1COMMAND(1P) POSIX Programmer's Manual COMMAND(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 command — execute a simple command
13
15 command [-p] command_name [argument...]
16 command [-p][-v|-V] command_name
18
20 The command utility shall cause the shell to treat the arguments as a
21 simple command, suppressing the shell function lookup that is described
22 in Section 2.9.1.1, Command Search and Execution, item 1b.
23 If the command_name is the same as the name of one of the special
25 built-in utilities, the special properties in the enumerated list at
26 the beginning of Section 2.14, Special Built-In Utilities shall not
27 occur. In every other respect, if command_name is not the name of a
28 function, the effect of command (with no options) shall be the same as
29 omitting command.
30 When the -v or -V option is used, the command utility shall provide
32 information concerning how a command name is interpreted by the shell.
33
35 The command utility shall conform to the Base Definitions volume of
36 POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.
37 The following options shall be supported:
39 -p Perform the command search using a default value for PATH
41 that is guaranteed to find all of the standard utilities.
42 -v Write a string to standard output that indicates the pathname
44 or command that will be used by the shell, in the current
45 shell execution environment (see Section 2.12, Shell Execu‐
46 tion Environment), to invoke command_name, but do not invoke
47 command_name.
48 * Utilities, regular built-in utilities, command_names
50 including a <slash> character, and any implementation-
51 defined functions that are found using the PATH variable
52 (as described in Section 2.9.1.1, Command Search and Exe‐
53 cution), shall be written as absolute pathnames.
54 * Shell functions, special built-in utilities, regular
56 built-in utilities not associated with a PATH search, and
57 shell reserved words shall be written as just their
58 names.
59 * An alias shall be written as a command line that repre‐
61 sents its alias definition.
62 * Otherwise, no output shall be written and the exit status
64 shall reflect that the name was not found.
65 -V Write a string to standard output that indicates how the name
67 given in the command_name operand will be interpreted by the
68 shell, in the current shell execution environment (see Sec‐
69 tion 2.12, Shell Execution Environment), but do not invoke
70 command_name. Although the format of this string is unspeci‐
71 fied, it shall indicate in which of the following categories
72 command_name falls and shall include the information stated:
73 * Utilities, regular built-in utilities, and any implemen‐
75 tation-defined functions that are found using the PATH
76 variable (as described in Section 2.9.1.1, Command Search
77 and Execution), shall be identified as such and include
78 the absolute pathname in the string.
79 * Other shell functions shall be identified as functions.
81 * Aliases shall be identified as aliases and their defini‐
83 tions included in the string.
84 * Special built-in utilities shall be identified as special
86 built-in utilities.
87 * Regular built-in utilities not associated with a PATH
89 search shall be identified as regular built-in utilities.
90 (The term ``regular'' need not be used.)
91 * Shell reserved words shall be identified as reserved
93 words.
94
96 The following operands shall be supported:
97 argument One of the strings treated as an argument to command_name.
99 command_name
101 The name of a utility or a special built-in utility.
102
104 Not used.
105
107 None.
108
110 The following environment variables shall affect the execution of com‐
111 mand:
112 LANG Provide a default value for the internationalization vari‐
114 ables that are unset or null. (See the Base Definitions vol‐
115 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
116 ables for the precedence of internationalization variables
117 used to determine the values of locale categories.)
118 LC_ALL If set to a non-empty string value, override the values of
120 all the other internationalization variables.
121 LC_CTYPE Determine the locale for the interpretation of sequences of
123 bytes of text data as characters (for example, single-byte as
124 opposed to multi-byte characters in arguments).
125 LC_MESSAGES
127 Determine the locale that should be used to affect the format
128 and contents of diagnostic messages written to standard error
129 and informative messages written to standard output.
130 NLSPATH Determine the location of message catalogs for the processing
132 of LC_MESSAGES.
133 PATH Determine the search path used during the command search
135 described in Section 2.9.1.1, Command Search and Execution,
136 except as described under the -p option.
