Simple primitive to monitor output of the command: DISC (do if something changed)

disc.sh

#!/bin/sh

FIRST=y
sh -c "$CMD" > .prev-output

while :
do
    if [[ "$FIRST" == "y" ]]; then
        FIRST="n"
        sleep "$SLEEP"
        continue
    fi

    sh -c "$CMD" > .output
    sh -c "comm -1 -3 .prev-output .output || < .output ${TRIGGER_CMD}"
    cp .output .prev-output
    sleep "$SLEEP"
done