Shell test conditions and exit codes
Exit Codes
Many times a script can only decide what to do next by looking at the exact outcome of some command or other program. Traditionally, this information is provided through exit status codes, also known as return statuses. Exit codes are integer numbers that commands or scripts output when they end. By historical convention, an exit status equal to zero means that the execution was successful, and any non-zero value indicates that something bad, or at least unexpected, happened. In the Bash environment, the exit status of the last instruction that was executed is stored in the special variable $?
.
Every well-written program (including your scripts) should output defined exit codes, one for each way in which the script could end. As a matter of fact, all Bash functions, pipes, and scripts always return an exit code even if their author did not set it explicitly. That number is the exit code of the last command executed by the function, pipe, or script.
However, it is good practice to make a script end with an informative exit code (see the box "Exit Codes to Avoid"). Doing so couldn't be simpler: Just use the exit
command, followed by the number designated as exit status:
exit 400 #
Another thing you should remember about exit status codes is that every time your script interprets a line, it produces a number that you may examine with numeric test operators to diagnose problems or decide what to do next. This means, among other things, that exit codes also provide the basis to quickly implement test operators for files or strings that may be more verbose, but also more sophisticated, than the built-in operators.
A Final Example
To learn another way to check if a filename or any other string contains spaces, type the following commands at the prompt:
#> TESTSPACE='Hello World' #> echo $TESTSPACE | egrep -q "[[:space:]]" #> echo $? #> TESTSPACE='HelloWorld' #> echo $TESTSPACE | egrep -q "[[:space:]]" #> echo $?
Exit Codes to Avoid
Certain exit codes have reserved meanings, which you must not use them unless you really want to break or confuse whoever runs your code (other scripts, other users, or even you). You can find these reserved exit codes listed online [5]. To give you an idea of the kind of messages reserved exit codes carry, here are two examples:
- 126: Command invoked cannot execute (not an executable program or permission problem)
- 127: Command not found (possible problem with
$PATH
or a typo)
Infos
- "Tutorials – Shell Flow Control" by Marco Fioretti, Linux Magazine, issue 221, April 2019, pp. 86-91
- Bash test constructs: https://www.tldp.org/LDP/abs/html/testconstructs.html
- Bash "if" statement conditions: https://linuxacademy.com/blog/linux/conditions-in-bash-scripting-if-statements/
- Bash file testing operators: http://www.tldp.org/LDP/abs/html/fto.html
- Bash reserved exit codes: http://www.tldp.org/LDP/abs/html/exitcodes.html#EXITCODESREF
« Previous 1 2 3
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
TUXEDO Computers Unveils Linux Laptop Featuring AMD Ryzen CPU
This latest release is the first laptop to include the new CPU from Ryzen and Linux preinstalled.
-
XZ Gets the All-Clear
The back door xz vulnerability has been officially reverted for Fedora 40 and versions 38 and 39 were never affected.
-
Canonical Collaborates with Qualcomm on New Venture
This new joint effort is geared toward bringing Ubuntu and Ubuntu Core to Qualcomm-powered devices.
-
Kodi 21.0 Open-Source Entertainment Hub Released
After a year of development, the award-winning Kodi cross-platform, media center software is now available with many new additions and improvements.
-
Linux Usage Increases in Two Key Areas
If market share is your thing, you'll be happy to know that Linux is on the rise in two areas that, if they keep climbing, could have serious meaning for Linux's future.
-
Vulnerability Discovered in xz Libraries
An urgent alert for Fedora 40 has been posted and users should pay attention.
-
Canonical Bumps LTS Support to 12 years
If you're worried that your Ubuntu LTS release won't be supported long enough to last, Canonical has a surprise for you in the form of 12 years of security coverage.
-
Fedora 40 Beta Released Soon
With the official release of Fedora 40 coming in April, it's almost time to download the beta and see what's new.
-
New Pentesting Distribution to Compete with Kali Linux
SnoopGod is now available for your testing needs
-
Juno Computers Launches Another Linux Laptop
If you're looking for a powerhouse laptop that runs Ubuntu, the Juno Computers Neptune 17 v6 should be on your radar.