How to enable tab command completion on linux

1 login to user in which you want to set command completion.

2 after that check its shell by typing following command

echo $SHELL

3 If you get /bin/sh than you need to change its shell and if you get /bin/bash than no need to do anything

4 change shell for user

5 chsh -s /bin/bash username

6 this will change the usermae user shell to /bin/bash

7 logout and login again to check