9.1. Introduction

An interactive shell is one in which the standard input, output, and errors are connected to a terminal. The C shell (csh) and TC shell (tcsh) provide a number of convenient features for interactive use that are not available with the Bourne shell, including filename completion, command aliasing, history substitution, job control, and more. These shells are almost identical when used as a programming language, but the TC shell has many more interactive features than its predecessor, the C shell. This section covers the common features of the C shell and TC shell for interactive use. If you are looking for some nice shortcuts at the command line, the new TC shell enhancements are covered in the second half of this chapter, beginning with "New Features of the Interactive TC Shell" on page 460.

9.1.1 C/TC Shell Startup

Before the shell displays a prompt, it is preceded by a number of processes. (See Figure 9.1.) After the system boots, the first process to run is called init; it is assigned process identification number (PID) 1. It gets instructions from a file called inittab (System V) or spawns a getty process (BSD). These processes are responsible for opening up the terminal ports, for providing a place where input comes from (stdin), where standard output (stdout) and error (stderr) go, and for putting a login prompt on your screen. The /bin/login program is then executed. The login program prompts for a password, encrypts and verifies your password, sets up an initial working environment, and then initiates the shell, /bin/csh or bin/tcsh for the C or TC shell, respectively.

When the shell is a login shell, this is the sequence of invocations: The C/TC shell looks in the /etc directory for system startup files, which are executed if they exist. For the C shell, for example, these files are /etc/csh.cshrc and /etc/csh.login. (See Chapter 16, "The System Administrator and the Shell," on page 1023 for details.) Then the C/TC shell looks in the user's home directory for a file called .cshrc or .tcshrc, respectively, which is an initialization file that allows you to customize the C/TC shell environment in which you will be working. After executing commands in the .cshrc or .tcshrc file, commands in the .login file are executed. The .cshrc or .tcshrc file will be executed every time a new C/TC shell is started. The .login file is executed only once when the user logs on, and also contains commands and variables to initialize the user's environment. After executing commands from those files, the prompt appears on your screen and the C/TC shell awaits commands. Most systems today start up with a graphical user interface, adding to the steps just mentioned. When you are in the desktop, such as CDE, you will see icons and menus. Once you select a terminal from the menu, the shell prompt will appear and you can start typing commands.