1TRAP(1P) POSIX Programmer's Manual TRAP(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
13 trap — trap signals
14
16 trap n [condition...]
17 trap [action condition...]
18
20 If the first operand is an unsigned decimal integer, the shell shall
21 treat all operands as conditions, and shall reset each condition to the
22 default value. Otherwise, if there are operands, the first is treated
23 as an action and the remaining as conditions.
24 If action is '−', the shell shall reset each condition to the default
26 value. If action is null (""), the shell shall ignore each specified
27 condition if it arises. Otherwise, the argument action shall be read
28 and executed by the shell when one of the corresponding conditions
29 arises. The action of trap shall override a previous action (either
30 default action or one explicitly set). The value of "$?" after the trap
31 action completes shall be the value it had before trap was invoked.
32 The condition can be EXIT, 0 (equivalent to EXIT), or a signal speci‐
34 fied using a symbolic name, without the SIG prefix, as listed in the
35 tables of signal names in the <signal.h> header defined in the Base
36 Definitions volume of POSIX.1‐2008, Chapter 13, Headers; for example,
37 HUP, INT, QUIT, TERM. Implementations may permit names with the SIG
38 prefix or ignore case in signal names as an extension. Setting a trap
39 for SIGKILL or SIGSTOP produces undefined results.
40 The environment in which the shell executes a trap on EXIT shall be
42 identical to the environment immediately after the last command exe‐
43 cuted before the trap on EXIT was taken.
44 Each time trap is invoked, the action argument shall be processed in a
46 manner equivalent to:
47 eval action
49 Signals that were ignored on entry to a non-interactive shell cannot be
51 trapped or reset, although no error need be reported when attempting to
52 do so. An interactive shell may reset or catch signals ignored on
53 entry. Traps shall remain in place for a given shell until explicitly
54 changed with another trap command.
55 When a subshell is entered, traps that are not being ignored shall be
57 set to the default actions, except in the case of a command substitu‐
58 tion containing only a single trap command, when the traps need not be
59 altered. Implementations may check for this case using only lexical
60 analysis; for example, if `trap` and $( trap -- ) do not alter the
61 traps in the subshell, cases such as assigning var=trap and then using
62 $($var) may still alter them. This does not imply that the trap command
63 cannot be used within the subshell to set new traps.
64 The trap command with no operands shall write to standard output a list
66 of commands associated with each condition. If the command is executed
67 in a subshell, the implementation does not perform the optional check
68 described above for a command substitution containing only a single
69 trap command, and no trap commands with operands have been executed
70 since entry to the subshell, the list shall contain the commands that
71 were associated with each condition immediately before the subshell
72 environment was entered. Otherwise, the list shall contain the com‐
73 mands currently associated with each condition. The format shall be:
74 "trap −− %s %s ...\n", <action>, <condition> ...
76 The shell shall format the output, including the proper use of quoting,
78 so that it is suitable for reinput to the shell as commands that
79 achieve the same trapping results. For example:
80 save_traps=$(trap)
82 ...
83 eval "$save_traps"
84 XSI-conformant systems also allow numeric signal numbers for the condi‐
86 tions corresponding to the following signal names:
87 1 SIGHUP
89 2 SIGINT
91 3 SIGQUIT
93 6 SIGABRT
95 9 SIGKILL
97 14 SIGALRM
99 15 SIGTERM
101 The trap special built-in shall conform to the Base Definitions volume
103 of POSIX.1‐2008, Section 12.2, Utility Syntax Guidelines.
104
106 None.
107
109 See the DESCRIPTION.
110
112 Not used.
113
115 None.
116
118 None.
119
121 Default.
122
124 See the DESCRIPTION.
125
127 The standard error shall be used only for diagnostic messages.
128
130 None.
131
133 None.
134
136 If the trap name or number is invalid, a non-zero exit status shall be
137 returned; otherwise, zero shall be returned. For both interactive and
138 non-interactive shells, invalid signal names or numbers shall not be
139 considered a syntax error and do not cause the shell to abort.
140
142 Default.
143 The following sections are informative.
145
147 None.
148
150 Write out a list of all traps and actions:
151 trap
153 Set a trap so the logout utility in the directory referred to by the
155 HOME environment variable executes when the shell terminates:
156 trap '"$HOME"/logout' EXIT
158 or:
160 trap '"$HOME"/logout' 0
162 Unset traps on INT, QUIT, TERM, and EXIT:
164 trap − INT QUIT TERM EXIT
166
168 Implementations may permit lowercase signal names as an extension.
169 Implementations may also accept the names with the SIG prefix; no known
170 historical shell does so. The trap and kill utilities in this volume of
171 POSIX.1‐2008 are now consistent in their omission of the SIG prefix for
172 signal names. Some kill implementations do not allow the prefix, and
173 kill −l lists the signals without prefixes.
174 Trapping SIGKILL or SIGSTOP is syntactically accepted by some histori‐
176 cal implementations, but it has no effect. Portable POSIX applications
177 cannot attempt to trap these signals.
178 The output format is not historical practice. Since the output of his‐
180 torical trap commands is not portable (because numeric signal values
181 are not portable) and had to change to become so, an opportunity was
182 taken to format the output in a way that a shell script could use to
183 save and then later reuse a trap if it wanted.
184 The KornShell uses an ERR trap that is triggered whenever set −e would
186 cause an exit. This is allowable as an extension, but was not mandated,
187 as other shells have not used it.
188 The text about the environment for the EXIT trap invalidates the behav‐
190 ior of some historical versions of interactive shells which, for exam‐
191 ple, close the standard input before executing a trap on 0. For exam‐
192 ple, in some historical interactive shell sessions the following trap
193 on 0 would always print "−−":
194 trap 'read foo; echo "−$foo−"' 0
196 The command:
198 trap 'eval " $cmd"' 0
200 causes the contents of the shell variable cmd to be executed as a com‐
202 mand when the shell exits. Using:
203 trap '$cmd' 0
205 does not work correctly if cmd contains any special characters such as
207 quoting or redirections. Using:
208 trap " $cmd" 0
210 also works (the leading <space> character protects against unlikely
212 cases where cmd is a decimal integer or begins with '−'), but it
213 expands the cmd variable when the trap command is executed, not when
214 the exit action is executed.
215
217 None.
218
220 Section 2.14, Special Built-In Utilities
221 The Base Definitions volume of POSIX.1‐2008, Section 12.2, Utility Syn‐
223 tax Guidelines, <signal.h>
224
226 Portions of this text are reprinted and reproduced in electronic form
227 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
228 -- Portable Operating System Interface (POSIX), The Open Group Base
229 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
230 cal and Electronics Engineers, Inc and The Open Group. (This is
231 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
232 event of any discrepancy between this version and the original IEEE and
233 The Open Group Standard, the original IEEE and The Open Group Standard
234 is the referee document. The original Standard can be obtained online
235 at http://www.unix.org/online.html .
236 Any typographical or formatting errors that appear in this page are
238 most likely to have been introduced during the conversion of the source
239 files to man page format. To report such errors, see https://www.ker‐
240 nel.org/doc/man-pages/reporting_bugs.html .
241IEEE/The Open Group 2013 TRAP(1P)