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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  corresponding
       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 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  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 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 condi-
       tions 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 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
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 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 .

IEEE/The Open Group                  2003                             TRAP(1P)