The shell is a program that serves as an interface between the user and the operating system, allowing two-way communication and flexibility. There are different types of shells like bash, csh, and tcsh, with bash being the default shell in Linux and csh and tcsh having similarities to the C programming language. The file /etc/shells lists the supported shells on a Linux system.

1. SHELL BASICS The use of a program that serves as an interface between user and operating system is called the SHELL. A shell is stared on the text console after the login session. This is called the login shell. Every shell can be started as a program and you can change at any time to a different shell.

2. What is a SHELL? The shell is a layer around the Linux operating system. The shell, on the other hand, is an advanced way to communicating with the system, because it allows for two-way conversation and taking initiative. The shell allows the user to handle a system in a very flexible way.

3. Types of SHELL 1. bash is the default shell in Linux. 2. csh or C shell has a syntax that resembles that of the highly popular C programming language and thus preferred by programmers. 3. tcsh or Turbo C shell is based on csh but also has programmable filename completion, command-line editing, a history mechanism and other features lacking in csh.

4. The file /etc/shells gives a list of supported shells on a Linux system. # cat /etc/shells /bin/bash /bin/sh /bin/tcsh /bin/csh # cat /etc/passwd