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
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