When writing shell scripts for Linux, you work with three types of variables:
Environment variables—Part of the system environment, you can use them in your shell program. You can define new variables, and you can also modify some of them, such as
PATH, within a shell program.
Built-in variables—Variables such as options used on the command (interpreted by the shell as a positional argument) are provided by Linux. Unlike environment variables, you cannot modify them.
User variables—Variables defined within a script ...