#1: top => Process Activity Command
The top provides a real-time process activity. It displays by default the most CPU consuming tasks runnning and it refreshs every 3 seconds.
There are some interactive commands very interesting
d: Delay time can be set to 1 sec
A: Sorts the display of various system resources
N: sort tasks by pid (numerically).
P: sort tasks by CPU usage (default).
M: sort tasks by resident memory usage.
T: sort tasks by time / cumulative time.
check man for further options
#2: w – Find Out Who Is Logged on And What They Are Doing
Displays information about users currently connected to the machine and thier processes
# w: username
#3: ps – Displays The Processes
Displays the current proces running
Print All Process On The Server
# ps ax
# ps axu
Find Out The Top 10 Memory Consuming Process
# ps -auxf | sort -nr -k 4 | head -10
#4: free – Memory Usage
Dsplays the total amount of used and free physical and swap memory in the system and the buffers used by the kernel.