nologin

Nologin is a program and a special login shell used on many Unix-like systems to prevent interactive logins for certain user accounts. On Linux, the executable is typically /sbin/nologin or /usr/sbin/nologin, and it is commonly assigned as the login shell for system users or accounts that should not have a normal interactive session.

When a user with nologin as their login shell attempts to log in, the login process runs the program, which prints a brief message such as "This account is currently not available" and then exits with a non-zero status. The resulting session is not started, effectively denying access. Nologin is preferred over simply disallowing login by other means because it provides a clear user-facing message while still allowing background processes to run under that account if needed.

Usage and administration: To disable an account, administrators set the user’s login shell to nologin using a command such as chsh -s /usr/sbin/nologin username or by editing the user entry in the system's account database. System accounts that do not require interactive logins are commonly configured with a nologin shell to reduce the surface area for abuse. In contrast, /bin/false can also be used to prevent logins but does not provide an informative message to the user.

/etc/nologin is another mechanism used by some systems. If this file exists, most login programs deny non-root logins and may display the contents of the file as a maintenance notice. The file is typically removed when maintenance is complete.

Notes: Paths and exact behavior can vary across Unix-like systems. Some distributions may treat root logins differently or allow local console access during maintenance. Nologin is one method among several for controlling interactive access for system and service accounts.