Linux Shell Scripting Tutorial (LSST) v1.05r3
Chapter 2: Getting started with Shell Programming

Why Process required

As You know Linux is multi-user, multitasking Os. It means you can run more than two process simultaneously if you wish. For e.g. To find how many files do you have on your system you may give command like:

$ ls / -R | wc -l

This command will take lot of time to search all files on your system. So you can run such command in Background or simultaneously by giving command like

$ ls / -R | wc -l &

The ampersand (&) at the end of command tells shells start process (ls / -R | wc -l) and run it in background takes next command immediately.

Process & PID defined as:
"An instance of running command is called process and the number printed by shell is called process-id (PID), this PID can be use to refer specific running process."

What is Processes
Linux Command(s) Related with Process