limitime.sh

#!/bin/bash
# this file is executed with root permissions every minute by cron

# so that users who belong to the admin group can change the created files
umask 002

# echo the $1 nth line of stdin
# if that line doesn't exist than echo $2
nth_or() { (cat; echo;)|head -n "$1" | tail -n 1 | sed 's/^\s*$/'"$2"'/'; }

for username in $(ls /home)
do
    if [ -e "limits/$username" ] && # if a roule is defined for user
        { who | grep "^$username" > /dev/null; } # user is logged on 
    then
        if [ ! -e "consumed/$username" ] ||
            [ $(( $(date -d "$(date +%Y/%m/%d)" +%s) >
                  $(stat "consumed/$username" -c %Y) )) == 1 ] # consumed/$username was not modified today
        then
            echo 0 > "consumed/$username"
        fi
        weekday=$(date +%u)
        # blank lines are interpreted as INFINITY (there are less than 9999min in a day)
        timepermited=$(nth_or "$weekday" 9999 < "limits/$username") 
        timeconsumed=$(cat "consumed/$username")
        if [ $((timeconsumed >= timepermited)) = 1 ]
        then
            # exit users session
            pkill -u "$username" -KILL
        else
            echo $((timeconsumed + 1)) > "consumed/$username"
        fi
    fi
done