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 .