What Is Shell

∟What Is Shell

This section provides a quick introduction of shell programs on Ubuntu computer. Bash (Bourne Again SHell) is the default shell program.

What Is Shell? - A shell is a command-line Interpreter that provides a text based user interface for you to interact with the operating system.

Ubuntu distribution comes with 3 shells pre-installed on your computer:

/bin/bash - The Bourne Again SHell (Bash) is an extension of the original Bourn Shell. Bash was created by Brian Fox in 1989 and is available on almost all Linux distributions. Bash provides many nice features for interactive user sessions and is the default shell on Ubuntu when users log in from a command window or remote terminal.
/bin/dash - The Debian Almquist SHell (Dash) is another popular shell on Linux distributions. Dash lacks interactive features and runs fast. It is the default shell on Ubuntu for executing shell scripts.
/bin/rbash - The Restricted Bash (RDash) is a restricted version of Bash to provide more secure interactive user sessions.

You can read the /etc/shells file to see what shells are installed on your Ubuntu computers.

herong$ more /etc/shells 

# /etc/shells: valid login shells
/bin/sh
/bin/bash
/bin/rbash
/bin/dash

Note that "/bin/sh" is a link to the Dash program, "/bin/dash". It is not representing the original Shell program.

herong$ ls -l /bin/sh 

lrwxrwxrwx 1 root root 4 Sep 17  2019 /bin/sh -> dash

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