PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       trap — trap signals

SYNOPSIS         top

       trap n [condition...]
       trap [action condition...]

DESCRIPTION         top

       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.1‐2008, 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.1‐2008, Section 12.2, Utility Syntax Guidelines.

OPTIONS         top

       None.

OPERANDS         top

       See the DESCRIPTION.

STDIN         top

       Not used.

INPUT FILES         top

       None.

ENVIRONMENT VARIABLES         top

       None.

ASYNCHRONOUS EVENTS         top

       Default.

STDOUT         top

       See the DESCRIPTION.

STDERR         top

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES         top

       None.

EXTENDED DESCRIPTION         top

       None.

EXIT STATUS         top

       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         top

       Default.

       The following sections are informative.

APPLICATION USAGE         top

       None.

EXAMPLES         top

       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         top

       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.1‐2008 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         top

       None.

SEE ALSO         top

       Section 2.14, Special Built-In Utilities

       The Base Definitions volume of POSIX.1‐2008, Section 12.2, Utility
       Syntax Guidelines, signal.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                            TRAP(1P)

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