Synopsis:
Unix: An introduction to Unix shells

Solution:
What is a shell?
A Unix shell is what gives Unix its powerful capabilities. The shell is the part of the system with which the user interacts, and as such it is arguably the most important part of a Unix system. A Unix shell interprets commands such as "pwd", "cd" or "traceroute" and sends the proper instructions to the actual operating system itself. Other functions of a shell include scripting capability, path memory, multitasking, and file handling.

Effective February 2, 2004 new Unix accounts (Undergraduate accounts excepted) will be set up with a default Unix shell of /bin/nologin that prohibits login access. If you wish to use a your Unix account, simply change this option by using the Enable Unix Account page. (Log in using your netID and LDAP password.) See How do I enable / change my default Unix shell for more information regarding your options.

Available Shells
There are several shells available on the OIT Unix systems: bash (Bourne Again), csh (C), ksh (Korn), rksh (restricted Korn), sh, tcsh (Turbo C) and zsh (Z). (Note that they all include the two letter sequence "sh"). By default, all enabled accounts are set up to use csh (C shell). For those who intend to use Unix more extensively, tcsh (Turbo C Shell) is a popular shell. Bash (Bourne Again Shell) is another popular shell, often used by beginning Computer Science students.

The C Shells
There are two C shells: Csh and Tcsh. Csh is the most common shell on the Princeton campus because it is the Princeton-specified default. Many commercial Unix systems set ksh as the default; Linux specifies bash. See the Help Desk solution: How to enable / change your Unix shell.

There are several nifty things about the C shells:

• Aliasing
• Prompt Customization
• History Recording
• Filename Completion (tcsh only)

The setenv command controls shell variables. For example, if you wanted to use the M/X Server to use Xterms and other sundry Unix windows in your dorm room, you could SSH to arizona as usual, and upon entering the shell type "setenv DISPLAY=yourNetID.student.princeton.edu:0.0". This sets the environment variable to display all window commands to your PC at home.

Another useful function of setenv variables is use in scripting. In a shell script, referring to

• $HOME

would be the same as replacing

• $HOME

with

• /u/yourNetID

These variables work the same way in almost all shells.

The Tcsh is an extension of csh, and allows command line editing, file name completion, plus previous command recall via the up-arrow (which are all very useful). Tcsh is very versatile, and has all the features that csh has. Instructions on how to change your shell are in the Help Desk Online KnowledgeBase. Tcsh offers conveniences.

For example, filename completion is an essential feature of tcsh. When typing a command, such as netscape, you can type

• netsc

and press [TAB] at this point. Since there are no other commands on the arizona systems that begin with "mythes", the shell finishes the command for you to

• mythesis_for_school

If you spend much time manipulating files in Unix, you will find this feature indispensable. It works with directory names, as well. In almost any situation, you can fill in filenames and commands using the [TAB] key.

The Bourne Again Shell
The Bash shell is a redundant acronym short for "Bourne Again Shell".
Bash is distributed by almost every archive of free software on the web, but the original authors were part of the Free Software Foundation, found at www.gnu.org. Now there exist ports of bash to several different flavors of Unix.

What advantages are there to using bash? One is that it is made by the Free Software Foundation. A feature of bash is its way of manipulating environment variables. For example, bash uses the "export" command to keep track of the $DISPLAY value (important for using X-terminals) instead of the csh/sh/tcsh-standard "setenv DISPLAY=" command.

So, if you were logged in via an ssh window and wanted to get windows on your x server from that session, you would type

export DISPLAY=$REMOTEHOST:0.0

That would set the DISPLAY variable to your logged in host.

When it really comes down to it, these shells are pretty much the same, except that bash uses somewhat different terminology.

One cool thing about bash is that system crackers do NOT like it. Bash keeps a record of what commands you execute, and stores it in a file called .bash_history in your home directory. Since this leaves traces of what commands are executed by a user of the account, a cracker would either delete this file or could be detected by looking at backed up copies of the file. Either way, crackers dislike bash. If you use bash, you can keep tabs on what goes on in your account.

Some technical notes about bash:
Bash, unlike its ancestor, sh, is completely POSIX-standards conformant. Sh was written before POSIX standards were in place. Bash also has the capability of using one-dimensional arrays, a history (unlike sh), argument negation using the ! character (a la C), and compound brackets and parentheses. Bash can also be used as a restricted shell, limited to several commands. One final noteworthy feature of bash is its customizability. Anyone can edit their .bash* files to suit their preferences. These files include .bashrc, .bash_profile, and .profile.

The Korn Shell
Unlike Bash, the Korn shell (ksh) and the Korn Restricted Shell (rksh) were written by David Korn of Bell Labs. What advantages are there to using Korn? Korn has a command history feature, a command alias feature, support for shell scripts, arithmetic expressions, filename completion (disabled by default), and command line editing (also disabled by default). The sheer number of things disabled by default makes it a chore to start using any Korn shell. To make matters worse, the K shells all use the "export" feature, instead of "setenv." (See Bash shell description for details.)

• ksh keeps a 128-command history file by default
• when editing a command at the prompt, you have to use ridiculously annoying keys, starting with ESC, then "h" for right and "l" for left. Arrow keys are not supported.
• You can't change the look of your command prompt.
• Korn shells don't support filename completion.

For more information regarding Unix shells, read the following links:

http://www.faqs.org/faqs/unix-faq/shell/shell-differences
(Internet FAQ Archive, July 1997)

http://www.computerbits.com/archive/1997/1100/lnx9711.html
(Computer Bits, November 1997)