Top

top will continuously show your machine's load and which programs are taking what percentage of resources, in real time.

Main commands in top:

s will change the delay (in seconds) between two refreshes of top. Split-seconds aren't allowed but for quick refresh you can use zero.

o followed by one of the following changes the sorting of the display by time of: TIME, RES, SIZE, CPU, PRI

k will send signals such as kill to specified tasks.

m toggles the display between CPU and I/O mode.

u show only the processes of a certain user

r renice a process for altering its speed. Some renice acts are restricted to root or su mode.

S toggle the display of system processes

t toggle the display of top itself

d change the number of time to refresh top before ending it.

q (at the command line only as "top -q", restricted to root only) to renice top to -20 for extra fast refresh

Most of the commands work the same way in the command line.