137
139 Default.
140
142 When the -v option is specified, standard output shall be formatted as:
143 "%s\n", <pathname or command>
146 When the -V option is specified, standard output shall be formatted as:
148 "%s\n", <unspecified>
151
153 The standard error shall be used only for diagnostic messages.
154
156 None.
157
159 None.
160
162 When the -v or -V options are specified, the following exit values
163 shall be returned:
164 0 Successful completion.
166 >0 The command_name could not be found or an error occurred.
168 Otherwise, the following exit values shall be returned:
170 126 The utility specified by command_name was found but could not be
172 invoked.
173 127 An error occurred in the command utility or the utility specified
175 by command_name could not be found.
176 Otherwise, the exit status of command shall be that of the simple com‐
178 mand specified by the arguments to command.
179
181 Default.
182 The following sections are informative.
184
186 The order for command search allows functions to override regular
187 built-ins and path searches. This utility is necessary to allow func‐
188 tions that have the same name as a utility to call the utility (instead
189 of a recursive call to the function).
190 The system default path is available using getconf; however, since get‐
192 conf may need to have the PATH set up before it can be called itself,
193 the following can be used:
194 command -p getconf PATH
197 There are some advantages to suppressing the special characteristics of
199 special built-ins on occasion. For example:
200 command exec > unwritable-file
203 does not cause a non-interactive script to abort, so that the output
205 status can be checked by the script.
206 The command, env, nohup, time, and xargs utilities have been specified
208 to use exit code 127 if an error occurs so that applications can dis‐
209 tinguish ``failure to find a utility'' from ``invoked utility exited
210 with an error indication''. The value 127 was chosen because it is not
211 commonly used for other meanings; most utilities use small values for
212 ``normal error conditions'' and the values above 128 can be confused
213 with termination due to receipt of a signal. The value 126 was chosen
214 in a similar manner to indicate that the utility could be found, but
215 not invoked. Some scripts produce meaningful error messages differenti‐
216 ating the 126 and 127 cases. The distinction between exit codes 126 and
217 127 is based on KornShell practice that uses 127 when all attempts to
218 exec the utility fail with [ENOENT], and uses 126 when any attempt to
219 exec the utility fails for any other reason.
220 Since the -v and -V options of command produce output in relation to
222 the current shell execution environment, command is generally provided
223 as a shell regular built-in. If it is called in a subshell or separate
224 utility execution environment, such as one of the following:
225 (PATH=foo command -v)
228 nohup command -v
229 it does not necessarily produce correct results. For example, when
231 called with nohup or an exec function, in a separate utility execution
232 environment, most implementations are not able to identify aliases,
233 functions, or special built-ins.
234 Two types of regular built-ins could be encountered on a system and
236 these are described separately by command. The description of command
237 search in Section 2.9.1.1, Command Search and Execution allows for a
238 standard utility to be implemented as a regular built-in as long as it
239 is found in the appropriate place in a PATH search. So, for example,
240 command -v true might yield /bin/true or some similar pathname. Other
241 implementation-defined utilities that are not defined by this volume of
242 POSIX.1‐2017 might exist only as built-ins and have no pathname associ‐
243 ated with them. These produce output identified as (regular) built-ins.
244 Applications encountering these are not able to count on execing them,
245 using them with nohup, overriding them with a different PATH, and so
246 on.
247
249 1. Make a version of cd that always prints out the new working direc‐
250 tory exactly once:
251 cd() {
254 command cd "$@" >/dev/null
255 pwd
256 }
257 2. Start off a ``secure shell script'' in which the script avoids
259 being spoofed by its parent:
260 IFS='
263 '
264 # The preceding value should be <space><tab><newline>.
265 # Set IFS to its default value.
266 \unalias -a
268 # Unset all possible aliases.
269 # Note that unalias is escaped to prevent an alias
270 # being used for unalias.
271 unset -f command
273 # Ensure command is not a user function.
274 PATH="$(command -p getconf PATH):$PATH"
276 # Put on a reliable PATH prefix.
277 # ...
