Linux man pages : login (1)
LOGIN(1) Linux Programmer's Manual LOGIN(1)
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
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
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).
-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
-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 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
hosts where members of that group are allowed access. These specifica-
tions may involve the use of classes defined in previous CLASSES sec-
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 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 @184.108.40.206/255.255.255.0
blue tty3 myclass2
This lets the user zacho login only on tty1 and from hosts with IP
addreses in the range 220.127.116.11 - 18.104.22.168, 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
o 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 nota-
tion, 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 @22.214.171.124/255.255.254.0 means that
the user may rlogin/telnet from any host whose IP address is in
the range 126.96.36.199 - 188.8.131.52.
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 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), shutdown(8)
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 (firstname.lastname@example.org)
Ported to Linux 0.12: Peter Orbaek (email@example.com)
Util-linux 1.6 4 November 1996 LOGIN(1)