1SET(P) POSIX Programmer's Manual SET(P)
2
6 set - set or unset options and positional parameters
7
9 set [-abCefmnuvx][-h][-o option][argument...]
10 set [+abCefmnuvx][+h][+o option][argument...]
12 set -- [argument...]
14 set -o
16 set +o
18
21 If no options or arguments are specified, set shall write the names and
22 values of all shell variables in the collation sequence of the current
23 locale. Each name shall start on a separate line, using the format:
24 "%s=%s\n", <name>, <value>
27 The value string shall be written with appropriate quoting; see the
29 description of shell quoting in Quoting . The output shall be suitable
30 for reinput to the shell, setting or resetting, as far as possible, the
31 variables that are currently set; read-only variables cannot be reset.
32 When options are specified, they shall set or unset attributes of the
34 shell, as described below. When arguments are specified, they cause
35 positional parameters to be set or unset, as described below. Setting
36 or unsetting attributes and positional parameters are not necessarily
37 related actions, but they can be combined in a single invocation of
38 set.
39 The set special built-in shall support the Base Definitions volume of
41 IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines except
42 that options can be specified with either a leading hyphen (meaning
43 enable the option) or plus sign (meaning disable it) unless otherwise
44 specified.
45 Implementations shall support the options in the following list in both
47 their hyphen and plus-sign forms. These options can also be specified
48 as options to sh.
49 -a When this option is on, the export attribute shall be set for
51 each variable to which an assignment is performed; see the Base
52 Definitions volume of IEEE Std 1003.1-2001, Section 4.21, Vari‐
53 able Assignment. If the assignment precedes a utility name in a
54 command, the export attribute shall not persist in the current
55 execution environment after the utility completes, with the
56 exception that preceding one of the special built-in utilities
57 causes the export attribute to persist after the built-in has
58 completed. If the assignment does not precede a utility name in
59 the command, or if the assignment is a result of the operation
60 of the getopts or read utilities, the export attribute shall
61 persist until the variable is unset.
62 -b This option shall be supported if the implementation supports
64 the User Portability Utilities option. It shall cause the shell
65 to notify the user asynchronously of background job completions.
66 The following message is written to standard error:
67 "[%d]%c %s%s\n", <job-number>, <current>, <status>, <job-name>
70 where the fields shall be as follows:
72 <current>
74 The character '+' identifies the job that would be used as a
75 default for the fg or bg utilities; this job can also be speci‐
76 fied using the job_id "%+" or "%%" . The character '-' identi‐
77 fies the job that would become the default if the current
78 default job were to exit; this job can also be specified using
79 the job_id "%-" . For other jobs, this field is a <space>. At
80 most one job can be identified with '+' and at most one job can
81 be identified with '-' . If there is any suspended job, then the
82 current job shall be a suspended job. If there are at least two
83 suspended jobs, then the previous job also shall be a suspended
84 job.
85 <job-number>
87 A number that can be used to identify the process group to the
88 wait, fg, bg, and kill utilities. Using these utilities, the job
89 can be identified by prefixing the job number with '%' .
90 <status>
92 Unspecified.
93 <job-name>
95 Unspecified.
96 When the shell notifies the user a job has been completed, it may
99 remove the job's process ID from the list of those known in the current
100 shell execution environment; see Asynchronous Lists . Asynchronous
101 notification shall not be enabled by default.
102 -C (Uppercase C.) Prevent existing files from being overwritten by
104 the shell's '>' redirection operator (see Redirecting Output );
105 the ">|" redirection operator shall override this noclobber
106 option for an individual file.
107 -e When this option is on, if a simple command fails for any of the
109 reasons listed in Consequences of Shell Errors or returns an
110 exit status value >0, and is not part of the compound list fol‐
111 lowing a while, until, or if keyword, and is not a part of an
112 AND or OR list, and is not a pipeline preceded by the ! reserved
113 word, then the shell shall immediately exit.
114 -f The shell shall disable pathname expansion.
116 -h Locate and remember utilities invoked by functions as those
118 functions are defined (the utilities are normally located when
119 the function is executed).
120 -m This option shall be supported if the implementation supports
122 the User Portability Utilities option. All jobs shall be run in
123 their own process groups. Immediately before the shell issues a
124 prompt after completion of the background job, a message report‐
125 ing the exit status of the background job shall be written to
126 standard error. If a foreground job stops, the shell shall write
127 a message to standard error to that effect, formatted as
128 described by the jobs utility. In addition, if a job changes
129 status other than exiting (for example, if it stops for input or
130 output or is stopped by a SIGSTOP signal), the shell shall write
131 a similar message immediately prior to writing the next prompt.
