Work the Shell - Handling Errors and Making Scripts Bulletproof

Shell scripts may be quick, easy and lightweight, but proper scripting includes the ability to anticipate and respond to error situations gracefully and without anything breaking. Dave explores some of the basic shell script error-handling options.
Avoiding Output Problems with noclobber

Finally, another thing to be aware of with the shell is that it's all too easy to zap important files with a redirect. For example, this shouldn't work:

$ who > who.output
$ ls > who.output

The second command should generate an error because the output file already exists, right? But it doesn't, and it simply trashes the who output without a warning or error—not good.

To avoid that problem, you'll want to set -o noclobber in scripts or, better, for your login shell, and let it be inherited by subshells, including those that run your shell scripts. A good place to put it could be in your .profile or .bashrc.

With noclobber set, the two commands behave differently:

$ ls > who.output
-bash: who.output: cannot overwrite existing file

That's useful for everyone, and doubly so for us shell script hackers, right?

Dave Taylor is a 26-year veteran of UNIX, creator of The Elm Mail System, and most recently author of both the best-selling Wicked Cool Shell Scripts and Teach Yourself Unix in 24 Hours, among his 16 technical books. His main Web site is at, and he also offers up tech support at Follow him on Twitter if you'd like:


Dave Taylor has been hacking shell scripts for over thirty years. Really. He's the author of the popular "Wicked Cool Shell Scripts" and can be found on Twitter as @DaveTaylor and more generally at