Variables in Bash shell scripting

What is variable ?

          Every variable contains meta data either is system defined or user defined.
      In every programming language variable play's important role,in Linux there are two types of variables System defined variable and User defined variable.

System defined variable:

         It defined by system,means every OS have some default variables called  system variables.
Example:
$PATH
$date
output
Sun May 12 11:56:34 IST 2019

User defined variable:

        It defined by user,means user need to create,define variable.
Example:
cpu=$( top -b -n1 | grep "Cpu(s)" | awk '{print $2 }'  | cut -d'.' -f1 )
here,we are taking CPU usage