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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 dif-
       fer (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 exe-
       cuted  by the shell when one of the corresponding conditions arises. The action of trap shall override a previ-
       ous 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 con-
       dition. 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 sig-
       nal 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 con-
       sidered 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  imple-
       mentations 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 inter-
       active 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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  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)