Bash script: `exit 0` fails to exit

Question

So I have this Bash script:

#!/bin/bash

PID=`ps -u ...`
if [ "$PID" = "" ]; then
    echo $(date) Server off: not backing up
    exit
else
    echo "say Server backup in 10 seconds..." >> fifo
    sleep 10

    STARTTIME="$(date +%s)"

    echo nosave >> fifo
    echo savenow >> fifo
    tail -n 3 -f server.log | while read line
    do
        if echo $line | grep -q 'save complete'; then
            echo $(date) Backing up...
            OF="./backups/backup $(date +%Y-%m-%d\ %H:%M:%S).tar.gz"
            tar -czhf "$OF" data

            echo autosave >> fifo
            echo "$(date) Backup complete, resuming..."
            echo "done"
            exit 0
            echo "done2"
        fi

        TIMEDIFF="$(($(date +%s)-STARTTIME))"
        if ((TIMEDIFF > 70)); then
            echo "Save took too long, canceling backup."
            exit 1
        fi
    done
fi

Basically, the server takes input from a fifo and outputs to server.log. The fifo is used to send stop/start commands to the server for autosaves. At the end, once it receives the message from the server that the server has completed a save, it tar's the data directory and starts saves again.

It's at the exit 0 line that I'm having trouble. Everything executes fine, but I get this output:

srv:scripts $ ./backup.sh
Sun Nov 24 22:42:09 EST 2013 Backing up...
Sun Nov 24 22:42:10 EST 2013 Backup complete, resuming...
done

But it hangs there. Notice how "done" echoes but "done2" fails. Something is causing it to hang on exit 0.

ADDENDUM: Just to avoid confusion for people looking at this in the future, it hangs at the exit line and never returns to the command prompt. Not sure if I was clear enough in my original description.

Any thoughts? This is the entire script, there's nothing else going on and I'm calling it direct from bash.

Answer 1

Here's a smaller, self contained example that exhibits the same behavior:

echo foo > file
tail -f file | while read; do exit; done

The problem is that since each part of the pipeline runs in a subshell, exit only exits the while read loop, not the entire script.

It will then hang until tail finds a new line, tries to write it, and discovers that the pipe is broken.

To fix it, you can replace

tail -n 3 -f server.log | while read line
    do
       ...
    done

with

while read line
do
   ...
done  <  <(tail -n 3 -f server.log)

By redirecting from a process substitution instead, the flow doesn't have to wait for tail to finish like it would in a pipeline, and it won't run in a subshell so that exit will actually exits the entire script.

Answer 2

But it hangs there. Notice how "done" echoes but "done2" fails.

done2 won't be printed at all since exit 0 has already ended your script with return code 0.