Processes and signals

The command you typed does not come back. This is the chapter about what to do then.

Each running program is called a process, and each one has a number (a PID). ps lists them.

• ps — the list of what is running; -o picks the columns
• pstree — see the parent and child relations as a tree
• top — watch, as it moves, what is eating the CPU and the memory
• kill — send a signal to a process
• & jobs fg — run something behind you, and call it back
• nice — run it with a lower priority

The name kill overdoes the threat a little; the tool really only sends a signal. The default signal (TERM) is a request to "tidy up and finish", and for something that will not listen you send -9 (KILL). Knowing that difference is how you stop things without breaking your data.

You have used Ctrl-C already. That too is sending a signal (INT). This chapter is where what was going on behind the button becomes clear.

Look at the family relations too. Every process branches off init (number 1), and your shell is one of those branches.