[kwlug-disc] bash subprocesses & traps

[William Park]

It could be that, in

     cat "$FIFO_FILE" & cat_pid=$!

cat "$FIFO_FILE" finishes too fast, and cat_pid=$! is pointing to PID 
that has already ended.


On 2024-07-28 12:18, John Steel via kwlug-disc wrote:
> I’m having trouble figuring out how to manage a subprocess… I’m probably overcomplicating this…
> 
> I’ve made a script called checksum.sh. I want it to keep moving data from a fifo to sha256sum until it’s terminated. But it seems when I kill it the cat and sha256sum are gone before my finalize method is called? If I can kill cat and let that sub shell exit gracefully I think I should see the sha256sum come out on stderr.
> 
> #!/bin/bash
> # This is checksum.sh
> 
> FIFO_FILE="${FIFO_FILE:-sha256sum_fifo}"
> CAT_PID_FILE="$(mktemp -p /dev/shm $$_cat_pid.XXXXXX)"
> export CAT_PID_FILE FIFO_FILE
> mkfifo "$FIFO_FILE"
> 
> finalize() {
>      pgrep -fl "cat fifo $FIFO_FILE" > /dev/stderr
>      pgrep -fl sha256sum > /dev/stderr
>      
>      cat_pid="$(cat "$CAT_PID_FILE")"
>      rm "$CAT_PID_FILE"
>      kill -s SIGTERM "$cat_pid" && wait "$cat_pid"
>      exit 0
> }
> 
> trap finalize SIGTERM
> trap finalize SIGHUP
> trap finalize SIGINT
> trap finalize SIGQUIT
> 
> # Start the cat process and capture its PID
> while [ -f "$CAT_PID_FILE" ]; do
>      cat "$FIFO_FILE" & cat_pid=$!
>      echo "$cat_pid" > "$CAT_PID_FILE"
>      wait "$cat_pid"
>      sleep 0.01
> done | sha256sum > /dev/stderr
> 
> Here’s how I’m running it:
> 
> docker run -it -v $PWD:/app alpine sh -xc '
>    apk add bash procps
>    export FIFO_FILE="$(mktemp -u -p /dev/shm fifo_$$.XXXXXX)"
>    bash -x /app/checksum.sh &
>    sleep 0.2
>    jobs
>    ps -ef --forest
>    echo "First data to hash" > $FIFO_FILE
>    sleep 0.2
>    ps -ef --forest
>    echo "More data to hash" > $FIFO_FILE
>    kill %1
>    sleep 1
> ‘
> 
> Am I missing something obvious? If it’s possible without the loop that’d be great too but I found that if I connect sha256sum directly to the fifo it exits after the first write to the fifo.
> 
> Should I learn how to use socat for this? From a few of the things I’ve read it sounds like it could make this simpler?
> 
> 
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