The UNIX system uses the term ``environment'' to refer to all the settings that influence the way you work on the computer.
You can define the following sort of information in your environment:
Each shell has certain control files that it reads when you log in. For the Bourne shell (sh), the control file is called .profile. The Korn shell (ksh) has both a .profile and a .kshrc, and the C shell (csh) has a .login and a .cshrc.
The difference between .profile and .kshrc, and between .login and .cshrc, is in when the files are read. The .profile and the .login are only read when you log in. However, the .rc files, .kshrc and .cshrc, are read each time you start a ksh or csh. (You can start a shell from the command line by typing the name of the shell just like you would type any command.)
These control files are shell scripts: ``shell'' because they are written in the shell programming language; ``scripts'' because they are text files that are read one line at a time, like a DOS batch file.
In shell scripts, you see commands you are already familiar with, as well as programming constructs for looping, branching, and setting variables.
For listings and explanations of a typical .profile, .kshrc, .login, and .cshrc, see ``Sample shell startup files''.