Kill a nested while loop

Question

I'm trying to make a basic loop, and I want to exit the nested loop when I loop the parent. The nested loop should restart each time the parent loops.

#!/bin/bash

loop1=0
i=0

while [[ $loop1 -eq "0" ]]; do

    let "i+=1"
    echo $i "--------------"

    loop2=0
    j=0

    while [[ $loop2 -eq "0" ]]; do

        let "j+=1"
        echo $j $i
        sleep .2    

    done &

    loop2=1
    sleep 2

done

My desired output would look like this:

1 --------------
1 1
2 1
3 1
4 1
5 1
6 1
7 1
8 1
9 1
10 1
2 --------------
1 2
2 2
3 2
4 2
5 2
6 2
7 2
8 2
9 2
10 2

I'm getting that output, sort of, but the original nested loop doesn't terminate, so I also get 11 1, 12 1, 13 1, and so on.

I'm sure this is dead simple, but I'm not getting it.

Answer 1

I'm not exactly sure what the practical use for this exercise is, but to kill background processes, I'd recommend saving the process id from $! and then just shooting it with kill.

The script element backgrounded with & runs in a subshell, a distinct shell process, so no changes to shell variables take any effect, in one direction or the other. (Usually this just comes up with changes in the child not showing in the parent, since usually we wait for the child process to complete.)

This:

#!/bin/bash

i=0
while true; do   # the loop runs forever, so just use 'true' here
    let "i+=1"
    echo $i "--------------"

    j=0
    while true; do   
        let "j+=1"
        echo $j $i
        sleep .2    
    done &

    pid=$!
    # disown   # remove the comment marker here
    sleep .5   # shorter timeout 
    kill $pid
done

prints:

1 --------------
1 1
2 1
3 1
2 --------------
1 2
2 2
3 2
loop.sh: line 19:  4113 Terminated              while true; do
    let "j+=1"; echo $j $i; sleep .2;
done
3 --------------
1 3
2 3
...

The shell prints out notices when it sees its child processes dying, we can get rid of that by disowning the child (remove the comment marker from the line).

Answer 2

Someone in the comments turned me onto the "jobs" command, and how to kill a process based on that. So this is what I ended up doing. Seemed the most straightforward.

i=0
while true; do
    let "i+=1"
    echo $i "--------------"

    j=0
    while true; do
        let "j+=1"
        echo $j $i
        sleep .2    
    done &

    sleep 2

    # jobs
    kill %"$i"
done