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Introduction to Unix

CHAPTER 5 Shells

The shell sits between you and the operating system, acting as a command interpreter. It reads your terminal input and translates the commands into actions taken by the system. The shell is analogous to command.com in DOS. When you log into the system you are given a default shell. When the shell starts up it reads its startup files and may set environment variables, command search paths, and command aliases, and executes any commands specified in these files.

The original shell was the Bourne shell, sh. Every Unix platform will either have the Bourne shell, or a Bourne compatible shell available. It has very good features for controlling input and output, but is not well suited for the interactive user. To meet the latter need the C shell, csh, was written and is now found on most, but not all, Unix systems. It uses C type syntax, the language Unix is written in, but has a more awkward input/output implementation. It has job control, so that you can reattach a job running in the background to the foreground. It also provides a history feature which allows you to modify and repeat previously executed commands.

The default prompt for the Bourne shell is $ (or #, for the root user). The default prompt for the C shell is %.

Numerous other shells are available from the network. Almost all of them are based on either sh or csh with extensions to provide job control to sh, allow in-line editing of commands, page through previously executed commands, provide command name completion and custom prompt, etc. Some of the more well known of these may be on your favorite Unix system: the Korn shell, ksh, by David Korn and the Bourne Again SHell, bash, from the Free Software Foundations GNU project, both based on sh, the T-C shell, tcsh, and the extended C shell, cshe, both based on csh. Below we will describe some of the features of sh and csh so that you can get started.

5.1 - Built-in Commands
5.2 - Environment Variables
5.3 - The Bourne Shell, sh
5.4 - The C Shell, csh
5.5 - Job Control
5.6 - History
5.7 - Changing your Shell

Introduction to Unix - 14 AUG 1996
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