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login

login - sign on

login [ name ]
login -p
login -h
hostname
login -f
name

login is used when signing onto a system. It can also be used to control who can login to the system.

If an argument is not given, login prompts for the username.

If the user is not root, and if /etc/nologin  exists, the contents of this file are printed to the screen, and the login is terminated. This is typically used to prevent logins when the system is being taken down.

If special access restrictions are specified for the user in /etc/usertty, these must be met, or the log in attempt will be denied and a syslog message will be generated. See the section on "Special Access Restrictions".

If the user is root, then the login must be occurring on a tty listed in /etc/securetty. Failures will be logged with the syslog facility.

After these conditions have been checked, the password will be requested and checked (if a password is required for this username). Ten attempts are allowed before login dies, but after the first three, the response starts to get very slow. Login failures are reported via the syslog facility. This facility is also used to report any successful root logins.

If the file .hushlogin  exists, then a "quiet" login is performed (this disables the checking of mail and the printing of the last login time and message of the day). Otherwise, if /var/log/lastlog  exists, the last login time is printed (and the current login is recorded).

Random administrative things, such as setting the UID and GID of the tty are performed. The TERM environment variable is preserved, if it exists (other environment variables are preserved if the -p option is used). Then the HOME, PATH, SHELL, TERM, MAIL, and LOGNAME environment variables are set. PATH defaults to /usr/local/bin:/bin:/usr/bin  for normal users, and to /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin  for root. Last, if this is not a "quiet" login, the message of the day is printed and the file with the user's name in /var/spool/mail  will be checked, and a message printed if it has non-zero length.

The user's shell is then 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).

Options

-p

Used by getty(8) to tell login not to destroy the environment

-f

Used to skip a second login authentication. This specifically does not work for root, and does not appear to work well under Linux.

-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.

Special Access Restrictions

The file /etc/securetty  lists the names of the ttys where root is allowed to log in. One name of a tty device without the /dev/ prefix must be specified on each line. If the file does not exist, root is allowed to log in on any tty.

On most modern Linux systems PAM (Pluggable Authentication Modules) is used. On systems that do not use PAM, the file /etc/usertty  specifies additional access restrictions for specific users. If this file does not exist, no additional access restrictions are imposed. The file consists of a sequence of sections. There are three possible section types: CLASSES, GROUPS and USERS. A CLASSES section defines classes of ttys and hostname patterns, A GROUPS section defines allowed ttys and hosts on a per group basis, and a USERS section defines allowed ttys and hosts on a per user basis.

Each line in this file in may be no longer than 255 characters. Comments start with # character and extend to the end of the line.

Files

/var/run/utmp /var/log/wtmp /var/log/lastlog /var/spool/mail/* /etc/motd /etc/passwd /etc/nologin /etc/usertty .hushlogin 

See Also

init(8), getty(8), mail(1), passwd(1), passwd(5), environ(7), shutdown(8)

Bugs

The undocumented BSD -r option is not supported. This may be required by some rlogind(8) programs.

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.

Author

Derived from BSD login 5.40 (5/9/89) by Michael Glad (glad@daimi.dk) for HP-UX
Ported to Linux 0.12: Peter Orbaek (poe@daimi.aau.dk)



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