1WAIT(1P) POSIX Programmer's Manual WAIT(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 wait — await process completion
13
15 wait [pid...]
16
18 When an asynchronous list (see Section 2.9.3.1, Examples) is started by
19 the shell, the process ID of the last command in each element of the
20 asynchronous list shall become known in the current shell execution
21 environment; see Section 2.12, Shell Execution Environment.
22 If the wait utility is invoked with no operands, it shall wait until
24 all process IDs known to the invoking shell have terminated and exit
25 with a zero exit status.
26 If one or more pid operands are specified that represent known process
28 IDs, the wait utility shall wait until all of them have terminated. If
29 one or more pid operands are specified that represent unknown process
30 IDs, wait shall treat them as if they were known process IDs that
31 exited with exit status 127. The exit status returned by the wait util‐
32 ity shall be the exit status of the process requested by the last pid
33 operand.
34 The known process IDs are applicable only for invocations of wait in
36 the current shell execution environment.
37
39 None.
40
42 The following operand shall be supported:
43 pid One of the following:
45 1. The unsigned decimal integer process ID of a command, for
47 which the utility is to wait for the termination.
48 2. A job control job ID (see the Base Definitions volume of
50 POSIX.1‐2017, Section 3.204, Job Control Job ID) that
51 identifies a background process group to be waited for.
52 The job control job ID notation is applicable only for
53 invocations of wait in the current shell execution envi‐
54 ronment; see Section 2.12, Shell Execution Environment.
55 The exit status of wait shall be determined by the last
56 command in the pipeline.
57 Note: The job control job ID type of pid is only
59 available on systems supporting the User Porta‐
60 bility Utilities option.
61
63 Not used.
64
66 None.
67
69 The following environment variables shall affect the execution of wait:
70 LANG Provide a default value for the internationalization vari‐
72 ables that are unset or null. (See the Base Definitions vol‐
73 ume of POSIX.1‐2017, Section 8.2, Internationalization Vari‐
74 ables for the precedence of internationalization variables
75 used to determine the values of locale categories.)
76 LC_ALL If set to a non-empty string value, override the values of
78 all the other internationalization variables.
79 LC_CTYPE Determine the locale for the interpretation of sequences of
81 bytes of text data as characters (for example, single-byte as
82 opposed to multi-byte characters in arguments).
83 LC_MESSAGES
85 Determine the locale that should be used to affect the format
86 and contents of diagnostic messages written to standard
87 error.
88 NLSPATH Determine the location of message catalogs for the processing
90 of LC_MESSAGES.
91
93 Default.
94
96 Not used.
97
99 The standard error shall be used only for diagnostic messages.
100
102 None.
103
105 None.
106
108 If one or more operands were specified, all of them have terminated or
109 were not known by the invoking shell, and the status of the last oper‐
110 and specified is known, then the exit status of wait shall be the exit
111 status information of the command indicated by the last operand speci‐
112 fied. If the process terminated abnormally due to the receipt of a sig‐
113 nal, the exit status shall be greater than 128 and shall be distinct
114 from the exit status generated by other signals, but the exact value is
115 unspecified. (See the kill -l option.) Otherwise, the wait utility
116 shall exit with one of the following values:
117 0 The wait utility was invoked with no operands and all process
119 IDs known by the invoking shell have terminated.
120 1‐126 The wait utility detected an error.
122 127 The command identified by the last pid operand specified is
124 unknown.
125
127 Default.
128 The following sections are informative.
130
132 On most implementations, wait is a shell built-in. If it is called in a
133 subshell or separate utility execution environment, such as one of the
134 following:
135 (wait)
138 nohup wait ...
139 find . -exec wait ... \;
140 it returns immediately because there are no known process IDs to wait
142 for in those environments.
143 Historical implementations of interactive shells have discarded the
145 exit status of terminated background processes before each shell
146 prompt. Therefore, the status of background processes was usually lost
147 unless it terminated while wait was waiting for it. This could be a
148 serious problem when a job that was expected to run for a long time
149 actually terminated quickly with a syntax or initialization error
150 because the exit status returned was usually zero if the requested
151 process ID was not found. This volume of POSIX.1‐2017 requires the
152 implementation to keep the status of terminated jobs available until
153 the status is requested, so that scripts like:
154 j1&
157 p1=$!
