[kwlug-disc] bash subprocesses & traps

[William Park]

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?
>>
>>
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