diff options
Diffstat (limited to 'man-pages-posix-2013/man1p/trap.1p')
| -rw-r--r-- | man-pages-posix-2013/man1p/trap.1p | 362 |
1 files changed, 362 insertions, 0 deletions
diff --git a/man-pages-posix-2013/man1p/trap.1p b/man-pages-posix-2013/man1p/trap.1p new file mode 100644 index 0000000..264ce1f --- /dev/null +++ b/man-pages-posix-2013/man1p/trap.1p @@ -0,0 +1,362 @@ +'\" et +.TH TRAP "1P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual" +.SH PROLOG +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. + +.SH NAME +trap +\(em trap signals +.SH SYNOPSIS +.LP +.nf +trap \fIn \fB[\fIcondition\fR...\fB]\fR +trap \fB[\fIaction condition\fR...\fB]\fR +.fi +.SH 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. +.P +If +.IR action +is +.BR '\(mi' , +the shell shall reset each +.IR condition +to the default value. If +.IR action +is null (\c +.BR \(dq\^\(dq ), +the shell shall ignore each specified +.IR condition +if it arises. Otherwise, the argument +.IR action +shall be read and executed by the shell when one of the corresponding +conditions arises. The action of +.IR trap +shall override a previous action (either default action or one +explicitly set). The value of +.BR \(dq$?\(dq +after the +.IR trap +action completes shall be the value it had before +.IR trap +was invoked. +.P +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 +.IR <signal.h> +header defined in the Base Definitions volume of POSIX.1\(hy2008, +.IR "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. +.P +The environment in which the shell executes a +.IR trap +on EXIT shall be identical to the environment immediately after the +last command executed before the +.IR trap +on EXIT was taken. +.P +Each time +.IR trap +is invoked, the +.IR action +argument shall be processed in a manner equivalent to: +.sp +.RS 4 +.nf +\fB +eval \fIaction\fR +.fi \fR +.P +.RE +.P +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 +.IR trap +command. +.P +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 +.IR trap +command, when the traps need not be altered. Implementations may check +for this case using only lexical analysis; for example, if +.IR `trap` +and +.IR "$( trap -- )" +do not alter the traps in the subshell, cases such as assigning +.IR var=trap +and then using +.IR $($var) +may still alter them. This does not imply that the +.IR trap +command cannot be used within the subshell to set new traps. +.P +The +.IR 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 +.IR trap +command, and no +.IR 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: +.sp +.RS 4 +.nf +\fB +"trap \(mi\|\(mi %s %s ...\en", <\fIaction\fR>, <\fIcondition\fR> ... +.fi \fR +.P +.RE +.P +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: +.sp +.RS 4 +.nf +\fB +save_traps=$(trap) +\&... +eval "$save_traps" +.fi \fR +.P +.RE +.P +XSI-conformant systems also allow numeric signal numbers for the +conditions corresponding to the following signal names: +.IP 1 6 +SIGHUP +.IP 2 6 +SIGINT +.IP 3 6 +SIGQUIT +.IP 6 6 +SIGABRT +.IP 9 6 +SIGKILL +.IP 14 6 +SIGALRM +.IP 15 6 +SIGTERM +.P +The +.IR trap +special built-in shall conform to the Base Definitions volume of POSIX.1\(hy2008, +.IR "Section 12.2" ", " "Utility Syntax Guidelines". +.SH OPTIONS +None. +.SH OPERANDS +See the DESCRIPTION. +.SH STDIN +Not used. +.SH "INPUT FILES" +None. +.SH "ENVIRONMENT VARIABLES" +None. +.SH "ASYNCHRONOUS EVENTS" +Default. +.SH STDOUT +See the DESCRIPTION. +.SH STDERR +The standard error shall be used only for diagnostic messages. +.SH "OUTPUT FILES" +None. +.SH "EXTENDED DESCRIPTION" +None. +.SH "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. +.SH "CONSEQUENCES OF ERRORS" +Default. +.LP +.IR "The following sections are informative." +.SH "APPLICATION USAGE" +None. +.SH EXAMPLES +Write out a list of all traps and actions: +.sp +.RS 4 +.nf +\fB +trap +.fi \fR +.P +.RE +.P +Set a trap so the +.IR logout +utility in the directory referred to by the +.IR HOME +environment variable executes when the shell terminates: +.sp +.RS 4 +.nf +\fB +trap '"$HOME"/logout' EXIT +.fi \fR +.P +.RE +.P +or: +.sp +.RS 4 +.nf +\fB +trap '"$HOME"/logout' 0 +.fi \fR +.P +.RE +.P +Unset traps on INT, QUIT, TERM, and EXIT: +.sp +.RS 4 +.nf +\fB +trap \(mi INT QUIT TERM EXIT +.fi \fR +.P +.RE +.SH "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 +.IR trap +and +.IR kill +utilities in this volume of POSIX.1\(hy2008 are now consistent in their omission of the SIG +prefix for signal names. Some +.IR kill +implementations do not allow the prefix, and +.IR kill +.BR \(mil +lists the signals without prefixes. +.P +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. +.P +The output format is not historical practice. Since the output of +historical +.IR 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. +.P +The KornShell uses an +.BR ERR +trap that is triggered whenever +.IR set +.BR \(mie +would cause an exit. This is allowable as an extension, but was not +mandated, as other shells have not used it. +.P +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 +.BR \(dq\(mi\|\(mi\(dq : +.sp +.RS 4 +.nf +\fB +trap 'read foo; echo "\(mi$foo\(mi"' 0 +.fi \fR +.P +.RE +.P +The command: +.sp +.RS 4 +.nf +\fB +trap 'eval " $cmd"' 0 +.fi \fR +.P +.RE +.P +causes the contents of the shell variable +.IR cmd +to be executed as a command when the shell exits. Using: +.sp +.RS 4 +.nf +\fB +trap '$cmd' 0 +.fi \fR +.P +.RE +.P +does not work correctly if +.IR cmd +contains any special characters such as quoting or redirections. Using: +.sp +.RS 4 +.nf +\fB +trap " $cmd" 0 +.fi \fR +.P +.RE +.P +also works (the leading +<space> +character protects against unlikely cases where +.IR cmd +is a decimal integer or begins with +.BR '\(mi' ), +but it expands the +.IR cmd +variable when the +.IR trap +command is executed, not when the exit action is executed. +.SH "FUTURE DIRECTIONS" +None. +.SH "SEE ALSO" +.IR "Section 2.14" ", " "Special Built-In Utilities" +.P +The Base Definitions volume of POSIX.1\(hy2008, +.IR "Section 12.2" ", " "Utility Syntax Guidelines", +.IR "\fB<signal.h>\fP" +.SH COPYRIGHT +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 . |