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

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(P)

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