login

Log into the computer.
If no user is specified, or if password authentication of the user fails, login prompts for a user name.

Syntax
       login [-fp] [-h hostname] [user]

Options
   -f	 The -f option is used when a user name is specified to indicate
	 that proper authentication has already been done and that no
	 password need be requested.  This option may only be used by the
	 super-user or when an already logged in user is logging in as
	 themselves.

   -h	 The -h option specifies the host from which the connection was
	 received. It is used by various daemons such as telnetd(8).
	 This option may only be used by the super-user.

   -p	 By default, login discards any previous environment. The -p
	 option disables this behavior.

If the file /etc/nologin exists, login dislays its contents to the user and exits. This is used by shutdown to prevent users from logging in when the system is about to go down.

Immediately after logging a user in, login displays the system copyright notice, the date and time the user last logged in, the message of the day as well as other information. If the file `.hushlogin' exists in the user's home directory, all of these messages are suppressed. This is to simplify logins for non-human users, such as uucp(1). Login then records an entry in the wtmp(5) and utmp(5) files and executes the user's command interpreter.

Login enters information into the environment (see environ(7)) specifying the user's home directory (HOME), command interpreter (SHELL), search path (PATH), terminal type (TERM) and user name (both LOGNAME and USER).

The standard shells, csh(1) and sh(1), do not fork before executing the login utility.

Startup files

   A login shell begins	by executing commands from the system files

          /etc/csh.cshrc 
          /etc/csh.login.

   It then executes commands from files in the user's home directory:
       first ~/.tcshrc  or, if not found, ~/.cshrc,
                
       then ~/.history (or the value of the histfile shell variable),

       then ~/.login, 
       and finally
            ~/.cshdirs (or the value of the dirsfile shell variable).

Compilation options
   The  shell  may  read /etc/csh.login before instead of after /etc/csh.cshrc,
   and ~/.login before instead of after ~/.tcshrc or ~/.cshrc  and  ~/.history,
   if so compiled; see the version shell variable.

Non-Login shells
       Non-login  shells read only 
       /etc/csh.cshrc 
       and ~/.tcshrc or ~/.cshrc on startup.

FILES
/etc/motd message-of-the-day
/etc/nologin disallows logins
/var/run/utmp current logins
/var/log/lastlog last login account records
/var/log/wtmp login account records
/var/mail/user system mailboxes
.hushlogin makes login quieter

“Leave behind all hope, ye who enter here“ ~ sign above the entrance to Dante's hell

Related:

login man page - Apple.com
passwd - Modify a user password
rlogin - Connect to remote host system


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