CODEWITHSUNDEEP
bashunixsignals
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Reputation: 121

How to restart a BASH script from itself with a signal?

For example I have script with an infinite loop printing something to stdout. I need to trap a signal (for example SIGHUP) so it will restart the script with different PID and the loop will start itself again from 0. Killing and starting doesn't work as expected: 

function traphup(){  
    kill $0  
    exec $0
}  
trap traphup HUP

Maybe I should place something in background or use nohup, but I am not familiar with this command.

Upvotes: 3

Views: 7679

Answers (1)

Jonathan Leffler
Jonathan Leffler

Reputation: 753970

In your function:

traphup(){
    $0 "$@" &
    exit 0
}

This starts a new process in the background with the original command name and arguments (vary arguments to suit your requirements) with a new process ID. The original shell then exits. Don't forget to sort out the PID file if your daemon uses one to identify itself - but the restart may do that anyway.

Note that using nohup would be the wrong direction; the first time you launched the daemon, it would respond to the HUP signal, but the one launched with nohup would ignore the signal, not restarting again - unless you explicitly overrode the 'ignore' status, which is a bad idea for various reasons.

Answering comment

I'm not quite sure what the trouble is.

When I run the following script, I only see one copy of the script in ps output, regardless of whether I start it as ./xx.sh or as ./xx.sh &.

#!/bin/bash

traphup()
{
    $0 "$$" &
    exit 0
}

trap traphup HUP
echo
sleep 1

i=1
while [ $i -lt 1000 ]
do
    echo "${1:-<none>}: $$: $i"
    sleep 1
    : $(( i++ ))
done

The output contains lines such as:

<none>: 1155: 21
<none>: 1155: 22
<none>: 1155: 23

1155: 1649: 1
1155: 1649: 2
1155: 1649: 3
1155: 1649: 4

The ones with '<none>' are the original process; the second set are the child process (1649) reporting its parent (1155). This output made it easy to track which process to send HUP signals to. (The initial echo and sleep gets the command line prompt out of the way of the output.)

My suspicion is that what you are seeing depends on the content of your script - in my case, the body of the loop is simple. But if I had a pipeline or something in there, then I might see a second process with the same name. But I don't think that would change depending on whether the original script is run in foreground or background.

Upvotes: 4

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