132 This option is enabled by default for interactive shells.
133 -n The shell shall read commands but does not execute them; this
135 can be used to check for shell script syntax errors. An interac‐
136 tive shell may ignore this option.
137 -o Write the current settings of the options to standard output in
139 an unspecified format.
140 +o Write the current option settings to standard output in a format
142 that is suitable for reinput to the shell as commands that
143 achieve the same options settings.
144 -o option
146 This option is supported if the system supports the User Porta‐
148 bility Utilities option. It shall set various options, many of
149 which shall be equivalent to the single option letters. The fol‐
150 lowing values of option shall be supported:
151 allexport
153 Equivalent to -a.
154 errexit
156 Equivalent to -e.
157 ignoreeof
159 Prevent an interactive shell from exiting on end-of-file. This
160 setting prevents accidental logouts when <control>-D is entered.
161 A user shall explicitly exit to leave the interactive shell.
162 monitor
164 Equivalent to -m. This option is supported if the system sup‐
165 ports the User Portability Utilities option.
166 noclobber
168 Equivalent to -C (uppercase C).
169 noglob
171 Equivalent to -f.
172 noexec
174 Equivalent to -n.
175 nolog
177 Prevent the entry of function definitions into the command his‐
178 tory; see Command History List .
179 notify
181 Equivalent to -b.
182 nounset
184 Equivalent to -u.
185 verbose
187 Equivalent to -v.
188 vi
190 Allow shell command line editing using the built-in vi editor.
191 Enabling vi mode shall disable any other command line editing
192 mode provided as an implementation extension.
193 It need not be possible to set vi mode on for certain block-mode
195 terminals.
196 xtrace
198 Equivalent to -x.
199 -u The shell shall write a message to standard error when it tries
202 to expand a variable that is not set and immediately exit. An
203 interactive shell shall not exit.
204 -v The shell shall write its input to standard error as it is read.
206 -x The shell shall write to standard error a trace for each command
208 after it expands the command and before it executes it. It is
209 unspecified whether the command that turns tracing off is
210 traced.
211 The default for all these options shall be off (unset) unless stated
214 otherwise in the description of the option or unless the shell was
215 invoked with them on; see sh.
216 The remaining arguments shall be assigned in order to the positional
218 parameters. The special parameter '#' shall be set to reflect the num‐
219 ber of positional parameters. All positional parameters shall be unset
220 before any new values are assigned.
221 The special argument "--" immediately following the set command name
223 can be used to delimit the arguments if the first argument begins with
224 '+' or '-' , or to prevent inadvertent listing of all shell variables
225 when there are no arguments. The command set -- without argument shall
226 unset all positional parameters and set the special parameter '#' to
227 zero.
228
230 See the DESCRIPTION.
231
233 See the DESCRIPTION.
234
236 Not used.
237
239 None.
240
242 None.
243
245 Default.
246
248 See the DESCRIPTION.
249
251 The standard error shall be used only for diagnostic messages.
252
254 None.
255
257 None.
258
260 Zero.
261
263 Default.
264 The following sections are informative.
266
268 None.
269
271 Write out all variables and their values:
272 set
275 Set $1, $2, and $3 and set "$#" to 3:
277 set c a b
280 Turn on the -x and -v options:
282 set -xv
285 Unset all positional parameters:
287 set --
290 Set $1 to the value of x, even if it begins with '-' or '+' :
292 set -- "$x"
295 Set the positional parameters to the expansion of x, even if x expands
297 with a leading '-' or '+' :
298 set -- $x
301
303 The set -- form is listed specifically in the SYNOPSIS even though this
304 usage is implied by the Utility Syntax Guidelines. The explanation of
305 this feature removes any ambiguity about whether the set -- form might
306 be misinterpreted as being equivalent to set without any options or
307 arguments. The functionality of this form has been adopted from the
308 KornShell. In System V, set -- only unsets parameters if there is at
309 least one argument; the only way to unset all parameters is to use
310 shift. Using the KornShell version should not affect System V scripts
311 because there should be no reason to issue it without arguments delib‐
312 erately; if it were issued as, for example:
313 set -- "$@"
316 and there were in fact no arguments resulting from "$@" , unsetting the
318 parameters would have no result.
