V-based systems; BSD
339 systems included an apply utility that provided functionality similar
340 to xargs -n number. The SVID lists xargs as a software development
341 extension. This volume of POSIX.1‐2017 does not share the view that it
342 is used only for development, and therefore it is not optional.
343 The classic application of the xargs utility is in conjunction with the
345 find utility to reduce the number of processes launched by a simplistic
346 use of the find -exec combination. The xargs utility is also used to
347 enforce an upper limit on memory required to launch a process. With
348 this basis in mind, this volume of POSIX.1‐2017 selected only the mini‐
349 mal features required.
350 Although the 255 exit status is mostly an accident of historical imple‐
352 mentations, it allows a utility being used by xargs to tell xargs to
353 terminate if it knows no further invocations using the current data
354 stream shall succeed. Any non-zero exit status from a utility falls
355 into the 1‐125 range when xargs exits. There is no statement of how the
356 various non-zero utility exit status codes are accumulated by xargs.
357 The value could be the addition of all codes, their highest value, the
358 last one received, or a single value such as 1. Since no algorithm is
359 arguably better than the others, and since many of the standard utili‐
360 ties say little more (portably) than ``pass/fail'', no new algorithm
361 was invented.
362 Several other xargs options were removed because simple alternatives
364 already exist within this volume of POSIX.1‐2017. For example, the -i
365 replstr option can be just as efficiently performed using a shell for
366 loop. Since xargs calls an exec function with each input line, the -i
367 option does not usually exploit the grouping capabilities of xargs.
368 The requirement that xargs never produces command lines such that invo‐
370 cation of utility is within 2048 bytes of hitting the POSIX exec
371 {ARG_MAX} limitations is intended to guarantee that the invoked utility
372 has room to modify its environment variables and command line arguments
373 and still be able to invoke another utility. Note that the minimum
374 {ARG_MAX} allowed by the System Interfaces volume of POSIX.1‐2017 is
375 4096 bytes and the minimum value allowed by this volume of POSIX.1‐2017
376 is 2048 bytes; therefore, the 2048 bytes difference seems reasonable.
377 Note, however, that xargs may never be able to invoke a utility if the
378 environment passed in to xargs comes close to using {ARG_MAX} bytes.
379 The version of xargs required by this volume of POSIX.1‐2017 is
381 required to wait for the completion of the invoked command before
382 invoking another command. This was done because historical scripts
383 using xargs assumed sequential execution. Implementations wanting to
384 provide parallel operation of the invoked utilities are encouraged to
385 add an option enabling parallel invocation, but should still wait for
386 termination of all of the children before xargs terminates normally.
387 The -e option was omitted from the ISO POSIX‐2:1993 standard in the
389 belief that the eofstr option-argument was recognized only when it was
390 on a line by itself and before quote and escape processing were per‐
391 formed, and that the logical end-of-file processing was only enabled if
392 a -e option was specified. In that case, a simple sed script could be
393 used to duplicate the -e functionality. Further investigation revealed
394 that:
395 * The logical end-of-file string was checked for after quote and
397 escape processing, making a sed script that provided equivalent
398 functionality much more difficult to write.
399 * The default was to perform logical end-of-file processing with an
401 <underscore> as the logical end-of-file string.
402 To correct this misunderstanding, the -E eofstr option was adopted from
404 the X/Open Portability Guide. Users should note that the description of
405 the -E option matches historical documentation of the -e option (which
406 was not adopted because it did not support the Utility Syntax Guide‐
407 lines), by saying that if eofstr is the null string, logical end-of-
408 file processing is disabled. Historical implementations of xargs actu‐
409 ally did not disable logical end-of-file processing; they treated a
410 null argument found in the input as a logical end-of-file string. (A
411 null string argument could be generated using single or double-quotes
412 ('' or ""). Since this behavior was not documented historically, it is
413 considered to be a bug.
414 The -I, -L, and -n options are mutually-exclusive. Some implementations
416 use the last one specified if more than one is given on a command line;
417 other implementations treat combinations of the options in different
418 ways.
419
421 None.
422
424 Chapter 2, Shell Command Language, diff, echo, find
425 The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
427 Variables, Section 12.2, Utility Syntax Guidelines
428 The System Interfaces volume of POSIX.1‐2017, exec
430
432 Portions of this text are reprinted and reproduced in electronic form
433 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
434 table Operating System Interface (POSIX), The Open Group Base Specifi‐
435 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
436 Electrical and Electronics Engineers, Inc and The Open Group. In the
437 event of any discrepancy between this version and the original IEEE and
438 The Open Group Standard, the original IEEE and The Open Group Standard
439 is the referee document. The original Standard can be obtained online
440 at http://www.opengroup.org/unix/online.html .
441 Any typographical or formatting errors that appear in this page are
443 most likely to have been introduced during the conversion of the source
444 files to man page format. To report such errors, see https://www.ker‐
445 nel.org/doc/man-pages/reporting_bugs.html .
446IEEE/The Open Group 2017 XARGS(1P)