The programs the user interfaces to are called shells. The standard shell for Linux is called bash and is similar to the Bourne shell. There are alternatives such as tcsh which are similar to the C shell. Although they provide the same basic facilities, they are different and shell scripts (batch files) written for one will not necessarily run on another. The commands described in this chapter are identified if they are specific to one shell or another. However, the detailed operations of the shells are so different that they are covered in separate chapters. The system administrator decides which shell the users see by specifying it in the user's entry in /etc/passwd.