login (1)


       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
       switch from one user to another at any time (most  modern  shells  have
       support for this feature built into them, however).

       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

       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 success-
       ful 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 /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).
              remote host to login so that it may be placed in utmp and  wtmp.
              Only the superuser may use this option.


       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 con-
       sists 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.  Com-
       ments start with # character and extend to the end of the line.

   The CLASSES Section
       A  CLASSES  section begins with the word CLASSES at the start of a line
       in all upper case. Each following line until the start of a new section
       or  the  end  of  the file consists of a sequence of words separated by
       tabs or spaces. Each line defines a class of ttys and host patterns.

       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 sec-
       tion.  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:

       myclass1       tty1 tty2
       myclass2       tty3 @.foo.com

       This defines the classes myclass1 and  myclass2  as  the  corresponding
       right hand sides.

   The GROUPS Section
       A  GROUPS  section  defines  allowed ttys and hosts on a per Unix group
       basis. If a user is a member of a Unix group according  to  /etc/passwd
       and  /etc/group  and  such  a group is mentioned in a GROUPS section in
       /etc/usertty then the user is granted access if the group is.

       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 sepa-
       rated 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

       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.

   The USERS Section
       A USERS section starts with the word USERS in all  upper  case  at  the
       start  of  a line, and each following line is a sequence of words sepa-
       rated by spaces or tabs. The first word on a line  is  a  username  and
       that user is allowed to log in on the ttys and from the hosts mentioned
       on the rest of the  line.  These  specifications  may  involve  classes
       defined  in  previous CLASSES sections.  If no section header is speci-
       fied at the top of the file, the first section defaults to be  a  USERS

       An example USERS section:

       zacho          tty1 @
       blue      tty3 myclass2

       This  lets  the  user  zacho  login only on tty1 and from hosts with IP
       addreses in the range -, and user  blue  is
       allowed  to  log  in  from  tty3 and whatever is specified in the class

       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

       The tty and host pattern specifications used in  the  specification  of
       classes, group and user access are called origins. An origin string may
       have one of these formats:

       o      The name of a tty device without the /dev/ prefix,  for  example
              tty1 or ttyS0.

       o      The  string @localhost, meaning that the user is allowed to tel-
              net/rlogin from the local host  to  the  same  host.  This  also
              allows  the  user  to  for  example  run  the  command: xterm -e

       o      A domain name suffix such as @.some.dom, meaning that  the  user
              may rlogin/telnet from any host whose domain name has the suffix

       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  spec-
       ify  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 behav-




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


       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  surround-
       ing  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 (glad@daimi.dk)
       for HP-UX
       Ported to Linux 0.12: Peter Orbaek (poe@daimi.aau.dk)