Avoiding C-Shell Alias Loops (Unix Power Tools, 3rd Edition)

29.8. Avoiding C-Shell Alias Loops

Section 27.9 has similar information for bash.

Here's a situation that came up on the Net a while ago. Someone wanted an exit (Section 24.4) alias that would run a ~/.exit file (Section 31.13) before leaving the shell. The obvious solution is:

alias exit "source ~/.exit; exit"

This doesn't work; when you use the exit alias, the C shell thinks that the alias is trying to execute itself. Recursive aliases aren't allowed on many shells, so the C shell prints an error message (Alias loop) and gives up.

There are many ways to break the loop. Here's the best (in my opinion):

alias exit 'source ~/.exit; ""exit'

Section 27.10 has the hairy details of what works and why. To summarize, if you need to use the alias's name within a C shell alias, you can use:

""name: Where name is the name of a built-in (Section 1.9) command or any "regular" command.
\name: Where name is the name of any "regular" command, but not a built-in command.

Tempting as this all may sound (and I have to admit, if it didn't sound a bit tempting, I wouldn't be writing this article), I can't really recommend the practice of "redefining" commands with aliases. You should leave the original commands as they are. The original author could have avoided all these problems by calling his alias quit rather than exit.

If you redefine commands with aliases and then use another account where your alias isn't defined, it's easy for things to go wrong. That's especially true for commands that do something permanent -- overwriting or removing files, for example. It also can cause problems if you let someone type a command on your account and the person isn't expecting an aliased version.

Let me give one more example to show you what problems you can have. Let's say you've aliased the exit command to source a .exit file before quitting. Fair enough. But now, let's say that you're not in your login shell, that you've set ignoreeof, and that, for no apparent reason, your .exit file disappears (maybe it develops a bad block, so the system can't read it; such things happen).

Now you're stuck. If you type exit, the source command will fail, and the "real" exit command will never be executed. You can't leave the shell. Of course, if you remember what you did, you can always type unalias exit and get the original command back. Or you can type " "exit. Or finally, you could simply write the alias such that it tests for the existence of the file before trying to read it. But if you've foisted this alias on a beginner, he or she might not know that. All of a sudden, you're stuck in some shell that you apparently can't get out of.

The biggest virtue of Unix is that it's infinitely extendable. However, you aren't helping if your extensions hide the basic operations that make everything work. So -- extend all you want. But when you write your extensions, give them new names. End of sermon.

-- ML