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TRAP(1P) POSIX Programmer's Manual TRAP(1P)
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.
trap — trap signals
trap n [condition...]
trap [action condition...]
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 (""), 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 "$?" 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.1‐2017, 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
Signals that were ignored on entry to a non-interactive shell
cannot be trapped or reset, although no error need be reported
when attempting to do so. An interactive shell may reset or catch
signals ignored on entry. Traps shall remain in place for a given
shell until explicitly changed with another trap command.
When a subshell is entered, traps that are not being ignored
shall be set to the default actions, except in the case of a
command substitution containing only a single trap command, when
the traps need not be altered. Implementations may check for this
case using only lexical analysis; for example, if `trap` and $(
trap -- ) do not alter the traps in the subshell, cases such as
assigning var=trap and then using $($var) may still alter them.
This does not imply that the trap command cannot be used within
the subshell to set new traps.
The trap command with no operands shall write to standard output
a list of commands associated with each condition. If the command
is executed in a subshell, the implementation does not perform
the optional check described above for a command substitution
containing only a single trap command, and no trap commands with
operands have been executed since entry to the subshell, the list
shall contain the commands that were associated with each
condition immediately before the subshell environment was
entered. Otherwise, the list shall contain the commands
currently associated with each condition. The format shall be:
"trap -- %s %s ...\n", <action>, <condition> ...
The shell shall format the output, including the proper use of
quoting, so that it is suitable for reinput to the shell as
commands that achieve the same trapping results. For example:
save_traps=$(trap)
...
eval "$save_traps"
XSI-conformant systems also allow numeric signal numbers for the
conditions corresponding to the following signal names:
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.1‐2017, Section 12.2, Utility Syntax Guidelines.
None.
See the DESCRIPTION.
Not used.
None.
None.
Default.
See the DESCRIPTION.
The standard error shall be used only for diagnostic messages.
None.
None.
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.
Default.
The following sections are informative.
None.
Write out a list of all traps and actions:
trap
Set a trap so the logout utility in the directory referred to by
the HOME environment variable executes when the shell terminates:
trap '"$HOME"/logout' EXIT
or:
trap '"$HOME"/logout' 0
Unset traps on INT, QUIT, TERM, and EXIT:
trap - INT QUIT TERM EXIT
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.1‐2017 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 "--":
trap 'read foo; echo "-$foo-"' 0
The command:
trap 'eval " $cmd"' 0
causes the contents of the shell variable cmd to be executed as a
command when the shell exits. Using:
trap '$cmd' 0
does not work correctly if cmd contains any special characters
such as quoting or redirections. Using:
trap " $cmd" 0
also works (the leading <space> character protects against
unlikely cases where cmd is a decimal integer or begins with
'-'), but it expands the cmd variable when the trap command is
executed, not when the exit action is executed.
None.
Section 2.14, Special Built-In Utilities
The Base Definitions volume of POSIX.1‐2017, Section 12.2,
Utility Syntax Guidelines, signal.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. 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.opengroup.org/unix/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 .
IEEE/The Open Group 2017 TRAP(1P)
Pages that refer to this page: sh(1p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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