#!/bin/bash
ME=myusername@mymailhost
ulistorig=( `cut -d: -f1 /etc/passwd | sort` )
dateorig=`date`
while true; do
  ulistdel=""
  ulistadd=""
  sleep 10
  ulistcurr=( `cut -d: -f1 /etc/passwd | sort` )
  i=0
  j=0
  while [[ $i -lt ${#ulistorig[@]} && $j -lt ${#ulistcurr[@]} ]]; do
    if [[ "${ulistorig[i]}" == "${ulistcurr[j]}" ]]; then
      i=$((i+1))
      j=$((j+1))
    elif [[ "${ulistorig[i]}" < "${ulistcurr[j]}" ]]; then
      ulistdel="$ulistdel ${ulistorig[i]}"
      i=$((i+1))
    else
      ulistadd="$ulistadd ${ulistcurr[j]}"
      j=$((j+1))
    fi
  done
  while [[ $i -lt ${#ulistorig[@]} ]]; do
    ulistdel="$ulistdel ${ulistorig[i]}"
    i=$((i+1))
  done
  while [[ $j -lt ${#ulistcurr[@]} ]]; do
    ulistadd="$ulistdel ${ulistorig[i]}"
    i=$((i+1))
  done
  if [[ -n "$ulistdel" || -n "$ulistadd" ]]; then
    mailx -s "file changed" $ME <<EOF
changes in file since $dateorig:
  deleted users: $ulistdel
  added users: $ulistadd
EOF
  # echo changes to stdout, too
    cat <<EOF
changes in file since $dateorig:
  deleted users: $ulistdel
  added users: $ulistadd
EOF
    ulistorig=( `echo ${ulistcurr[@]}` )
    dateorig=`date`
  else
    # optional no changes message (for debugging/testing)
    echo "no changes in file since $dateorig"
  fi
done