login - Unix, Linux Command
login - sign on
login [ name ]
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 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
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).
/var/log/lastlog exists, the last login time is printed (and the current login is
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 users name in
/var/spool/mail will be checked, and a message printed if it has non-zero length.
The users shell is then started. If no shell is specified for the
/bin/sh is used. If there is no directory specified in
/ is used (the home directory is checked for the
.hushlogin file described above).
login not to destroy the environment
Used to skip a second login authentication. This specifically does
not work for root, and does not appear to work well under Linux.
Used by other servers (i.e.,
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
SPECIAL ACCESS RESTRICTIONS
/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
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 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
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:
myclass1 tty1 tty2
myclass2 tty3 @.foo.com
This defines the classes
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/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 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.
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
stud may log in from hosts/ttys specified in the class myclass1 or from
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
separated 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
specified at the top of the file, the first section defaults to be a
An example USERS section:
zacho tty1 @220.127.116.11/255.255.255.0
blue tty3 myclass2
This lets the user zacho login only on tty1 and from hosts with IP
addreses in the range 18.104.22.168 - 22.214.171.124, 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:
Any of the above origins may be prefixed by a time specification
according to the syntax:
The name of a tty device without the /dev/ prefix, for example tty1 or
The string @localhost, meaning that the user is allowed to
telnet/rlogin from the local host to the same host. This also allows
the user to for example run the command: xterm -e /bin/login.
A domain name suffix such as @.some.dom, meaning that the user may
rlogin/telnet from any host whose domain name has the suffix
A range of IPv4 addresses, written @x.x.x.x/y.y.y.y where x.x.x.x is
the IP address in the usual dotted quad decimal notation, and y.y.y.y
is a bitmask in the same notation specifying which bits in the address
to compare with the IP address of the remote host. For example
@126.96.36.199/255.255.254.0 means that the user may rlogin/telnet from
any host whose IP address is in the range 188.8.131.52 -
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.
The undocumented BSD
-r option is not supported. This may be required by some
A recursive login, as used to be possible in the good old days,
no longer works; for most purposes
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 its 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 (email@example.com)
Ported to Linux 0.12: Peter Orbaek (firstname.lastname@example.org)