Detecting (or not) the use of -l and -c together in Bourne shells

Many Bourne shells go slightly beyond the POSIX sh specification to also support a ‘-l’ option that makes the shell act as a ‘login shell’. POSIX’s omission of -l isn’t only because it doesn’t really talk about login shells at all, it’s also because Unix has a special way of marking login shells that goes back very far in its history. The -l option isn’t necessarily what login and sshd and so on use, it’s something that you can use if you specifically want to get a login shell in an unusual circumstance.

Bourne shells also have a ‘-c <command string>’ option that causes the shell to execute the command string rather than be interactive (this is a long standing option that is in POSIX). It may surprise you to hear that most or all Bourne shells that support -l also allow you to use -l and -c together. Basically all Bourne shells interpret this as first executing your .profile and so on, then executing the command string instead of going interactive. One use for this is to non-interactively run a command line in the context of your fully set up shell, with $PATH and other environment variables ready for use.

↫ Chris Siebenmann

Now, what if you want to detect the use of these two options combined, for instance to make it so certain parts of your .profile are ignored? It turns out very few Bourne shells actually support this, and that’s what Siebenmann’s latest post is about.