Quite a few UNIX commands set themselves up by reading the environment. For example, the vi editor reads startup commands from the EXINIT environment variable. Sometimes, you'll want to override the setting of an environment variable for just one command. There's an easier way than setting the variable to a different value and changing it back after you run the command:
For example, if your EXINIT variable has:
set wrapscan showmatch number
and you want to add
For a great example of this technique, see article 6.7 .