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

  1. "Tutorials – Shell Flow Control" by Marco Fioretti, Linux Magazine, issue 221, April 2019, pp. 86-91
  2. Bash test constructs: https://www.tldp.org/LDP/abs/html/testconstructs.html
  3. Bash "if" statement conditions: https://linuxacademy.com/blog/linux/conditions-in-bash-scripting-if-statements/
  4. Bash file testing operators: http://www.tldp.org/LDP/abs/html/fto.html
  5. Bash reserved exit codes: http://www.tldp.org/LDP/abs/html/exitcodes.html#EXITCODESREF

The Author

Marco Fioretti (http://mfioretti.com) is a freelance author, trainer, and researcher based in Rome, Italy. He has been working with free/open source software since 1995 and on open digital standards since 2005. Marco also is a Board Member of the Free Knowledge Institute (http://freeknowledge.eu).

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