Shell Programming and Scripting

# Number of sessions allowed per user.
MAX_SESSIONS=2
# You could use a list here instead of a file...
# You should add a case for each shell also...
case $SHELL in
/usr/local/bin/bash)
   USERS=($(cat /etc/users.txt))
   ;;
/usr/bin/ksh)
   set -A USERS $(cat /etc/users.txt)
   ;;
esac
for u in ${USERS[*]}; do
   if [[ $LOGIN = $u ]]; then
      sleep 2
      SESSIONS=$(who -u | grep -i $u | grep -v grep | wc -l)
      if [[ $SESSIONS -gt $MAX_SESSIONS ]]; then
         echo "ERROR:  You have reached your limits ($MAX_SESSIONS sessions)."
         exit 1
      fi
   fi
done