279 At this point, given correct permissions on the directories called
281 by PATH, the script has the ability to ensure that any utility it
282 calls is the intended one. It is being very cautious because it
283 assumes that implementation extensions may be present that would
284 allow user functions to exist when it is invoked; this capability
285 is not specified by this volume of POSIX.1‐2017, but it is not pro‐
286 hibited as an extension. For example, the ENV variable precedes the
287 invocation of the script with a user start-up script. Such a script
288 could define functions to spoof the application.
289
291 Since command is a regular built-in utility it is always found prior to
292 the PATH search.
293 There is nothing in the description of command that implies the command
295 line is parsed any differently from that of any other simple command.
296 For example:
297 command a | b ; c
300 is not parsed in any special way that causes '|' or ';' to be treated
302 other than a pipe operator or <semicolon> or that prevents function
303 lookup on b or c.
304 The command utility is somewhat similar to the Eighth Edition shell
306 builtin command, but since command also goes to the file system to
307 search for utilities, the name builtin would not be intuitive.
308 The command utility is most likely to be provided as a regular built-
310 in. It is not listed as a special built-in for the following reasons:
311 * The removal of exportable functions made the special precedence of
313 a special built-in unnecessary.
314 * A special built-in has special properties (see Section 2.14, Spe‐
316 cial Built-In Utilities) that were inappropriate for invoking other
317 utilities. For example, two commands such as:
318 date > unwritable-file
321 command date > unwritable-file
323 would have entirely different results; in a non-interactive script,
325 the former would continue to execute the next command, the latter
326 would abort. Introducing this semantic difference along with sup‐
327 pressing functions was seen to be non-intuitive.
328 The -p option is present because it is useful to be able to ensure a
330 safe path search that finds all the standard utilities. This search
331 might not be identical to the one that occurs through one of the exec
332 functions (as defined in the System Interfaces volume of POSIX.1‐2017)
333 when PATH is unset. At the very least, this feature is required to
334 allow the script to access the correct version of getconf so that the
335 value of the default path can be accurately retrieved.
336 The command -v and -V options were added to satisfy requirements from
338 users that are currently accomplished by three different historical
339 utilities: type in the System V shell, whence in the KornShell, and
340 which in the C shell. Since there is no historical agreement on how and
341 what to accomplish here, the POSIX command utility was enhanced and the
342 historical utilities were left unmodified. The C shell which merely
343 conducts a path search. The KornShell whence is more elaborate—in addi‐
344 tion to the categories required by POSIX, it also reports on tracked
345 aliases, exported aliases, and undefined functions.
346 The output format of -V was left mostly unspecified because human users
348 are its only audience. Applications should not be written to care
349 about this information; they can use the output of -v to differentiate
350 between various types of commands, but the additional information that
351 may be emitted by the more verbose -V is not needed and should not be
352 arbitrarily constrained in its verbosity or localization for applica‐
353 tion parsing reasons.
354
356 None.
357
359 Section 2.9.1.1, Command Search and Execution, Section 2.12, Shell Exe‐
360 cution Environment, Section 2.14, Special Built-In Utilities, sh, type
361 The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment
363 Variables, Section 12.2, Utility Syntax Guidelines
364 The System Interfaces volume of POSIX.1‐2017, exec
366
368 Portions of this text are reprinted and reproduced in electronic form
369 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
370 table Operating System Interface (POSIX), The Open Group Base Specifi‐
371 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
372 Electrical and Electronics Engineers, Inc and The Open Group. In the
373 event of any discrepancy between this version and the original IEEE and
374 The Open Group Standard, the original IEEE and The Open Group Standard
375 is the referee document. The original Standard can be obtained online
376 at http://www.opengroup.org/unix/online.html .
377 Any typographical or formatting errors that appear in this page are
379 most likely to have been introduced during the conversion of the source
380 files to man page format. To report such errors, see https://www.ker‐
381 nel.org/doc/man-pages/reporting_bugs.html .
382IEEE/The Open Group 2017 COMMAND(1P)