trap(1) User Commands trap(1)NAME
trap, onintr - shell built-in functions to respond to (hardware) sig‐
nals
SYNOPSIS
sh
trap [argument n [n2]...]
csh
onintr [-| label]
ksh
*trap [arg sig [sig2...]]
ksh93
+trap [-p] [action condition...]
DESCRIPTION
sh
The trap command argument is to be read and executed when the shell
receives numeric or symbolic signal(s) (n). (Note: argument is scanned
once when the trap is set and once when the trap is taken.) Trap com‐
mands are executed in order of signal number or corresponding symbolic
names. Any attempt to set a trap on a signal that was ignored on entry
to the current shell is ineffective. An attempt to trap on signal 11
(memory fault) produces an error. If argument is absent all trap(s) n
are reset to their original values. If argument is the null string this
signal is ignored by the shell and by the commands it invokes. If n is
0 the command argument is executed on exit from the shell. The trap
command with no arguments prints a list of commands associated with
each signal number.
csh
onintr controls the action of the shell on interrupts. With no argu‐
ments, onintr restores the default action of the shell on interrupts.
(The shell terminates shell scripts and returns to the terminal command
input level). With the − argument, the shell ignores all interrupts.
With a label argument, the shell executes a goto label when an inter‐
rupt is received or a child process terminates because it was inter‐
rupted.
ksh
trap uses arg as a command to be read and executed when the shell
receives signal(s) sig. arg is scanned once when the trap is set and
once when the trap is taken. Each sig can be specified as a number or
as the name of the signal. trap commands are executed in order of sig‐
nal number. Any attempt to set a trap on a signal that was ignored on
entry to the current shell is ineffective. If arg is omitted or is −,
then the trap(s) for each sig are reset to their original values. If
arg is the null (the empty string, for example, "") string then this
signal is ignored by the shell and by the commands it invokes. If sig
is ERR then arg are executed whenever a command has a non-zero exit
status. If sig is DEBUG then arg are executed after each command. If
sig is 0 or EXIT for a trap set outside any function then the command
arg is executed on exit from the shell. The trap command with no argu‐
ments prints a list of commands associated with each signal number.
On this manual page, ksh(1) commands that are preceded by one or two *
(asterisks) are treated specially in the following ways:
1. Variable assignment lists preceding the command remain in
effect when the command completes.
2. I/O redirections are processed after variable assignments.
3. Errors cause a script that contains them to abort.
4. Words, following a command preceded by ** that are in the
format of a variable assignment, are expanded with the same
rules as a variable assignment. This means that tilde sub‐
stitution is performed after the = sign and word splitting
and file name generation are not performed.
ksh93
trap is a special built-in that defines actions to be taken when condi‐
tions such as receiving a signal occur. trap can also be used to dis‐
play the current trap settings on standard output.
If action is -, trap resets each condition to the default value. If
action is an empty string, the shell ignores each of the conditions if
they arise. Otherwise, the argument action is read and executed by the
shell as if it were processed by eval when one of the corresponding
conditions arise. The action of the trap overrides any previous action
associated with each specified condition. The value of $? is not
altered by the trap execution.
condition can be the name or number of a signal, or one of the follow‐
ing:
EXIT Execute this trap when the shell exits. If defined within a
function with the function reserved word, executes the trap in
the caller's environment when the function returns. The trap
action is restored to the value it had when it called the
function.
0 Same as EXIT.
DEBUG Execute before each simple command is executed but after the
arguments are expanded.
ERR Execute whenever set -e would cause the shell to exit.
KEYBD Execute when a key is entered from a terminal device.
Signal names are case insensitive and the sig prefix is optional. Sig‐
nals that were ignored on entry to a non-interactive shell cannot
trapped or reset although doing so does not report an error. The use of
signal numbers other than 1, 2, 3, 6, 9, 14, and 15 are not portable.
Although trap is a special built-in, specifying a condition that the
shell does not know about causes trap to exit with a non-zero exit sta‐
tus, but does not terminate the invoking shell.
If no action or conditions are specified then all the current trap set‐
tings are written to standard output.
The following options are supported by the trap built-in command in
ksh93:
-p Causes the current traps to be output in a format that can be
processed as input to the shell to recreate the current traps.
The trap built-in in ksh93 exits with one of the following values:
0 Successful completion.
>0 An error occurred.
On this manual page, ksh93(1) commands that are preceded by one or two
+ (plus signs) are treated specially in the following ways:
1. Variable assignment lists preceding the command remain in
effect when the command completes.
2. I/O redirections are processed after variable assignments.
3. Errors cause a script that contains them to abort.
4. They are not valid function names.
5. Words, following a command preceded by ++ that are in the
format of a variable assignment, are expanded with the same
rules as a variable assignment. This means that tilde sub‐
stitution is performed after the = sign and word splitting
and file name generation are not performed.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Availability │SUNWcs │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOcsh(1), eval(1), exit(1), ksh(1), ksh93(1), sh(1), attributes(5)SunOS 5.11 20 Nov 2007 trap(1)
