What is signal 0 in a trap command?

Question

I'm following this guide on how to set up passwordless SSH authentication with ssh-agent.

To start up ssh-agent the author recommends the following code in .bash_profile:

SSHAGENT=/usr/bin/ssh-agent
SSHAGENTARGS="-s"
if [ -z "$SSH_AUTH_SOCK" -a -x "$SSHAGENT" ]; then
    eval `$SSHAGENT $SSHAGENTARGS`
    trap "kill $SSH_AGENT_PID" 0
fi

I don't understand why it is trapping signal 0. According to man 7 signal there is no such signal.

Is this just a typo or bug or does this really achieve something?

score 21 · Accepted Answer · answered Jul 24 '11 at 12:47

21

From the bash manual:

trap [-lp] [[arg] sigspec ...]

... If a sigspec is EXIT (0) the command arg is executed on exit from the shell.

answered Jul 24 '11 at 12:47

Mat

52,586

14

Correct. In case a reader came here because of the question's title, note that this is the meaning of 0 in a shell trap command. When sending a signal with kill, a signal number of 0 means not to send a signal after all; this is a way of testing the existence of the target process. – Gilles 'SO- stop being evil' Jul 24 '11 at 14:00
@Mat: Thanks a lot. I was reading man 1 trap... – Christoph Wurm Jul 24 '11 at 14:30
1

But it's not in the list output by trap -l! – amphetamachine Jul 25 '11 at 08:59
can you please address the case when the script is sourced? – Eric May 01 '19 at 08:31

score 2 · Answer 2 · answered Nov 25 '14 at 19:38

As @Mat shows in his answer, a SIGSPEC of 0, when used on the trap command will cause the trap command to run when the script exits.

This example illustrates what happens.

$ cat tr.bash 
#!/bin/bash

echo "PID: $$"

trap 'echo hi; exit 1' 0 1 2 15

while [ 1 ]; do
    sleep 3
done

When we run this:

$ ./tr.bash 
PID: 24086

It sits here waiting indefinitely. In another window if we now send kill signals to it you'll see that a kill -0 will not kill the process, even though signal 0 is listed in the trap command.

$ kill -0 $(pgrep tr.bash)
$

However if we kill the script using signal 1, kill -1:

$ kill -1 $(pgrep tr.bash)
$

We'll see that the script exits, and prints the message, "hi" 2 times. The first for signal 1, and the second because the script exited.

$ ./tr.bash 
PID: 24086
hi
hi

dbrower · Answer 3 · 2014-09-05T17:42:02.090

0

trap on 0 is run when the shell exits. It is commonly used to clean up tmp files in one place that is always executed:

tmp=/tmp/myscript.$$

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

do_a_bunch_of_stuff

exit

the exit at the end of the trap exits the shell at the cleanup with the right status.

edited Sep 05 '14 at 17:42

answered Sep 05 '14 at 17:27

dbrower

1,017

What is signal 0 in a trap command?

3 Answers3

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