The top command is the traditional way to view your system’s resource usage and see the processes that are taking up the most system resources. Top displays a list of processes, with the ones using the most CPU at the top.

To exit top or htop, use the Ctrl-C keyboard shortcut. This keyboard shortcut usually kills the currently running process in the terminal.

The htop command is an improved top. It’s not installed by default on most Linux distributions — here’s the command you’ll need to install it on Ubuntu: sudo apt-get install htop

htop displays the same information with an easier-to-understand layout. It also lets you select processes with the arrow keys and perform actions, such as killing them or changing their priority, with the F keys.

The ps command lists running processes. The following command lists all processes running on your system:

ps -A

This may be too many processes to read at one time, so you can pipe the output through the less command to scroll through them at your own pace:

ps -A | less

Press q to exit when you’re done. You could also pipe the output through grep to search for a specific process without using any other commands. The following command would search for the Firefox process:

ps -A | grep firefox

The pstree command is another way of visualizing processes. It displays them in tree format. So, for example, your X server and graphical environment would appear under the display manager that spawned them.

killPID

Technically speaking, the kill command can send any signal to a process. You can use kill -KILL or kill -9 instead to kill a stubborn process.

Given a search term, pgrep returns the process IDs that match it. For example, you could use the following command to find Firefox’s PID:

pgrep firefox

You can also combine this command with kill to kill a specific process. Using pkill or killall is simpler, though.

The pkill and killall commands can kill a process, given its name. Use either command to kill Firefox:

pkill firefox

killall firefox

The renice command changes the nice value of an already running process. The nice value determines what priority the process runs with. A value of -19 is very high priority, while a value of 19 is very low priority. A value of 0 is the default priority.

The renice command requires a process’s PID. The following command makes a process run with very low priority:

renice 19 PID

You can use the pgrep trick above with renice, too.

If you’re making a process run at a higher priority, you’ll require root permissions. On Ubuntu, use sudo for that:

sudo renice -19 #

The xkill command is a way of easily killing graphical programs. Run it and your cursor will turn into an x sign. Click a program’s window to kill that program. If you don’t want to kill a program, you can back out of xkill by right-clicking instead. You don’t have to run this command from a terminal — you can also press Alt-F2, type xkill and press Enter to use it from a graphical desktop.