Operating Environment Customization

Introduction

rom the FAQ maintained by the ITC helpdesk:

The shell is a command line interpreter between you and the Unix operating system. The shell accepts your commands and then executes them. The shell has a large number of built in commands as well as the capability of being a programming language

Changing your shell

Fortunately there are many different shells around. If you don't like your current login shell (i.e., the one you get when you log in) you can always replace it with a better one. More information on shell differences and how to change your shell.

Shells available in the department

These are some of the department-supported shells. They reside in /bin.

ksh - Korn shell
sh - Bourne shell
csh - C shell
tcsh- A better C shell

Some popular shells are user-contributed and live in /usr/cs/contrib/bin:

bash - a shell by Free Software Foundation
zsh - the Z shell
wish and - Tcl/Tk based windowing shells

Shell scripts

Any UNIX shell is capable of interpreting programs written in the language of that shell. Such programs are called shell scripts.

Here is an example of a Korn shell script taken from part two of comp.os.unix FAQ. It renames all files "*.foo" to "*.bar".

        #!/bin/ksh
        for f in *.foo; do
            mv $f ${f%foo}bar
        done
        #end

The first line of this example tells your current shell that the Korn shell must be invoked to execute the script. For more examples and information on writing scripts see UNIX scripts FAQ maintained by ITC helpdesk.

Any shell is a command language interpreter. Though some of the shells are considered more suitable for script programming than others. For example, you should be aware that csh programming might be a dangerous thing.

.login, .profile and other "dot" files

When a UNIX shell starts or terminates it automatically executes certain scripts. Names of the script files always begin with a dot. A list of "dot" files that various shells use. You can put in these files whatever commands you want to execute at start-up and/or termination of every UNIX session.

Environment Variables

Any shell maintains a set of string variables called environment variables. Those variables are available to any program started from that shell. Some of them are used by the shell itself. Here are some important variables:

PATH - a list of directories that shell scans when it looks for a program file to execute. The shell always scans that list from the beginning to end so the order is important.
DISPLAY - this variable is used by X-Window applications to determine what X server should they connect to. This variable is set for you automatically in the system startup script. It usually contain the name of your machine followed by something like :0.0 If this variable is set incorrectly, then you may be unable to run your X applications.
TERM - editors and other programs check this variable to determine what kind of terminal you have. This variable may contain something like "VT100" if you are working from an alphanumeric terminal or using telnet. It usually contains "xterm" if you are using X Windows.