Mastering Linux Shell Scripting - Second Edition

Environment variables

So far, we have used variables that we didn't define, such as $BASH_VERSION, $HOME, $PATH, and $USER. You might wonder, as we didn't declare these variables, where did they come from?

These variables are defined by the shell for your use and they are called environment variables.

There are many environment variables. If you want to list them, you can use the printenv command.

Also, you can print a specific environment variable by specifying it to the printenv command:

$ printenv HOME

We can use any of these variables in our bash scripts.

Note that all environment variables are written in capital letters, so you can declare your variables as lower case to make it easy to differentiate your variables from environment variables. ...