AGETTY(8) System Administration AGETTY(8)
NAME
agetty - alternative Linux getty
SYNOPSIS
agetty [options] port [baud_rate...] [term]
DESCRIPTION
agetty opens a tty port, prompts for a login name and invokes the
/bin/login command. It is normally invoked by init(8).
agetty has several non-standard features that are useful for hard-wired
and for dial-in lines:
o Adapts the tty settings to parity bits and to erase, kill, end-
of-line and uppercase characters when it reads a login name.
The program can handle 7-bit characters with even, odd, none or
space parity, and 8-bit characters with no parity. The following
special characters are recognized: Control-U (kill); DEL and
back space (erase); carriage return and line feed (end of line).
See also --erase-chars and --kill-chars options.
o Optionally deduces the baud rate from the CONNECT messages pro-
duced by Hayes(tm)-compatible modems.
o Optionally does not hang up when it is given an already opened
line (useful for call-back applications).
o Optionally does not display the contents of the /etc/issue file.
o Optionally displays an alternative issue file instead of
/etc/issue.
o Optionally does not ask for a login name.
o Optionally invokes a non-standard login program instead of
/bin/login.
o Optionally turns on hard-ware flow control
o Optionally forces the line to be local with no need for carrier
detect.
This program does not use the /etc/gettydefs (System V) or /etc/get-
tytab (SunOS 4) files.
ARGUMENTS
port A path name relative to the /dev directory. If a "-" is speci-
fied, agetty assumes that its standard input is already con-
nected to a tty port and that a connection to a remote user has
already been established.
Under System V, a "-" port argument should be preceded by a
"--".
baud_rate,...
A comma-separated list of one or more baud rates. Each time
agetty receives a BREAK character it advances through the list,
which is treated as if it were circular.
Baud rates should be specified in descending order, so that the
null character (Ctrl-@) can also be used for baud rate switch-
ing.
This argument is optional and unnecessary for virtual terminals.
The default for serial terminals is '9600'.
term The value to be used for the TERM environment variable. This
overrides whatever init(8) may have set, and is inherited by
login and the shell.
The default is 'vt100', or 'linux' for Linux on virtual termi-
nal, or 'hurd' for GNU Hurd on virtual terminal.
OPTIONS
-8, --8bits
Assume that the tty is 8-bit clean, hence disable parity detec-
tion.
-a, --autologin username
Log the specified user automatically in without asking for a
login name and password. The -f username option is added to the
/bin/login command line by default. The --login-options option
changes this default behaviour and then only \u is replaced by
the username and no other option is added to the login command
line.
Note that --autologin may affect the way how agetty initializes
the serial line, because on auto-login agetty does not read from
the line and it has no opportunity optimize the line setting.
-c, --noreset
Don't reset terminal cflags (control modes). See termios(3) for
more details.
-E, --remote
If -H fakehost option is given then -r fakehost options is added
to the /bin/login command line.
-f, --issue-file issue_file
Display the contents of issue_file instead of /etc/issue. This
allows custom messages to be displayed on different terminals.
The -i option will override this option.
-h, --flow-control
Enable hardware (RTS/CTS) flow control. It is left up to the
application to disable software (XON/XOFF) flow protocol where
appropriate.
-H, --host login_host
Write the specified login_host into the utmp file. (Normally, no
login host is given, since agetty is used for local hardwired
connections and consoles. However, this option can be useful for
identifying terminal concentrators and the like.
-i, --noissue
Do not display the contents of /etc/issue (or other) before
writing the login prompt. Terminals or communications hardware
may become confused when receiving lots of text at the wrong
baud rate; dial-up scripts may fail if the login prompt is pre-
ceded by too much text.
-I, --init-string initstring
Set an initial string to be sent to the tty or modem before
sending anything else. This may be used to initialize a modem.
Non printable characters may be sent by writing their octal code
preceded by a backslash (\). For example to send a linefeed
character (ASCII 10, octal 012) write \012.
-l, --login-program login_program
Invoke the specified login_program instead of /bin/login. This
allows the use of a non-standard login program (for example, one
that asks for a dial-up password or that uses a different pass-
word file).
-L, --local-line
Force the line to be a local line with no need for carrier
detect. This can be useful when you have a locally attached ter-
minal where the serial line does not set the carrier detect sig-
nal.
-m, --extract-baud
Try to extract the baud rate the CONNECT status message produced
by Hayes(tm)-compatible modems. These status messages are of the
form: "<junk><speed><junk>". agetty assumes that the modem
emits its status message at the same speed as specified with
(the first) baud_rate value on the command line.
Since the -m feature may fail on heavily-loaded systems, you
still should enable BREAK processing by enumerating all expected
baud rates on the command line.
