We stated that a Linux login attempt can fail for many reasons. The information provided in this chapter should enable the reader to troubleshoot login problems more effectively:
If a user can't log in, use chage to see whether login for the account is disabled due to password aging settings.
If login only fails for the root user, try logging in as a normal user and using su to switch to root because /etc/securetty might exist but contain no devices.
If at least one user can log in, the global login restriction file /etc/nologin is not the trouble. If all users are having login problems, check for /etc/nologin and look for PAM module problems.
If you are still having problems, you could try creating a new user with the default dot configuration files to verify that customization of .bash_profile or some other dot file is not preventing successful logins.