onchange.sh

#!/bin/sh

COMMAND=
ONCHANGE=
INTERVAL=2
MD5COMMAND=
ONCE=NO

if which md5 >/dev/null 2>&1; then
  MD5COMMAND=md5
elif which md5sum >/dev/null 2>&1; then
  MD5COMMAND=md5sum
fi

while [[ $# -gt 0 ]]; do
  case $1 in
    --interval|-n)
      shift
      INTERVAL=$1
      ;;
    --onchange|-c)
      shift
      ONCHANGE=$1
      ;;
    --once|-o)
      ONCE=YES
      ;;
    *)
      if [[ -z "$COMMAND" ]]; then
        COMMAND=$1
      else
        echo "Too many commands in arguments; using first."
      fi
      ;;
  esac
  shift
done

if [[ -z "$COMMAND" ]]; then
  echo "$(basename $0): No command supplied."
  exit 1
fi

if [[ -z "$ONCHANGE" ]]; then
  echo "$(basename $0): No on-change command supplied."
  exit 1
fi

LASTSTATE=$(eval "$COMMAND" | $MD5COMMAND)

while true; do
    NEWSTATE=$(eval "$COMMAND" | $MD5COMMAND)
    if [ "$NEWSTATE" != "$LASTSTATE" ]; then
        eval "$ONCHANGE"
        if [[ "$ONCE" == "YES" ]]; then
          exit 0
        fi
        LASTSTATE="$NEWSTATE"
    fi
    sleep $INTERVAL
done