What is Shell ??

A command line interpreter to execute the commands.

sh
BASH - broune again
csh c
ksh korn
tcsh

Which shell is there?
[root@vikas ~]# echo $SHELL
/bin/bash
[root@vikas ~]# cat /etc/shells
/bin/sh
/bin/bash
/sbin/nologin
[root@vikas ~]# ps
PID TTY TIME CMD
5831 pts/1 00:00:00 bash
7629 pts/1 00:00:00 ps
[root@vikas ~]#