Originální popis anglicky:
trap - trap signals
Návod, kniha: POSIX Programmer's Manual
trap [action condition ...]
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
"$?" after the
trap action completes shall be the
value it had before
trap was invoked.
The condition can be EXIT, 0 (equivalent to EXIT), or a signal specified using a
symbolic name, without the SIG prefix, as listed in the tables of signal names
in the
<signal.h> header defined in the Base Definitions volume
of IEEE Std 1003.1-2001, Chapter 13, Headers; for example, HUP,
INT, QUIT, TERM. Implementations may permit names with the SIG prefix or
ignore case in signal names as an extension. Setting a trap for SIGKILL or
SIGSTOP produces undefined results.
The environment in which the shell executes a
trap on EXIT shall be
identical to the environment immediately after the last command executed
before the
trap on EXIT was taken.
Each time
trap is invoked, the
action argument shall be processed
in a manner equivalent to:
Signals that were ignored on entry to a non-interactive shell cannot be trapped
or reset, although no error need be reported when attempting to do so. An
interactive shell may reset or catch signals ignored on entry. Traps shall
remain in place for a given shell until explicitly changed with another
trap command.
When a subshell is entered, traps that are not being ignored are set to the
default actions. This does not imply that the
trap command cannot be
used within the subshell to set new traps.
The
trap command with no arguments shall write to standard output a list
of commands associated with each condition. The format shall be:
"trap -- %s %s ...\n", <action>, <condition> ...
The shell shall format the output, including the proper use of quoting, so that
it is suitable for reinput to the shell as commands that achieve the same
trapping results. For example:
save_traps=$(trap)
...
eval "$save_traps"
XSI-conformant systems also allow numeric signal numbers for the conditions
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.
None.
See the DESCRIPTION.
Not used.
None.
None.
Default.
See the DESCRIPTION.
The standard error shall be used only for diagnostic messages.
None.
None.
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.
Default.
The following sections are informative.
None.
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:
or:
Unset traps on INT, QUIT, TERM, and EXIT:
trap - INT QUIT TERM EXIT
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 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
None.
Special Built-In Utilities
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
.
