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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.1‐2008, 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
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.1‐2008, 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.1‐2008 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 "−−":
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.1‐2008, 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, 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 .
IEEE/The Open Group 2013 TRAP(1P)
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