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PROGRAM:

NAME

trap — trap signals

SYNOPSIS

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 (""), 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.

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 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.

FUTURE DIRECTIONS

None.

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