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 [-t][-p]][-E eofstr][-I replstr][-L number][-n number [-x]]
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 * The logical end-of-file string (see the -E eofstr option) is found
29 on standard input after double-quote processing, apostrophe process‐
30 ing, and backslash escape processing (see next paragraph).
31 * An invocation of a constructed command line returns an exit status
33 of 255.
34 The application shall ensure that arguments in the standard input are
36 separated by unquoted <blank>s, unescaped <blank>s, or <newline>s. A
37 string of zero or more non-double-quote ( ' )' characters and non-
38 <newline>s can be quoted by enclosing them in double-quotes. A string
39 of zero or more non-apostrophe ( '" ) characters and non- <newline>s
40 can be quoted by enclosing them in apostrophes. Any unquoted character
41 can be escaped by preceding it with a backslash. The utility named by
42 utility shall be executed one or more times until the end-of-file is
43 reached or the logical end-of file string is found. The results are
44 unspecified if the utility named by utility attempts to read from its
45 standard input.
46 The generated command line length shall be the sum of the size in bytes
48 of the utility name and each argument treated as strings, including a
49 null byte terminator for each of these strings. The xargs utility
50 shall limit the command line length such that when the command line is
51 invoked, the combined argument and environment lists (see the exec fam‐
52 ily of functions in the System Interfaces volume of
53 IEEE Std 1003.1-2001) shall not exceed {ARG_MAX}-2048 bytes. Within
54 this constraint, if neither the -n nor the -s option is specified, the
55 default command line length shall be at least {LINE_MAX}.
56
58 The xargs utility shall conform to the Base Definitions volume of
59 IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
60 The following options shall be supported:
62 -E eofstr
64 Use eofstr as the logical end-of-file string. If -E is not spec‐
65 ified, it is unspecified whether the logical end-of-file string
66 is the underscore character ( '_' ) or the end-of-file string
67 capability is disabled. When eofstr is the null string, the log‐
68 ical end-of-file string capability shall be disabled and under‐
69 score characters shall be taken literally.
70 -I replstr
72 Insert mode: utility is executed for each line from standard
73 input, taking the entire line as a single argument, inserting it
74 in arguments for each occurrence of replstr. A maximum of five
75 arguments in arguments can each contain one or more instances of
76 replstr. Any <blank>s at the beginning of each line shall be
77 ignored. Constructed arguments cannot grow larger than 255
78 bytes. Option -x shall be forced on.
79 -L number
81 The utility shall be executed for each non-empty number lines of
82 arguments from standard input. The last invocation of utility
83 shall be with fewer lines of arguments if fewer than number
84 remain. A line is considered to end with the first <newline>
85 unless the last character of the line is a <blank>; a trailing
86 <blank> signals continuation to the next non-empty line, inclu‐
87 sive. The -L and -n options are mutually-exclusive; the last one
88 specified shall take effect.
89 -n number
91 Invoke utility using as many standard input arguments as possi‐
92 ble, up to number (a positive decimal integer) arguments maxi‐
93 mum. Fewer arguments shall be used if:
94 * The command line length accumulated exceeds the size speci‐
96 fied by the -s option (or {LINE_MAX} if there is no -s
97 option).
98 * The last iteration has fewer than number, but not zero, oper‐
100 ands remaining.
101 -p Prompt mode: the user is asked whether to execute utility at
103 each invocation. Trace mode ( -t) is turned on to write the com‐
104 mand instance to be executed, followed by a prompt to standard
105 error. An affirmative response read from /dev/tty shall execute
106 the command; otherwise, that particular invocation of utility
107 shall be skipped.
108 -s size
110 Invoke utility using as many standard input arguments as possi‐
111 ble yielding a command line length less than size (a positive
112 decimal integer) bytes. Fewer arguments shall be used if:
113 * The total number of arguments exceeds that specified by the
115 -n option.
116 * The total number of lines exceeds that specified by the -L
118 option.
119 * End-of-file is encountered on standard input before size
121 bytes are accumulated.
122 Values of size up to at least {LINE_MAX} bytes shall be supported, pro‐
124 vided that the constraints specified in the DESCRIPTION are met. It
125 shall not be considered an error if a value larger than that supported
126 by the implementation or exceeding the constraints specified in the
127 DESCRIPTION is given; xargs shall use the largest value it supports
128 within the constraints.
