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TRAP
Section: POSIX Programmer's Manual (1P)Updated: 2013
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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.NAME
trap --- trap signalsSYNOPSIS
trap n [condition...] trap [action condition...]
DESCRIPTION
If the first operand is an unsigned decimal integer, the shell shall treat all operands as conditions, and shall reset each condition to the default value. Otherwise, if there are operands, the first is treated as an action and the remaining as conditions. If action is '-', the shell shall reset each condition to the default value. If action is null (dqdq), the shell shall ignore each specified condition if it arises. Otherwise, the argument action shall be read and executed by the shell when one of the corresponding conditions arises. The action of trap shall override a previous action (either default action or one explicitly set). The value of dq$?dq after the trap action completes shall be the value it had before trap was invoked. The condition can be EXIT, 0 (equivalent to EXIT), or a signal specified using a symbolic name, without the SIG prefix, as listed in the tables of signal names in the <signal.h> header defined in the Base Definitions volume of POSIX.1-2008, Chapter 13, Headers; for example, HUP, INT, QUIT, TERM. Implementations may permit names with the SIG prefix or ignore case in signal names as an extension. Setting a trap for SIGKILL or SIGSTOP produces undefined results. The environment in which the shell executes a trap on EXIT shall be identical to the environment immediately after the last command executed before the trap on EXIT was taken. Each time trap is invoked, the action argument shall be processed in a manner equivalent to:
-
eval action
-
"trap -- %s %s ...\n", <action>, <condition> ...
-
save_traps=$(trap) ... eval "$save_traps"
- 1
- SIGHUP
- 2
- SIGINT
- 3
- SIGQUIT
- 6
- SIGABRT
- 9
- SIGKILL
- 14
- SIGALRM
- 15
- SIGTERM The trap special built-in shall conform to the Base Definitions volume of POSIX.1-2008, Section 12.2, Utility Syntax Guidelines.
OPTIONS
None.OPERANDS
See the DESCRIPTION.STDIN
Not used.INPUT FILES
None.ENVIRONMENT VARIABLES
None.ASYNCHRONOUS EVENTS
Default.STDOUT
See the DESCRIPTION.STDERR
The standard error shall be used only for diagnostic messages.OUTPUT FILES
None.EXTENDED DESCRIPTION
None.EXIT STATUS
If the trap name or number is invalid, a non-zero exit status shall be returned; otherwise, zero shall be returned. For both interactive and non-interactive shells, invalid signal names or numbers shall not be considered a syntax error and do not cause the shell to abort.CONSEQUENCES OF ERRORS
Default.The following sections are informative.
APPLICATION USAGE
None.EXAMPLES
Write out a list of all traps and actions:
-
trap
-
trap '"$HOME"/logout' EXIT
-
trap '"$HOME"/logout' 0
-
trap - INT QUIT TERM EXIT
RATIONALE
Implementations may permit lowercase signal names as an extension. Implementations may also accept the names with the SIG prefix; no known historical shell does so. The trap and kill utilities in this volume of POSIX.1-2008 are now consistent in their omission of the SIG prefix for signal names. Some kill implementations do not allow the prefix, and kill -l lists the signals without prefixes. Trapping SIGKILL or SIGSTOP is syntactically accepted by some historical implementations, but it has no effect. Portable POSIX applications cannot attempt to trap these signals. The output format is not historical practice. Since the output of historical trap commands is not portable (because numeric signal values are not portable) and had to change to become so, an opportunity was taken to format the output in a way that a shell script could use to save and then later reuse a trap if it wanted. The KornShell uses an ERR trap that is triggered whenever set -e would cause an exit. This is allowable as an extension, but was not mandated, as other shells have not used it. The text about the environment for the EXIT trap invalidates the behavior of some historical versions of interactive shells which, for example, close the standard input before executing a trap on 0. For example, in some historical interactive shell sessions the following trap on 0 would always print dq--dq:
-
trap 'read foo; echo "-$foo-"' 0
-
trap 'eval " $cmd"' 0
-
trap '$cmd' 0
-
trap " $cmd" 0
FUTURE DIRECTIONS
None.SEE ALSO
Section 2.14, Special Built-In Utilities The Base Definitions volume of POSIX.1-2008, Section 12.2, Utility Syntax Guidelines, <signal.h>COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .