du

Display directory space usage

df

Display disk usage

cal

To display calender

whoami

It shows your login name

uname -a

Used to display kernal information

date

It shows current date and time

uptime

This command shows system current uptime

cat /proc/meminfo

It shows memory information

cat /proc/cpuinfo

It shows cpu information

cat /proc/version

It shows information about Linux System

free

Display memory swape usage

kill [PID]

kill process by pid [Proces ID]

ps

It shows all running process on your system