Periodically monitor a command's output for changes.

diffmon

#!/usr/bin/env bash
# Monitor a command's output for changes.
set -eu
usage="diffmon [-D] [-t INTERVAL] CMD ARGS...
 -t INTERVAL  Interval to sleep between command executions
 -b           Print a blank line between changes instead of the date"
interval=1
show_date=yes
while getopts "hbt:" o; do
    case "$o" in
        t) interval=$OPTARG;;
        b) show_date='';;
        h) echo "$usage"; exit;;
        *) echo "$usage" 1>&2; exit;;
    esac
done
shift $((OPTIND - 1))
if [[ $# -lt 1 ]]; then
    echo "$usage" 1>&2
    exit
fi

old=/tmp/diffmon.old.$(date '+%s')
new=/tmp/diffmon.new.$(date '+%s')
trap "rm $old $new" TERM

"$@" > $old
while true; do
    "$@" > $new || true
    diff -N -u0 $old $new | sed -re '/^(---|\+\+\+|@@)/d'
    if [[ ${PIPESTATUS[0]} -eq 1 ]]; then
        if [[ -n $show_date ]]; then
            echo -n '@'; date -Iseconds
        else
            echo
        fi
    fi
    mv $new $old
    sleep $interval
done