[kwlug-disc] bash subprocesses & traps

[Chris Frey]

I'd recommend using some other mechanism of "end of stream" rather
than killing your script, since, as you found out, it halts all processing.

Perhaps instead of kill, rename the file:

	echo "More data to hash" > $FIFO_FILE
	# rename it so while loop exits
	mv "$FIFIO_FILE" "$FIFO_FILE-finished"
	# send empty data to trigger a fifo pipe close on the waiting cat
	cat /dev/null > "$FIFO_FILE-finished"

- Chris


On Sun, Jul 28, 2024 at 12:18:43PM -0400, 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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