TRAP(1P)						     POSIX Programmer's Manual							  TRAP(1P)

PROLOG

       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

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  speci-
       fied  condition	if  it arises. Otherwise, the argument action shall be read and executed by the shell when one of the corresponding condi-
       tions 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  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 attempt-
       ing to do so. An interactive shell may reset or catch signals ignored on entry. Traps shall remain in place for a given shell until explic-
       itly 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 can-
       not 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 ...
", <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 man-
       dated, 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.

SEE ALSO

       Special Built-In Utilities

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- 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 .

IEEE
/The Open Group						       2003								  TRAP(1P)