Linux in a Nutshell, Third Edition

Name

trap

Synopsis

trap [- l] [ [commands] signals]

Description

Execute commands if any of signals is received. Common signals include 0, 1, 2, and 15. Multiple commands should be quoted as a group and separated by semicolons internally. If commands is the null string (i.e., trap “”signals), then signals will be ignored by the shell. If commands is omitted entirely, reset processing of specified signals to the default action. If both commands and signals are omitted, list current trap assignments. See examples at the end of this entry and under exec.

Option

-l

List signals.

Signals

Signals are listed along with what triggers them.

0

Exit from shell (usually when shell script finishes).

1

Hang up (usually logout).

2

Interrupt (usually through Ctrl-C).

3

Quit.

4

Illegal instruction.

5

Trace trap.

6

Abort.

7

Unused.

8

Floating-point exception.

9

Termination.

10

User-defined.

11

Reference to invalid memory.

12

User-defined.

13

Write to a pipe without a process to read it.

14

Alarm timeout.

15

Software termination (usually via kill).

16

Unused.

17

Termination of child process.

18

Continue (if stopped).

19

Stop process.

20

Process suspended (usually through Ctrl-Z).

21

Background process has tty input.

22

Background process has tty output.

23

Unused.

24

Unused.

25

Unused.

26

Unused.

27

Unused.

28

Unused.

29

I/O possible on a channel.

Examples

                     trap "" 2     
                     Ignore signal 2 (interrupts)
                     trap 2        
                     Obey interrupts again

Remove a $tmp file when the shell program exits or if the user logs out, presses Ctrl-C, or does a kill:

                     trap "rm -f $tmp; exit" 0 1 2 15