Work the Shell - Understanding Exit Codes

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In the real world, things don't always work out as you hoped. That's what exit codes are for—letting you know when things went wrong.
Hiding Error Messages

Now that you know how to capture and analyze the exit codes from system commands, what if you want to have the error message be one from your script, not one from the command itself?

That's done with another new shorthand notation: >&, which redirects the stderr/error output stream. Here's how I use that to hide all error messages from the mkdir command being used in our sample scripts:


mkdir /usr >& /dev/null

You also can use &> or 2>&1 instead of >&.

If you don't test the results of the command, of course, you seriously can hose things up, but this makes the output more elegant for sure:

$ ./test.sh
mkdir /usr failed: we have an exit code of 0

Hmmm...I'm still getting that false 0. Oh! I haven't added the code to save the exit code value as “error”. One slight tweak later and:

$ ./test.sh
mkdir /usr failed: we have an exit code of 1

That's more like it!

I'm going to call this a wrap for this month. Next month, I'll demonstrate how the exit command lets you send exit codes back to the calling program from procedures and functions, just as if they were separate Linux commands rather than part of the same shell script.

Dave Taylor has been hacking shell scripts for a really long time, 30 years. He's the author of the popular Wicked Cool Shell Scripts, and he can be found on Twitter as @DaveTaylor and more generally at www.DaveTaylorOnline.com.

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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 www.DaveTaylorOnline.com.

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