Welcome back to our mini-series on square brackets. In the previous article, we looked at various ways square brackets are used at the command line, including globbing. If you’ve not read that article, you might want to start there.

Square brackets can also be used as a command. Yep, for example, in:

 [ "a" = "a" ]

which is, by the way, a valid command that you can execute, [ ... ] is a command. Notice that there are spaces between the opening bracket [ and the parameters "a" = "a", and then between the parameters and the closing bracket ]. That is precisely because the brackets here act as a command, and you are separating the command from its parameters.

You would read the above line as “test whether the string “a” is the same as string “a”“. If the premise is true, the [ ... ] command finishes with an exit status of 0. If not, the exit status is 1.