319 The set + form in early proposals was omitted as being an unnecessary
321 duplication of set alone and not widespread historical practice.
322 The noclobber option was changed to allow set -C as well as the set -o
324 noclobber option. The single-letter version was added so that the his‐
325 torical "$-" paradigm would not be broken; see Special Parameters .
326 The -h flag is related to command name hashing and is only required on
328 XSI-conformant systems.
329 The following set flags were omitted intentionally with the following
331 rationale:
332 -k The -k flag was originally added by the author of the Bourne
334 shell to make it easier for users of pre-release versions of the
335 shell. In early versions of the Bourne shell the construct set
336 name= value had to be used to assign values to shell variables.
337 The problem with -k is that the behavior affects parsing, virtu‐
338 ally precluding writing any compilers. To explain the behavior
339 of -k, it is necessary to describe the parsing algorithm, which
340 is implementation-defined. For example:
341 set -k; echo name=value
344 and:
346 set -k
349 echo name=value
350 behave differently. The interaction with functions is even more com‐
352 plex. What is more, the -k flag is never needed, since the command
353 line could have been reordered.
354 -t The -t flag is hard to specify and almost never used. The only
356 known use could be done with here-documents. Moreover, the
357 behavior with ksh and sh differs. The reference page says that
358 it exits after reading and executing one command. What is one
359 command? If the input is date; date, sh executes both date com‐
360 mands while ksh does only the first.
361 Consideration was given to rewriting set to simplify its confusing syn‐
364 tax. A specific suggestion was that the unset utility should be used to
365 unset options instead of using the non- getopt() -able + option syntax.
366 However, the conclusion was reached that the historical practice of
367 using + option was satisfactory and that there was no compelling reason
368 to modify such widespread historical practice.
369 The -o option was adopted from the KornShell to address user needs. In
371 addition to its generally friendly interface, -o is needed to provide
372 the vi command line editing mode, for which historical practice yields
373 no single-letter option name. (Although it might have been possible to
374 invent such a letter, it was recognized that other editing modes would
375 be developed and -o provides ample name space for describing such
376 extensions.)
377 Historical implementations are inconsistent in the format used for -o
379 option status reporting. The +o format without an option-argument was
380 added to allow portable access to the options that can be saved and
381 then later restored using, for instance, a dot script.
382 Historically, sh did trace the command set +x, but ksh did not.
384 The ignoreeof setting prevents accidental logouts when the end-of-file
386 character (typically <control>-D) is entered. A user shall explicitly
387 exit to leave the interactive shell.
388 The set -m option was added to apply only to the UPE because it applies
390 primarily to interactive use, not shell script applications.
391 The ability to do asynchronous notification became available in the
393 1988 version of the KornShell. To have it occur, the user had to issue
394 the command:
395 trap "jobs -n" CLD
398 The C shell provides two different levels of an asynchronous notifica‐
400 tion capability. The environment variable notify is analogous to what
401 is done in set -b or set -o notify. When set, it notifies the user
402 immediately of background job completions. When unset, this capability
403 is turned off.
404 The other notification ability comes through the built-in utility
406 notify. The syntax is:
407 notify [%job ... ]
410 By issuing notify with no operands, it causes the C shell to notify the
412 user asynchronously when the state of the current job changes. If given
413 operands, notify asynchronously informs the user of changes in the
414 states of the specified jobs.
415 To add asynchronous notification to the POSIX shell, neither the Korn‐
417 Shell extensions to trap, nor the C shell notify environment variable
418 seemed appropriate ( notify is not a proper POSIX environment variable
419 name).
420 The set -b option was selected as a compromise.
422 The notify built-in was considered to have more functionality than was
424 required for simple asynchronous notification.
425
427 None.
428
430 Special Built-In Utilities
431
433 Portions of this text are reprinted and reproduced in electronic form
434 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
435 -- Portable Operating System Interface (POSIX), The Open Group Base
436 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
437 Electrical and Electronics Engineers, Inc and The Open Group. In the
438 event of any discrepancy between this version and the original IEEE and
439 The Open Group Standard, the original IEEE and The Open Group Standard
440 is the referee document. The original Standard can be obtained online
441 at http://www.opengroup.org/unix/online.html .
442IEEE/The Open Group 2003 SET(P)