What Is a Shell?
A shell is a program that provides an interface between a user and an operating system (OS) kernel. An OS starts a shell for each user when the user logs in or opens a terminal or console window.
A kernel is a program that:
- Controls all computer operations.
- Coordinates all executing utilities
- Ensures that executing utilities do not interfere with each other or consume all system resources.
- Schedules and manages all system processes.
By interfacing with a kernel, a shell provides a way for a user to execute utilities and programs.
User Environment
The shell also provides a user environment that you can customize using initialization files. These files contain settings for user environment characteristics, such as:
- Search paths for finding commands.
- Default permissions on new files.
- Values for variables that other programs use.
- Values that you can customize.
What are the different Shells?
The following sections describe OS shells mostly available on UNIX/Linux Operating system. Shell features and their default prompts are also described.
1. The Bourne Shell
The Bourne shell (sh), written by Steve Bourne at AT&T Bell Labs, is the original UNIX shell. It is the preferred shell for shell programming because of its compactness and speed. A Bourne shell drawback is that it lacks features for interactive use, such as the ability to recall previous commands (history). The Bourne shell also lacks built-in arithmetic and logical expression handling.
The Bourne shell is the Solaris OS default shell. It is the standard shell for Solaris system administration scripts. For the Bourne shell the:
- Command full-path name is /bin/sh and /sbin/sh.
- Non-root user default prompt is $.
- Root user default prompt is #.
2. The C Shell
The C shell (csh):
- Is a UNIX enhancement written by Bill Joy at the University of California at Berkeley.
- Incorporated features for interactive use, such as aliases and command history.
- Includes convenient programming features, such as built-in arithmetic and a C-like expression syntax.
For the C shell the:
- Command full-path name is /bin/csh.
- Non-root user default prompt is hostname %.
- Root user default prompt is hostname #.
3. The Korn Shell
The Korn shell (ksh):
- Was written by David Korn at AT&T Bell Labs
- Is a superset of the Bourne shell.
- Supports everything in the Bourne shell.
- Has interactive features comparable to those in the C shell.
- Includes convenient programming features like built-in arithmetic and C-like arrays, functions, and string-manipulation facilities.
- Is faster than the C shell.
- Runs scripts written for the Bourne shell.
For the Korn shell the:
- Command full-path name is /bin/ksh.
- Non-root user default prompt is $.
- Root user default prompt is #.
4. The GNU Bourne-Again Shell
The GNU Bourne-Again shell (bash):
- Is compatible to the Bourne shell.
- Incorporates useful features from the Korn and C shells.
- Has arrow keys that are automatically mapped for command recall and editing.
For the GNU Bourne-Again shell the:
- Command full-path name is /bin/bash.
- Default prompt for a non-root user is bash-x.xx$. (Where x.xx indicates the shell version number. For example, bash-3.50$)
- Root user default prompt is bash-x.xx#. (Where x.xx indicates the shell version number. For example, bash-3.50$#)
Here is a short comparison of the all 4 shells and their properties.
| Shell | Path | Default Prompt (non-root user) | Default Prompt (Root user) |
|---|---|---|---|
| The Bourne Shell (sh) | /bin/sh and /sbin/sh | $ | # |
| The C Shell (csh) | /bin/csh | % | # |
| The Korn Shell (ksh) | /bin/ksh | $ | # |
| The GNU Bourne-Again Shell (Bash) | /bin/bash | bash-x.xx$ | bash-x.xx# |