trap(1p) — Linux manual page


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

       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:

           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         top


OPERANDS         top

       See the DESCRIPTION.

STDIN         top

       Not used.

INPUT FILES         top






STDOUT         top

       See the DESCRIPTION.

STDERR         top

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES         top




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.



       The following sections are informative.



EXAMPLES         top

       Write out a list of all traps and actions:


       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


           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.1‐2017 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

       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.



SEE ALSO         top

       Section 2.14, Special Built-In Utilities

       The Base Definitions volume of POSIX.1‐2017, 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 1003.1-2017, 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 .

       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 .

IEEE/The Open Group               2017                          TRAP(1P)

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