Checking the operating system type in shell script

Nov 5, 2016

Last week I learned about a tool called ShellCheck, a tool for static analysis of shell scripts. It reports errors like missing double quotes, use of deprecated syntax, etc.

I decided to check some projects I contribute to, and the first issue I found was in Apache Jena:

kinow@localhost:~/Development/java/jena/jena/apache-jena/bin$ shellcheck arq

In arq line 8:
    case "$OSTYPE" in
          ^-- SC2039: In POSIX sh, OSTYPE is not supported.

So, in summary, the OSTYPE variable should not be available in POSIX shell. The case in question, where OSTYPE is being used, checks for the Darwin OS type (i.e. Mac OS). Knowing how things get weird when you use different operating systems, I decided to check and learn how OSTYPE works. Here’s what I found.

In Ubuntu, with /bin/bash, OSTYPE works fine (linux-gnu)
In Ubuntu, with /bin/sh, OSTYPE is not set
In Mac OS, with /bin/sh, OSTYPE is set (darwin15)

I checked the shells to make sure they were not pointing to symbolic links - some distributions use a different default shell, and replace /bin/bash and/or /bin/sh by a link to another shell. Looks like Mac OS has a POSIX shell that behaves different than Ubuntu’s.

Instead of trying to find a way to use OSTYPE, I decided to spend some time looking at how other projects do the same thing. And the best example I could find was git.

Instead of relying on OSTYPE, git uses uname.

I will spend some time during the next days working on a proposal to replace the OSTYPE from Apache Jena scripts, but then may have to submit more changes for the other issues found by ShellCheck.

Happy hacking!

Categories: Blog

Tags: Shell Script