Recognizing non-interactive shells and 'shell levels'

The issue of when bash sources your .bashrc does raise the obvious question of how you can tell in your .bashrc what case you're dealing with, so that you can do different things in each of them.

The traditional approach to do this is to set environment variables to signal what state you're in and how many things have been initialized. One traditional 'have I been initialized' variable is $PS1, but this has the tiny weakness that su'ing to the account from another account that already has $PS1 set will not trigger your initializations.

(To keep clutter down, your 'am I a login shell' environment variable doesn't need to be exported, and indeed can be unset immediately after you use it. Other flag variables generally need to persist into future sub-shells.)

If you need to know interactive versus non-interactive, there are two approaches. First, you can check the shell's built in $- variable to see if the 'i' flag is part of the shell's flags, and second, you can use test to see if standard input is connected to a terminal. Which test is better depends in part on what you're trying to do; for example, if you want to run stty, you really care only about whether you're hooked up to a terminal.

The good news about modern systems is that all of this is much less important than it used to be; these days the overhead of re-doing redundant initializations, even relatively complex ones, is usually pretty low.

(Netbooks and other small machines may yet change this back, of course.)