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



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

NAME
       trap -- trap signals

SYNOPSIS
       trap _n [[condition...]]
       trap [[action condition...]]

DESCRIPTION
       If the first operand is an unsigned decimal integer, the shell shall
       treat all operands as conditions, and shall reset each condition to the
       default value. Otherwise, if there are operands, the first is treated
       as an action and the remaining as conditions.

       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 POSIX.1-2017, 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 shall be
       set to the default actions, except in the case of a command
       substitution containing only a single trap command, when the traps need
       not be altered. Implementations may check for this case using only
       lexical analysis; for example, if `trap` and $( trap -- _) do not alter
       the traps in the subshell, cases such as assigning var=trap and then
       using $($var) may still alter them. This does not imply that the trap
       command cannot be used within the subshell to set new traps.

       The trap command with no operands shall write to standard output a list
       of commands associated with each condition. If the command is executed
       in a subshell, the implementation does not perform the optional check
       described above for a command substitution containing only a single
       trap command, and no trap commands with operands have been executed
       since entry to the subshell, the list shall contain the commands that
       were associated with each condition immediately before the subshell
       environment was entered.  Otherwise, the list shall contain the
       commands currently 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:

       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 POSIX.1-2017, 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
       POSIX.1-2017 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

       The command:


           trap 'eval " $cmd"' 0

       causes the contents of the shell variable cmd to be executed as a
       command when the shell exits. Using:


           trap '$cmd' 0

       does not work correctly if cmd contains any special characters such as
       quoting or redirections. Using:


           trap " $cmd" 0

       also works (the leading <space> character protects against unlikely
       cases where cmd is a decimal integer or begins with ''--''), but it
       expands the cmd variable when the trap command is executed, not when
       the exit action is executed.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.14, Special Built-In Utilities

       The Base Definitions volume of POSIX.1-2017, Section 12.2, Utility
       Syntax Guidelines, <signal.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                              TRAP(1)
