Shell Programming

Type of Shell

There are several different type shells for UNIX. We will discuss some of these shells.

1. Bourne shell(sh)
2. C shell(csh)
3. TC shell(tcsh)
4. Korn shell(ksh)
5. Bourne Again shell(bash)
6. Almquist shell(ash)
7. Debian Almquist shell(dash)
8. Friendly interactive shell(fish)
9. Es
10. Rc
11. Scsh
12. sash
13. zsh

Bourne shell
This is the original shell by Steve Bourne of Bell Labs. It is not interactive like C shell and korn shell. It is intended as a scripting language and contains all the features to produces structured programs. It is available on all UNIX systems. It is small and fast. It is good for I/O control. Its default prompt is $.

C shell
It was written at the University of California, Berkeley. It is C-like syntax which provides shell scripting. It is a command language interpreter. It provides some additional features as command history, command alias, filename completion and job control. Its default prompt is %.

TC shell
It is enhanced version of C shell. It is a command language interpreter used as interactive login shell as well as shell script command processor. It provides all the features of the C shell together with emacs style editing of the command line, command history, command alias, filename completion and job control.

Korn shell
It was developed by David Korn at Bell Labs. It is considered as standard shell on UNIX systems. It provides all the features of the C and TC shells together. It is backward-compatible with the Bourne shell. The Korn Shell language is also a complete, powerful, high-level programming language for writing applications, and hence suitable for prototyping.

Bourne Again shell
It was written by Brian Fox for the GNU Project as free software. It is default shell on LINUX operating system. It is a command processor and run in text window. It incorporates features from the C Shell and the Korn Shell and conforms to the POSTIX 2 shell specifications.

Shell and Shell Script

Shell

Shell is a command interpreter that executes commands reads from the keyboard or from a file. It is user program and not a part of kernel, but uses the system kernel to execute programs.It is a user program that is ASCII based, that allows the user to specify operations in a certain sequence.

Some aspects of shell
1. User interacts with the system using shell.
2. It is a full featured programming language, with variables, conditional statements, and the ability to execute other programs.
3. It allows you to easily create a new program with all of the privileges of any other UNIX program.

Shell script Shell script is series of commands written in plain text file. It is just like a batch file.
Whenever we give command to shell through keyboard, it accepts command and executes them. But when we write command one by one then we can store this sequence of commands to text file and tell the shell to execute this file instead of entering command. This is called as shell script.

Use of Shell Script
1. Shell script is used for manipulation of text or files.
2. Useful to create our own commands.
3. To automate some task of day today life.
4. System Administration part can be also automated.
5. Creating simple application