I am a new Linux user and can you tell me a command name to change the default login shell in Linux or UNIX operating system?

You don’t have to edit /etc/passwd file to change your shell. You need to use the chsh command. It changes the user login shell. This determines the name of the users’ initial login command. A normal user may only change the login shell for his/her own account, the super user i.e. root user may change the login shell for any account.

chsh command syntax

Following is syntax for chsh command:
chsh -s {shell-name} {user-name}
Where,

• -s {shell-name} : Specify your login shell name. You can obtained list of avialble shell from /etc/shells file.
• User-name : It is optional, useful if you are a root user.

First, find out available shell list:
$ less /etc/shells
Sample Outputs:

/bin/ash
/bin/csh
/bin/sh
/usr/bin/es
/bin/ksh
/bin/tcsh
/bin/sash
/bin/zsh
/bin/dash
/usr/bin/screen
/bin/bash
/bin/rbash

Example: Changing default shell in Linux to /bin/tcsh

To change your shell name to /bin/tcsh:
$ chsh -s /bin/tcsh
Sample outputs:

Password:

When promoted for password, type your own password. You must log out and log back in to see this change. If you just type chsh command, it will prompt for shell name interactively:
$ chsh
Sample outputs:

Password:
Changing the login shell for tv
Enter the new value, or press ENTER for the default
       Login Shell [/bin/bash]:

This command will change the default login shell to /bin/tcsh permanently. In this example, first find out path for bash shell, enter:
$ type -s bash
$ which bash
Sample outputs:

bash is /usr/local/bin/bash

Now use the below command to set shell to bash for a user named vivek:
$ chsh -s /usr/local/bin/bash vivek
OR if you know sysadmin root passoword:
$ sudo chsh -s /usr/local/bin/bash vivek