What is a shell command?

When you open the Terminal application, you are presented with the shell to which you can type commands. Your commands are read then executed by the shell, usually by launching a program corresponding to the command’s name.

When you select a default application for a file type or the application to use for a rule, you can also choose to use a shell command by selecting the Shell Command menu item. You will be presented the command editor where you can write the command that will be issued when opening a file.