Work the Shell - Understanding Exit Codes
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.
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.
|A Project to Guarantee Better Security for Open-Source Projects||Aug 27, 2015|
|Concerning Containers' Connections: on Docker Networking||Aug 26, 2015|
|My Network Go-Bag||Aug 24, 2015|
|Doing Astronomy with Python||Aug 19, 2015|
|Build a “Virtual SuperComputer” with Process Virtualization||Aug 18, 2015|
|Firefox Security Exploit Targets Linux Users and Web Developers||Aug 17, 2015|
- Concerning Containers' Connections: on Docker Networking
- A Project to Guarantee Better Security for Open-Source Projects
- Problems with Ubuntu's Software Center and How Canonical Plans to Fix Them
- My Network Go-Bag
- Firefox Security Exploit Targets Linux Users and Web Developers
- Doing Astronomy with Python
- Build a “Virtual SuperComputer” with Process Virtualization
- diff -u: What's New in Kernel Development
- Three More Lessons
- August 2015 Issue of Linux Journal: Programming