• sh

1. Bourne Shell, developed by Steve Bourne @ AT&T
2. The standard shell on UNIX based system.
3. The first popular shell.

• csh

1. Developed by Bill Joy.
2. Bill built csh, vi, BSD.

• tcsh

1. An improved version of csh.
2. Compared with csh, adding Command-line completion.

• ash

1. Light-weighted Shell
2. Compatible with bash.

• bash

1. bash is compatible with sh(Bourne Shell).
   Difference between bash and sh
   bash expands some params, and commands

• GNU/Linux
  Use commands to exminate available shells:

$ cat /etc/shells
/bin/sh
/bin/bash
/sbin/nologin
/usr/bin/sh
/usr/bin/bash
/usr/sbin/nologin
/bin/tcsh
/bin/csh

On GNU/Linux, sh is replaced with bash

$ echo $SHELL
/bin/bash

SHELL is an environment variable under GNU/Linux, it indicates the path of current shell we use.