129 -t Enable trace mode. Each generated command line shall be written
131 to standard error just prior to invocation.
132 -x Terminate if a command line containing number arguments (see the
134 -n option above) or number lines (see the -L option above)
135 will not fit in the implied or specified size (see the -s option
136 above).
137
140 The following operands shall be supported:
141 utility
143 The name of the utility to be invoked, found by search path
144 using the PATH environment variable, described in the Base Defi‐
145 nitions volume of IEEE Std 1003.1-2001, Chapter 8, Environment
146 Variables. If utility is omitted, the default shall be the echo
147 utility. If the utility operand names any of the special built-
148 in utilities in Special Built-In Utilities, the results are
149 undefined.
150 argument
152 An initial option or operand for the invocation of utility.
153
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 variables
169 that are unset or null. (See the Base Definitions volume of
170 IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
171 ables for the precedence of internationalization variables used
172 to determine the values of locale categories.)
173 LC_ALL If set to a non-empty string value, override the values of all
175 the other internationalization variables.
176 LC_COLLATE
178 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 key‐
182 word in the LC_MESSAGES category.
183 LC_CTYPE
185 Determine the locale for the interpretation of sequences of
186 bytes of text data as characters (for example, single-byte as
187 opposed to multi-byte characters in arguments and input files)
188 and the behavior of character classes used in the extended regu‐
189 lar expression defined for the yesexpr locale keyword in the
190 LC_MESSAGES category.
191 LC_MESSAGES
193 Determine the locale for the processing of affirmative responses
194 and that should be used to affect the format and contents of
195 diagnostic messages written to standard error.
196 NLSPATH
198 Determine the location of message catalogs for the processing of
199 LC_MESSAGES .
200 PATH Determine the location of utility, as described in the Base Def‐
202 initions volume of IEEE Std 1003.1-2001, Chapter 8, Environment
203 Variables.
204
207 Default.
208
210 Not used.
211
213 The standard error shall be used for diagnostic messages and the -t and
214 -p options. If the -t option is specified, the utility and its con‐
215 structed argument list shall be written to standard error, as it will
216 be invoked, prior to invocation. If -p is specified, a prompt of the
217 following format shall be written (in the POSIX locale):
218 "?..."
221 at the end of the line of the output from -t.
223
225 None.
226
228 None.
229
231 The following exit values shall be returned:
232 0 All invocations of utility returned exit status zero.
234 1-125 A command line meeting the specified requirements could not be
236 assembled, one or more of the invocations of utility returned a
237 non-zero exit status, or some other error occurred.
238 126 The utility specified by utility was found but could not be
240 invoked.
241 127 The utility specified by utility could not be found.
243
246 If a command line meeting the specified requirements cannot be assem‐
247 bled, the utility cannot be invoked, an invocation of the utility is
248 terminated by a signal, or an invocation of the utility exits with exit
249 status 255, the xargs utility shall write a diagnostic message and exit
250 without processing any remaining input.
251 The following sections are informative.
253
255 The 255 exit status allows a utility being used by xargs to tell xargs
256 to terminate if it knows no further invocations using the current data
257 stream will succeed. Thus, utility should explicitly exit with an
258 appropriate value to avoid accidentally returning with 255.
259 Note that input is parsed as lines; <blank>s separate arguments. If
261 xargs is used to bundle output of commands like find dir -print or ls
262 into commands to be executed, unexpected results are likely if any
263 filenames contain any <blank>s or <newline>s. This can be fixed by
264 using find to call a script that converts each file found into a quoted
265 string that is then piped to xargs. Note that the quoting rules used by
266 xargs are not the same as in the shell. They were not made consistent
267 here because existing applications depend on the current rules and the
268 shell syntax is not fully compatible with it. An easy rule that can be
269 used to transform any string into a quoted form that xargs interprets
270 correctly is to precede each character in the string with a backslash.
