Variables are temporary placeholders for information, either numbers or sequences of characters (called strings). The system (that is, the shell) has a number of useful predefined variables, such as the user name, home directory, and system name. Using the env command generates a listing of all persistent environment variables (meaning that they don't stop existing when the current command or script ends, instead they exist for your Linux login environment as a whole) defined in your shell. Listing 21.3 is the output generated by the env command on my machine. This will vary somewhat from machine to machine, of course.
[jwalter@jansmachine samples_22]$ env USERNAME= ...