[kwlug-disc] bash subprocesses & traps

[John Steel]

Hrm… The goal was to try to avoid running multiple sha256sum processes, I would expect that putting it in the loop would change the behaviour right?

> On Aug 7, 2024, at 5:08 PM, William Park via kwlug-disc <kwlug-disc at kwlug.org> wrote:
> 
> No, that ain't it.  Inside the while loop, cat "$FIFO_FILE" finishes alright, but the while loop itself has not.  So, sha256sum is still reading the pipe.
> 
> Solution:  Move sha256sum inside the loop.  In fact, you can replace cat with sha256sum, eg.
> 	sha256sum "$FIFO_FILE" & cat_pid=$!
> 
> On 2024-08-07 02:14, William Park via kwlug-disc wrote:
>> It could be that, in
>>     cat "$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?
>>> 
>>> 
>>> _______________________________________________
>>> kwlug-disc mailing list
>>> To unsubscribe, send an email to kwlug-disc-leave at kwlug.org
>>> with the subject "unsubscribe", or email
>>> kwlug-disc-owner at kwlug.org to contact a human being.
>> _______________________________________________
>> kwlug-disc mailing list
>> To unsubscribe, send an email to kwlug-disc-leave at kwlug.org
>> with the subject "unsubscribe", or email
>> kwlug-disc-owner at kwlug.org to contact a human being.
> 
> _______________________________________________
> kwlug-disc mailing list
> To unsubscribe, send an email to kwlug-disc-leave at kwlug.org
> with the subject "unsubscribe", or email
> kwlug-disc-owner at kwlug.org to contact a human being.