The Shell Game 2

Kevin O'BrienWashtenaw Linux Users Group

http://www.lugwash.org

Varieties of Shell

● In Linux, you always have choices, whether you like it or not ☺

● With shells you have choices● The Bourne-Again Shell (bash) is most likely

the default you have on your system now● But you can use others as well

Why use different shells?

● Like any other choice, you would select different shells because they have special capabilities you want to make use of

● They may have special commands, or use less resources, or maybe you just feel more comfortable with one shell rather than another

Bourne shell (sh)

● The original Unix shell● Written by Stephen Bourne at Bell Labs in

1974● A simple shell, with small size and few

features● Every Unix-like system either has sh, or has a

shell that incorporates everything in sh

Bourne-Again Shell (bash)

● The default for all Linux systems● Also runs on virtually all Unix-like systems,

and there is even a version available for Windows

● Bash is a superset of sh, that is, it incorporates everything that is in sh, but then adds to it

● It is very flexible, and a good choice for beginners. The rest of this series will use bash to illustrate using a shell.

Almquist Shell (ash)

● Basically a clone of sh● Very small memory requirements● Thus it is useful for small embedded systems

C Shell (csh)

● Created by Bill Joy while he was at UC Berkeley

● Syntax very similar to the C programming language

Korn shell (ksh)

● Developed by David Korn at Bell Labs in 1983● Superset of SH, with many features of the C

Shell as well● Advanced scripting capabilities similar to what

is in awk, sed, and perl

TENEX C Shell (tcsh)

● Based on C Shell● Adds features not found in C Shell● Now the default shell on some BSD systems

(FreeBSD and Darwin)

Z Shell

● Written by Paul Falstad around 1990● Simlar to ksh, but has features from csh as

well● Attempt to use the programmability of the ksh

with csh features

You can choose

● You can make a temporary switch of your shell

● Or you can change the default if you find one you like better

Temporary switch

● A shell is an executable file (everything in Linux is a file)

● So you just run it● For example, to change to the original Bourne

shell, just type “sh” at the prompt● To go back to the Bourne-Again shell, just

type “bash” at the prompt● When you change, notice that the prompt itself

looks different

Changing the default 1

● Suppose you find a shell you like better, and you want that to be the shell you always want to see when you boot up your computer

● First, find out the full path of the shell, i.e., what is the full path, starting from the root, to the executable file that contains the shell

● To get clues, open up /etc/shells and etc/passwd

● For example, bash is usually /bin/bash

Changing the Default 2

● Once you know the full path to the shell you now want to be the default, use the change shell command (chsh)

● This will open a brief dialog to make the change, and will ask you for your password to authorize it

Whose shell is this, anyway?

● One thing you need to remember is that the choice of shell is only being made for the person logged in

● You could have multiple users on a system, and have each one make their own shell choice

● Tip: always try out a shell temporarily before making a permanent change