Work the Shell - Understanding Exit Codes
Last month, we looked at signals, the rudimentary mechanism that processes on a Linux box can use to communicate events and state changes. We talked about how each of the signals can be sent manually to a running process with the kill command, and how shell scripts then can catch and respond to specific signals (though not all of them—some cannot be caught because they're actually handled by the operating system itself).
Analogous to signals, exit codes turn out to be an easy way for processes to communicate state back to the calling parent process, in a way that most Linux users just ignore. Not anymore—this month, we're going to take a closer look.
Let's start with a simple Linux command that everyone's probably already mastered: mv, which moves a file or directory from one spot in the filesystem to another (and/or renames it).
As you know, you can generate errors if the target is missing, destination is missing and so on. Here's quick example:
$ mv missing ~/missing2 mv: cannot move `missing' to `.../missing2': No such file or directory
You see an error; obviously, it didn't work. Ah, but behind the scenes, a numeric “return code” variable has been set in the shell too, something you can test and respond to within a shell script. Check out this sequence:
$ echo $? 0 $ mv missing ~/missing2 mv: cannot move `missing' to `.../missing2': No such file or directory $ echo $? 1 $ echo test me test me $ echo $? 0
If no error occurs when executing a command, the exit code (which we reference in the shell with the shorthand $?) will have the value of 0: no error. Now, if I run a command that fails, the exit code will have a nonzero value. In the case of the failing mv command above, the error code will have the value of 1. And, if I now run yet another command, one which runs without error, the error code is reset to zero.
Now, let's take a peek at the mv man page, paying particular attention to the latter part of the doc. Close examination reveals: “The mv utility exits 0 on success, and >0 if an error occurs.”
That's not so interesting, really. The grep command has more interesting diagnostics, actually: “Normally, exit status is 0 if selected lines are found and 1 otherwise. But the exit status is 2 if an error occurred, unless the -q or --quiet or --silent option is used and a selected line is found.”
There is a set of system exit codes that are defined, although it's possible you'll never need the information. Here's a list of the codes and their meanings:
1: general errors
2: misuse of shell builtins (pretty rare)
126: cannot invoke requested command
127: command not found error
128: invalid argument to “exit”
128+n: fatal error signal “n” (for example, kill -9 = 137).
130: script terminated by Ctrl-C
I'd never actually seen this list until I started digging into the issue of exit codes, so you can continue on your merry shell-scripting path safely without worrying about the data above.
The most common situation in which you analyze and respond to an exit code is in error handling in a script.
Here's a simple snippet where you want to create a directory. If it fails, you want to output an error message and quit:
#!/bin/sh mkdir /usr echo \$? = $? if [ $? -ne 0 ] ; then echo "mkdir /usr failed: we have an exit code of $?" exit 1 fi echo "made the requested directory. Why is '/' world writable?" exit 0
It turns out, there's a nuance to working with the $? that I've already alluded to—one that makes output statements like the first “echo” quite problematic. You can see why in the output:
$ ./test.sh mkdir: /usr: File exists $? = 1 made the requested directory. Why is '/' world writable?
Can you see what happened? The exit code = 1 immediately after the mkdir, which makes sense as /usr already exists, but when we again test the exit code in the conditional, it's not a nonzero value!
Why? Because at that point, it indicates the exit code of the echo statement, not the mkdir command. Oops.
You can verify this simply by commenting out the first echo statement, in which case you now see this as the command output:
$ !. ./test.sh mkdir: /usr: File exists mkdir /usr failed: we have an exit code of 0
That makes more sense, doesn't it?
Because this can be tricky, a common thing I see in really bulletproof shell scripts with lots of error handling is something like this:
#!/bin/sh mkdir /usr error=$? if [ $error -ne 0 ] ; then echo "mkdir /usr failed: we have an exit code of $error" exit 1 fi
This is one instance where a local variable to hold a system or global variable makes a lot of sense, and it also lets you do things like have an error message show up on-screen and be handed off to something like syslog() to ensure that the admin sees it at some point.
Of course, error handling doesn't always just need to print a message and exit. Another scenario might be the following:
alternates=' http://www.example.com/test.pdf http://www.example2.com/test.pdf http://www.example3.com/test.pdf ' gotit=0 for file in $alternates do wget $file if [ $? -ne 0 ]; then echo "Unable to get $file else gotit=1 break fi done ...
Here, we try to retrieve a file from one of multiple alternate locations and exit the loop only when we succeed (or when we've run out of possibilities).
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.
Fast/Flexible Linux OS Recovery
On Demand Now
In this live one-hour webinar, learn how to enhance your existing backup strategies for complete disaster recovery preparedness using Storix System Backup Administrator (SBAdmin), a highly flexible full-system recovery solution for UNIX and Linux systems.
Join Linux Journal's Shawn Powers and David Huffman, President/CEO, Storix, Inc.
Free to Linux Journal readers.Register Now!
- Back to Backups
- Download "Linux Management with Red Hat Satellite: Measuring Business Impact and ROI"
- A New Version of Rust Hits the Streets
- Google's Abacus Project: It's All about Trust
- Secure Desktops with Qubes: Introduction
- Seeing Red and Getting Sleep
- Fancy Tricks for Changing Numeric Base
- Secure Desktops with Qubes: Installation
- Working with Command Arguments
- Linux Mint 18
Until recently, IBM’s Power Platform was looked upon as being the system that hosted IBM’s flavor of UNIX and proprietary operating system called IBM i. These servers often are found in medium-size businesses running ERP, CRM and financials for on-premise customers. By enabling the Power platform to run the Linux OS, IBM now has positioned Power to be the platform of choice for those already running Linux that are facing scalability issues, especially customers looking at analytics, big data or cloud computing.
￼Running Linux on IBM’s Power hardware offers some obvious benefits, including improved processing speed and memory bandwidth, inherent security, and simpler deployment and management. But if you look beyond the impressive architecture, you’ll also find an open ecosystem that has given rise to a strong, innovative community, as well as an inventory of system and network management applications that really help leverage the benefits offered by running Linux on Power.Get the Guide