LOGIN(1) Linux Administrator's Manual LOGIN(1)
login - Begin session on the system
login [ -p ] [ -h host ] [ -H ] [ -f username | username ]
login is used when signing onto a system. If no argument is given, login prompts for the username.
The user is then prompted for a password, where approprate. Echoing is disabled to prevent revealing the password. Only a small number of
password failures are permitted before login exits and the communications link is severed.
If password aging has been enabled for the account, the user may be prompted for a new password before proceeding. He will be forced to
provide his old password and the new password before continuing. Please refer to passwd(1) for more information.
The user and group ID will be set according to their values in the file. There is one exception if the user ID is zero: in this case, only
the primary group ID of the account is set. This should prevent that the system adminitrator cannot login in case of network problems. The
value for $HOME, $SHELL, $PATH, $LOGNAME, and $MAIL are set according to the appropriate fields in the password entry. $PATH defaults to
/usr/local/bin:/bin:/usr/bin:. for normal users, and to /sbin:/bin:/usr/sbin:/usr/bin for root if not other configured. The environment
variable $TERM will be preserved, if it exists (other environment variables are preserved if the -p option is given) or be initialize to
the terminal type on your tty line, as specified in /etc/ttytype.
Then the user's shell is started. If no shell is specified for the user in /etc/passwd, then /bin/sh is used. If there is no directory
specified in /etc/passwd, then / is used (the home directory is checked for the .hushlogin file described above).
login reads the /etc/login.defs(5) configuration file. Please refer to this documenation for options which could be set.
-p Used by getty(8) to tell login not to destroy the environment
-f Used to skip a second login authentication. This option is deprecated and should not be used. It does specifically not work for
root. Using this option also means, that not all PAM functions are called.
-h Used by other servers (i.e., telnetd(8)) to pass the name of the remote host to login so that it may be placed in utmp and wtmp.
Only the superuser may use this option.
-H Used by other servers (i.e., telnetd(8)) to tell login that printing the hostname should be suppressed in the login: prompt.
/var/run/utmp - list of current login sessins
/var/log/wtmp - list of previous login sessions
/etc/passwd - user account information
/etc/shadow - encrypted passwords and age information
/etc/motd - system message file
/etc/login.defs - configuration file
init(8), getty(8), mail(1), passwd(1), passwd(5), environ(7), shutdown(8), login.defs(5)
A recursive login, as used to be possible in the good old days, no longer works; for most purposes su(1) is a satisfactory substitute.
Indeed, for security reasons, login does a vhangup() system call to remove any possible listening processes on the tty. This is to avoid
password sniffing. If one uses the command "login", then the surrounding shell gets killed by vhangup() because it's no longer the true
owner of the tty. This can be avoided by using "exec login" in a top-level shell or xterm.
Derived from BSD login 5.40 (5/9/89) by Michael Glad (firstname.lastname@example.org) for HP-UX
Ported to Linux 0.12: Peter Orbaek (email@example.com)
Added new features: Thorsten Kukuk (firstname.lastname@example.org)
PAM Login 3.32 2. May 2007 LOGIN(1)