TRAP(1P) POSIX Programmer's Manual TRAP(1P)PROLOG
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NAMEtrap — trap signals
SYNOPSIStrap 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 (""), 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 speci‐
fied 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 exe‐
cuted 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 substitu‐
tion 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 com‐
mands 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 condi‐
tions 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.
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
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
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 histori‐
cal 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 his‐
torical 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 behav‐
ior of some historical versions of interactive shells which, for exam‐
ple, close the standard input before executing a trap on 0. For exam‐
ple, in some historical interactive shell sessions the following trap
on 0 would always print "−−":
trap 'read foo; echo "−$foo−"' 0
The command:
trap 'eval " $cmd"' 0
causes the contents of the shell variable cmd to be executed as a com‐
mand when the shell exits. Using:
trap '$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.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.14, Special Built-In Utilities
The Base Definitions volume of POSIX.1‐2008, Section 12.2, Utility Syn‐
tax 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 Electri‐
cal 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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 TRAP(1P)
