What Is the Bash Shell, and Why Is It So Important to Linux? What Is a Shell? When you open a terminal window and type commands, something has to take what you’ve typed, figure out what you intended, and run the tasks you asked for. The software that does this is the shell. A shell is a command interpreter. It scans what you’ve typed and picks out the commands, directory names, file names, and program names so that it can figure out what you’re trying to achieve. People often use the phrases “terminal windows,” “command line,” and “shell” interchangeably, but they’re three distinct things. A terminal window is a software representation of a physical teletype terminal. It gives you a connection to the computer. In order to do anything useful, you must be able to type instructions at a command line. The command line is provided by the shell, and the terminal window lets you access the shell.