1TRAP(P) POSIX Programmer's Manual TRAP(P)
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6 trap - trap signals
7
9 trap [action condition ...]
10
12 If action is '-' , the shell shall reset each condition to the default
13 value. If action is null ( "" ), the shell shall ignore each specified
14 condition if it arises. Otherwise, the argument action shall be read
15 and executed by the shell when one of the corresponding conditions
16 arises. The action of trap shall override a previous action (either
17 default action or one explicitly set). The value of "$?" after the trap
18 action completes shall be the value it had before trap was invoked.
19 The condition can be EXIT, 0 (equivalent to EXIT), or a signal speci‐
21 fied using a symbolic name, without the SIG prefix, as listed in the
22 tables of signal names in the <signal.h> header defined in the Base
23 Definitions volume of IEEE Std 1003.1-2001, Chapter 13, Headers; for
24 example, HUP, INT, QUIT, TERM. Implementations may permit names with
25 the SIG prefix or ignore case in signal names as an extension. Setting
26 a trap for SIGKILL or SIGSTOP produces undefined results.
27 The environment in which the shell executes a trap on EXIT shall be
29 identical to the environment immediately after the last command exe‐
30 cuted before the trap on EXIT was taken.
31 Each time trap is invoked, the action argument shall be processed in a
33 manner equivalent to:
34 eval action
37 Signals that were ignored on entry to a non-interactive shell cannot be
39 trapped or reset, although no error need be reported when attempting to
40 do so. An interactive shell may reset or catch signals ignored on
41 entry. Traps shall remain in place for a given shell until explicitly
42 changed with another trap command.
43 When a subshell is entered, traps that are not being ignored are set to
45 the default actions. This does not imply that the trap command cannot
46 be used within the subshell to set new traps.
47 The trap command with no arguments shall write to standard output a
49 list of commands associated with each condition. The format shall be:
50 "trap -- %s %s ...\n", <action>, <condition> ...
53 The shell shall format the output, including the proper use of quoting,
55 so that it is suitable for reinput to the shell as commands that
56 achieve the same trapping results. For example:
57 save_traps=$(trap)
60 ...
61 eval "$save_traps"
62 XSI-conformant systems also allow numeric signal numbers for the condi‐
64 tions corresponding to the following signal names:
65 Signal Number Signal Name
68 1 SIGHUP
69 2 SIGINT
70 3 SIGQUIT
71 6 SIGABRT
72 9 SIGKILL
73 14 SIGALRM
74 15 SIGTERM
75 The trap special built-in shall conform to the Base Definitions volume
77 of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
78
80 None.
81
83 See the DESCRIPTION.
84
86 Not used.
87
89 None.
90
92 None.
93
95 Default.
96
98 See the DESCRIPTION.
99
101 The standard error shall be used only for diagnostic messages.
102
104 None.
105
107 None.
108
110 If the trap name or number is invalid, a non-zero exit status shall
111 be returned; otherwise, zero shall be returned. For both interactive
112 and non-interactive shells, invalid signal names or numbers shall
113 not be considered a syntax error and do not cause the shell to abort.
114
116 Default.
117 The following sections are informative.
119
121 None.
122
124 Write out a list of all traps and actions:
125 trap
128 Set a trap so the logout utility in the directory referred to by the
130 HOME environment variable executes when the shell terminates:
131 trap '$HOME/logout' EXIT
134 or:
136 trap '$HOME/logout' 0
139 Unset traps on INT, QUIT, TERM, and EXIT:
141 trap - INT QUIT TERM EXIT
144
146 Implementations may permit lowercase signal names as an extension.
147 Implementations may also accept the names with the SIG prefix; no known
148 historical shell does so. The trap and kill utilities in this volume of
149 IEEE Std 1003.1-2001 are now consistent in their omission of the SIG
150 prefix for signal names. Some kill implementations do not allow the
151 prefix, and kill -l lists the signals without prefixes.
152 Trapping SIGKILL or SIGSTOP is syntactically accepted by some histori‐
154 cal implementations, but it has no effect. Portable POSIX applications
155 cannot attempt to trap these signals.
156 The output format is not historical practice. Since the output of his‐
158 torical trap commands is not portable (because numeric signal values
159 are not portable) and had to change to become so, an opportunity was
160 taken to format the output in a way that a shell script could use to
161 save and then later reuse a trap if it wanted.
162 The KornShell uses an ERR trap that is triggered whenever set -e would
164 cause an exit. This is allowable as an extension, but was not mandated,
165 as other shells have not used it.
166 The text about the environment for the EXIT trap invalidates the behav‐
168 ior of some historical versions of interactive shells which, for exam‐
169 ple, close the standard input before executing a trap on 0. For exam‐
170 ple, in some historical interactive shell sessions the following trap
171 on 0 would always print "--" :
172 trap 'read foo; echo "-$foo-"' 0
175
177 None.
178
180 Special Built-In Utilities
181
183 Portions of this text are reprinted and reproduced in electronic form
184 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
185 -- Portable Operating System Interface (POSIX), The Open Group Base
186 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
187 Electrical and Electronics Engineers, Inc and The Open Group. In the
188 event of any discrepancy between this version and the original IEEE and
189 The Open Group Standard, the original IEEE and The Open Group Standard
190 is the referee document. The original Standard can be obtained online
191 at http://www.opengroup.org/unix/online.html .
192IEEE/The Open Group 2003 TRAP(P)