The 'login' command initiates a new session on the system and provides the authentication mechanism for users to access their accounts. It's a fundamental component of the Linux/Unix authentication system, typically invoked automatically by the system when a user connects to a terminal, but it can also be used manually to switch users or start new sessions.
Key features of the login command:
1. Authentication Management: login handles the user authentication process, prompting for and validating usernames and passwords against the system's authentication database.
2. Session Initialization: After successful authentication, login establishes a new user session, setting up the environment variables, access permissions, and other session parameters.
3. Login Accounting: The command records login information in system logs, creating an audit trail of user access that can be reviewed for security purposes.
4. Password Enforcement: login enforces password policies, such as expiration, complexity requirements, and login restrictions configured on the system.
5. Environment Control: It sets up the initial environment for the user session, including variables like HOME, SHELL, PATH, and TERM, though many of these may be modified by the user's shell startup files.
6. Remote Login Support: With appropriate options, login can handle remote login sessions, setting the proper host information in system logs.
7. PAM Integration: On modern systems, login integrates with the Pluggable Authentication Modules (PAM) framework, allowing for flexible and configurable authentication methods.
Common use cases for the login command include:
- Switching to another user account during a terminal session
- Logging in after a system boot on a console terminal
- Authenticating users in scripts that require a full login session
- Testing authentication and login configuration changes
- Administering user sessions on multi-user systems
- Forcing a new login environment when the current one has become corrupted
It's important to note that the login command operates at a very low level in the system's authentication stack. In most everyday scenarios, users don't directly interact with login, as it's wrapped by higher-level interfaces like display managers (for graphical logins), ssh (for remote access), or automatically invoked by getty processes on virtual consoles.
For system administrators, understanding the login command and its interaction with other authentication components (like PAM, /etc/passwd, /etc/shadow, and various system logs) is essential for managing user access, troubleshooting authentication problems, and maintaining system security.