To Block User Logins Using /etc/nologin File
This can be used to block user login by manually creating the file as follows.
# vi /etc/nologin
Add the message below to the file, which will be shown to users attempting to log on to the system.
The Server is down for a routine maintenance. We apologize for any inconvenience caused, the system will be up and running in 1 hours time. For more information, contact the system admin email@example.com.
Now you can test if it all works; as you can see from the screenshot below, a normal user
ucartz is not able to log in.
To Block User Logins Using no login Shell
This method works a little differently: it only blocks a user from accessing a shell. But he or she can log on to the system via programs such as ftp that do not necessarily require a shell for the user to connect to a system.
Additionally, it can allow you to block shell access to specific users in special scenarios.
Simply use chsh (change shell) command to change the user's shell in /etc/passwd file from something like
/sbin/nologin meaning refuse a login.
# chsh -s /bin/nologin ucartz
Here, you have to use /bin/false file. The command below changes the user ucartz’s shell to
/bin/falsemeaning do nothing (after the user provides login credentials):
$ sudo chsh -s /bin/false ucartz