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).
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.
The word at the beginning of a line becomes defined as a collective name for the ttys and host patterns specified at the rest of the line. This collective name can be used in any subsequent GROUPS or USERS section. No such class name must occur as part of the definition of a class in order to avoid problems with recursive classes.
An example CLASSES section:
CLASSES myclass1 tty1 tty2 myclass2 tty3 @.foo.com
This defines the classes myclass1 and myclass2 as the corresponding right hand sides.
A GROUPS section starts with the word GROUPS in all upper case at the start of a line, and each following line is a sequence of words separated by spaces or tabs. The first word on a line is the name of the group and the rest of the words on the line specifies the ttys and hosts where members of that group are allowed access. These specifications may involve the use of classes defined in previous CLASSES sections.
An example GROUPS section.
GROUPS sys tty1 @.bar.edu stud myclass1 tty4
This example specifies that members of group sys may log in on tty1 and from hosts in the bar.edu domain. Users in group stud may log in from hosts/ttys specified in the class myclass1 or from tty4.
An example USERS section:
USERS zacho tty1 @22.214.171.124/255.255.255.0 blue tty3 myclass2
This lets the user zacho login only on tty1 and from hosts with IP addreses in the range 126.96.36.199 - 188.8.131.52, and user blue is allowed to log in from tty3 and whatever is specified in the class myclass2.
There may be a line in a USERS section starting with a username of *. This is a default rule and it will be applied to any user not matching any other line.
If both a USERS line and GROUPS line match a user then the user is allowed access from the union of all the ttys/hosts mentioned in these specifications.
Any of the above origins may be prefixed by a time specification according to the syntax:
timespec ::= '[' <day-or-hour> [':' <day-or-hour>]* ']' day ::= 'mon' | 'tue' | 'wed' | 'thu' | 'fri' | 'sat' | 'sun' hour ::= '0' | '1' | ... | '23' hourspec ::= <hour> | <hour> '-' <hour> day-or-hour ::= <day> | <hourspec>
For example, the origin [mon:tue:wed:thu:fri:8-17]tty3 means that log in is allowed on mondays through fridays between 8:00 and 17:59 (5:59 pm) on tty3. This also shows that an hour range a-b includes all moments between a:00 and b:59. A single hour specification (such as 10) means the time span between 10:00 and 10:59.
Not specifying any time prefix for a tty or host means log in from that origin is allowed any time. If you give a time prefix be sure to specify both a set of days and one or more hours or hour ranges. A time specification may not include any white space.
If no default rule is given then users not matching any line /etc/usertty are allowed to log in from anywhere as is standard behavior.
/var/run/utmp /var/log/wtmp /var/log/lastlog /var/spool/mail/* /etc/motd /etc/passwd /etc/nologin /etc/usertty .hushlogin
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.