Shell types and BASH shell
 


Different types of shell are

Bourne Shell
The original Bourne shell is named after its developer at Bell Labs, Steve Bourne. It was the first shell used for the Unix operating system, and it has been largely surpassed in functionality by many of the more recent shells. However, all Unix and many Linux versions allow users to switch to the original Bourne Shell, known simply as "sh," if they choose to forgo features such as file name completion and command histories that later shells have added.

C Shell
The C shell, as its name might imply, was designed to allow users to write shell script programs using a syntax very similar to that of the C programming language. It is known as "csh."

TC Shell
TC shell is an expansion upon the C shell. It has all the same features, but adds the ability to use keystrokes from the Emacs word processor program to edit text on the command line. For example, users can press Esc-D to delete the rest of the highlighted word. It is also known as "tcsh."

Korn Shell
Korn Shell was also written by a developer at Bell Labs, David Korn. It attempts to merge the features of the C shell, TC shell and Bourne shell under one package. It also includes the ability for developers to create new shell commands as the need arises. It is known as "ksh."

Bourne-Again Shell
The Bourne-Again shell is an updated version of the original Bourne shell that was created by the Free Software Foundation for its open source GNU project. For this reason, it is a widely used shell in the open source community.

Its syntax is similar to that used by the Bourne shell, however it incorporates some of the more advanced features found in the C, TC and Korn shells.

Among the added features that Bourne lacked are the ability to complete file names by pressing the TAB key, the ability to remember a history of recent commands and the ability to run multiple programs in the background at once. It is known as "bash."

 

BASH Shell

Features only found in bash

Invocation
In addition to the single-character shell command line options which can generally be configured using the set shell built-in command, there are several multi-character options that you can use. We will come across a couple of the more popular options in this and the following chapters; the complete list can be found in the Bash info pages, Bash features->Invoking Bash.

Bash startup files
Startup files are scripts that are read and executed by Bash when it starts. The following subsections describe different ways to start the shell, and the startup files that are read consequently.

Invoked as an interactive login shell, or with `--login'
Interactive means you can enter commands. The shell is not running because a script has been activated. A login shell means that you got the shell after authenticating to the system, usually by giving your user name and password.
Files read:

    /etc/profile

    ~/.bash_profile, ~/.bash_login or ~/.profile: first existing readable file is read

    ~/.bash_logout upon logout.

Error messages are printed if configuration files exist but are not readable. If a file does not exist, bash searches for the next.

Invoked as an interactive non-login shell
A non-login shell means that you did not have to authenticate to the system. For instance, when you open a terminal using an icon, or a menu item, that is a non-login shell.
Files read:

    ~/.bashrc

This file is usually referred to in ~/.bash_profile:

if [ -f ~/.bashrc ]; then . ~/.bashrc; fi


Invoked non-interactively
All scripts use non-interactive shells. They are programmed to do certain tasks and cannot be instructed to do other jobs than those for which they are programmed.

Files read:

    defined by BASH_ENV

PATH is not used to search for this file, so if you want to use it, best refer to it by giving the full path and file name.

Invoked with the sh command
Bash tries to behave as the historical Bourne sh program while conforming to the POSIX standard as well.

Files read:

    /etc/profile

    ~/.profile

When invoked interactively, the ENV variable can point to extra startup information.

POSIX mode
This option is enabled either using the set built-in:

set -o posix

or by calling the bash program with the --posix option. Bash will then try to behave as compliant as possible to the POSIX standard for shells. Setting the POSIXLY_CORRECT variable does the same.

Files read:

    defined by ENV variable.

Invoked remotely
Files read when invoked by rshd:

    ~/.bashrc