'\" 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 .