[kwlug-disc] bash subprocesses & traps

[John Steel]

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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