158 j2&
159 wait $p1
160 echo Job 1 exited with status $?
161 wait $!
162 echo Job 2 exited with status $?
163 work without losing status on any of the jobs. The shell is allowed to
165 discard the status of any process if it determines that the application
166 cannot get the process ID for that process from the shell. It is also
167 required to remember only {CHILD_MAX} number of processes in this way.
168 Since the only way to get the process ID from the shell is by using the
169 '!' shell parameter, the shell is allowed to discard the status of an
170 asynchronous list if "$!" was not referenced before another asynchro‐
171 nous list was started. (This means that the shell only has to keep the
172 status of the last asynchronous list started if the application did not
173 reference "$!". If the implementation of the shell is smart enough to
174 determine that a reference to "$!" was not saved anywhere that the
175 application can retrieve it later, it can use this information to trim
176 the list of saved information. Note also that a successful call to wait
177 with no operands discards the exit status of all asynchronous lists.)
178 If the exit status of wait is greater than 128, there is no way for the
180 application to know if the waited-for process exited with that value or
181 was killed by a signal. Since most utilities exit with small values,
182 there is seldom any ambiguity. Even in the ambiguous cases, most appli‐
183 cations just need to know that the asynchronous job failed; it does not
184 matter whether it detected an error and failed or was killed and did
185 not complete its job normally.
186
188 Although the exact value used when a process is terminated by a signal
189 is unspecified, if it is known that a signal terminated a process, a
190 script can still reliably determine which signal by using kill as shown
191 by the following script:
192 sleep 1000&
195 pid=$!
196 kill -kill $pid
197 wait $pid
198 echo $pid was terminated by a SIG$(kill -l $?) signal.
199 If the following sequence of commands is run in less than 31 seconds:
201 sleep 257 | sleep 31 &
204 jobs -l %%
205 either of the following commands returns the exit status of the second
207 sleep in the pipeline:
208 wait <pid of sleep 31>
211 wait %%
212
214 The description of wait does not refer to the waitpid() function from
215 the System Interfaces volume of POSIX.1‐2017 because that would need‐
216 lessly overspecify this interface. However, the wording means that wait
217 is required to wait for an explicit process when it is given an argu‐
218 ment so that the status information of other processes is not consumed.
219 Historical implementations use the wait() function defined in the Sys‐
220 tem Interfaces volume of POSIX.1‐2017 until wait() returns the
221 requested process ID or finds that the requested process does not
222 exist. Because this means that a shell script could not reliably get
223 the status of all background children if a second background job was
224 ever started before the first job finished, it is recommended that the
225 wait utility use a method such as the functionality provided by the
226 waitpid() function.
227 The ability to wait for multiple pid operands was adopted from the
229 KornShell.
230 This new functionality was added because it is needed to determine the
232 exit status of any asynchronous list accurately. The only compatibility
233 problem that this change creates is for a script like
234 while sleep 60 do
237 job& echo Job started $(date) as $! done
238 which causes the shell to monitor all of the jobs started until the
240 script terminates or runs out of memory. This would not be a problem if
241 the loop did not reference "$!" or if the script would occasionally
242 wait for jobs it started.
243
245 None.
246
248 Chapter 2, Shell Command Language, kill, sh
249 The Base Definitions volume of POSIX.1‐2017, Section 3.204, Job Control
251 Job ID, Chapter 8, Environment Variables
252 The System Interfaces volume of POSIX.1‐2017, wait()
254
256 Portions of this text are reprinted and reproduced in electronic form
257 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
258 table Operating System Interface (POSIX), The Open Group Base Specifi‐
259 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
260 Electrical and Electronics Engineers, Inc and The Open Group. In the
261 event of any discrepancy between this version and the original IEEE and
262 The Open Group Standard, the original IEEE and The Open Group Standard
263 is the referee document. The original Standard can be obtained online
264 at http://www.opengroup.org/unix/online.html .
265 Any typographical or formatting errors that appear in this page are
267 most likely to have been introduced during the conversion of the source
268 files to man page format. To report such errors, see https://www.ker‐
269 nel.org/doc/man-pages/reporting_bugs.html .
270IEEE/The Open Group 2017 WAIT(1P)