Understanding Bashisms

A bashism is any feature that’s specific to bash, and that won’t work with other shells. Let’s look at a few examples.

Using Portable Tests

For our first example, try running this command on your Fedora virtual machine:

donnie@fedora:~$ [[ -x /bin/ls ]] && echo "This file is installed.";
This file is installed.
donnie@fedora:~$

Here, I’m testing for the presence of the ls executable file in the /bin/ directory. The file is there, so the echo command is invoked. Now, let’s run the same command on a FreeBSD virtual machine:

donnie@freebsd14:~ $ [[ -f /bin/ls ]] && echo "This file is installed."
-sh: [[: not found
donnie@freebsd14:~ $

This time I get an error, because the default user login shell on FreeBSD is sh, instead of bash. The problem here is that the [[. . .]] construct isn’t supported on the FreeBSD implementation of sh. Let’s see if we can fix...