shell uptime

Bash has many reserved variables, but one of the most fun for screen junkies is the $SECONDS variable, which displays “the number of seconds since the shell was started”.

$ echo $SECONDS
2012339

$ echo $(($SECONDS/60)) minutes
33541 minutes

$ echo $(($SECONDS/60/60)) hours
559 hours

$ echo $(($SECONDS/60/60/24)) days 
23 days

http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_02.html