271 On implementations with a large value for {ARG_MAX}, xargs may produce
273 command lines longer than {LINE_MAX}. For invocation of utilities, this
274 is not a problem. If xargs is being used to create a text file, users
275 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 con‐
284 fused 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 parenthesised com‐
294 mands onto one line, which is then written to the end-of-file log:
295 (logname; date; printf "%s\n" "$0 $*") | xargs >>log
298 2. The following command invokes diff with successive pairs of argu‐
300 ments originally typed as command line arguments (assuming there
301 are no embedded <blank>s in the elements of the original argument
302 list):
303 printf "%s\n" "$*" | xargs -n 2 -x diff
306 3. In the following commands, the user is asked which files in the
308 current directory are to be archived. The files are archived into
309 arch; a, one at a time, or b, many at a time.
310 a. ls | xargs -p -L 1 ar -r arch
313 b. ls | xargs -p -L 1 | xargs ar -r arch
316 4. The following executes with successive pairs of arguments origi‐
318 nally typed as command line arguments:
319 echo $* | xargs -n 2 diff
322 5. On XSI-conformant systems, the following moves all files from
324 directory $1 to directory $2, and echoes each move command just
325 before doing it:
326 ls $1 | xargs -I {} -t mv $1/{} $2/{}
329
331 The xargs utility was usually found only in System V-based systems; BSD
332 systems included an apply utility that provided functionality similar
333 to xargs -n number. The SVID lists xargs as a software development
334 extension. This volume of IEEE Std 1003.1-2001 does not share the view
335 that it is used only for development, and therefore it is not optional.
336 The classic application of the xargs utility is in conjunction with the
338 find utility to reduce the number of processes launched by a simplistic
339 use of the find -exec combination. The xargs utility is also used to
340 enforce an upper limit on memory required to launch a process. With
341 this basis in mind, this volume of IEEE Std 1003.1-2001 selected only
342 the minimal features required.
343 Although the 255 exit status is mostly an accident of historical imple‐
345 mentations, it allows a utility being used by xargs to tell xargs to
346 terminate if it knows no further invocations using the current data
347 stream shall succeed. Any non-zero exit status from a utility falls
348 into the 1-125 range when xargs exits. There is no statement of how the
349 various non-zero utility exit status codes are accumulated by xargs.
350 The value could be the addition of all codes, their highest value, the
351 last one received, or a single value such as 1. Since no algorithm is
352 arguably better than the others, and since many of the standard utili‐
353 ties say little more (portably) than "pass/fail", no new algorithm was
354 invented.
355 Several other xargs options were withdrawn because simple alternatives
357 already exist within this volume of IEEE Std 1003.1-2001. For example,
358 the -i replstr option can be just as efficiently performed using a
359 shell for loop. Since xargs calls an exec function with each input
360 line, the -i option does not usually exploit the grouping capabilities
361 of xargs.
362 The requirement that xargs never produces command lines such that invo‐
364 cation of utility is within 2048 bytes of hitting the POSIX exec
365 {ARG_MAX} limitations is intended to guarantee that the invoked utility
366 has room to modify its environment variables and command line arguments
367 and still be able to invoke another utility. Note that the minimum
368 {ARG_MAX} allowed by the System Interfaces volume of
369 IEEE Std 1003.1-2001 is 4096 bytes and the minimum value allowed by
370 this volume of IEEE Std 1003.1-2001 is 2048 bytes; therefore, the 2048
371 bytes difference seems reasonable. Note, however, that xargs may never
372 be able to invoke a utility if the environment passed in to xargs comes
373 close to using {ARG_MAX} bytes.
374 The version of xargs required by this volume of IEEE Std 1003.1-2001 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
411 None.
412
414 Shell Command Language, echo, find, the System Interfaces volume of
415 IEEE Std 1003.1-2001, exec
416
418 Portions of this text are reprinted and reproduced in electronic form
419 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
420 -- Portable Operating System Interface (POSIX), The Open Group Base
421 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
422 Electrical and Electronics Engineers, Inc and The Open Group. In the
423 event of any discrepancy between this version and the original IEEE and
424 The Open Group Standard, the original IEEE and The Open Group Standard
425 is the referee document. The original Standard can be obtained online
426 at http://www.opengroup.org/unix/online.html .
427IEEE/The Open Group 2003 XARGS(1P)