Ubuntu Manpage: trap - trap signals

NAME

       trap - trap signals

SYNOPSIS

       trap [action condition ...]

DESCRIPTION

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 IEEE Std 1003.1-2001, 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 are set to the default actions. This does
not imply that the trap command cannot be used within the subshell to set new traps.

The trap command with no arguments shall write to standard output a list of commands 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:

Signal Number Signal Name
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 IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001 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

FUTURE DIRECTIONS

       None.

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications Issue 6, Copyright (C) 2001-2003 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 .