-n, --skip-login
Do not prompt the user for a login name. This can be used in
connection with -l option to invoke a non-standard login process
such as a BBS system. Note that with the -n option, agetty gets
no input from user who logs in and therefore won't be able to
figure out parity, character size, and newline processing of the
connection. It defaults to space parity, 7 bit characters, and
ASCII CR (13) end-of-line character. Beware that the program
that agetty starts (usually /bin/login) is run as root.
-o, --login-options "login_options"
Options that are passed to the login program. \u is replaced
by the login name. The default /bin/login command line is
"/bin/login -- <username>".
Please read the SECURITY NOTICE below if you want to use this.
-p, --login-pause
Wait for any key before dropping to the login prompt. Can be
combined with --autologin to save memory by lazily spawning
shells.
-r, --chroot "directory"
Change root to the specified directory.
-R, --hangup
Do call vhangup() for a virtually hangup of the specified termi-
nal.
-s, --keep-baud
Try to keep the existing baud rate. The baud rates from the com-
mand line are used when agetty receives a BREAK character.
-t, --timeout timeout
Terminate if no user name could be read within timeout seconds.
This option should probably not be used with hard-wired lines.
-U, --detect-case
Turn on support for detecting an uppercase only terminal. This
setting will detect a login name containing only capitals as
indicating an uppercase only terminal and turn on some upper to
lower case conversions. Note that this has no support for any
unicode characters.
-w, --wait-cr
Wait for the user or the modem to send a carriage-return or a
linefeed character before sending the /etc/issue (or other) file
and the login prompt. Very useful in connection with the -I
option.
--noclear
Do not clear the screen before prompting for the login name (the
screen is normally cleared).
--nohints
Do not print hints about Num, Caps and Scroll Locks.
--nonewline
Do not print a newline before writing out /etc/issue.
--nohostname
By default the hostname will be printed. With this option
enabled, no hostname at all will be shown.
--long-hostname
By default the hostname is only printed until the first dot.
With this option enabled, the fully qualified hostname by geth-
ostname() or (if not found) by getaddrinfo() is shown.
--erase-chars string
This option specifies additional characters that should be
interpreted as a backspace ("ignore the previous character")
when the user types the login name. The default additional
'erase' has been '#', but since util-linux 2.23 no additional
erase characters are enabled by default.
--kill-chars string
This option specifies additional characters that should be
interpreted as a kill ("ignore all previous characters") when
the user types the login name. The default additional 'kill'
has been '@', but since util-linux 2.23 no additional kill char-
acters are enabled by default.
--version
Display version information and exit.
--help Display help text and exit.
EXAMPLES
This section shows examples for the process field of an entry in the
/etc/inittab file. You'll have to prepend appropriate values for the
other fields. See inittab(5) for more details.
For a hard-wired line or a console tty:
/sbin/agetty 9600 ttyS1
For a directly connected terminal without proper carriage detect
wiring: (try this if your terminal just sleeps instead of giving you a
password: prompt.)
/sbin/agetty -L 9600 ttyS1 vt100
For a old style dial-in line with a 9600/2400/1200 baud modem:
/sbin/agetty -mt60 ttyS1 9600,2400,1200
For a Hayes modem with a fixed 115200 bps interface to the machine:
(the example init string turns off modem echo and result codes, makes
modem/computer DCD track modem/modem DCD, makes a DTR drop cause a dis-
connection and turn on auto-answer after 1 ring.)
/sbin/agetty -w -I 'ATE0Q1&D2&C1S0=1\015' 115200 ttyS1
SECURITY NOTICE
If you use the --login-program and --login-options options, be aware
that a malicious user may try to enter lognames with embedded options,
which then get passed to the used login program. Agetty does check for
a leading "-" and makes sure the logname gets passed as one parameter
(so embedded spaces will not create yet another parameter), but depend-
ing on how the login binary parses the command line that might not be
sufficient. Check that the used login program can not be abused this
way.
Some programs use "--" to indicate that the rest of the commandline
should not be interpreted as options. Use this feature if available by
passing "--" before the username gets passed by \u.
ISSUE ESCAPES
The issue-file (/etc/issue or the file set with the -f option) may con-
tain certain escape codes to display the system name, date and time
etc. All escape codes consist of a backslash (\) immediately followed
by one of the letters explained below.
4 or 4{interface}
Insert the IPv4 address of the machine hostname or IPv4 address
the configured network interface if the interface argument is
specified (e.g. \4{eth0}).
6 or 6{interface}
Insert the IPv6 address of the machine hostname or IPv6 address
the configured network interface if the interface argument is
specified (e.g. \6{eth0}}
b Insert the baudrate of the current line.
d Insert the current date.
s Insert the system name, the name of the operating system. Same
as `uname -s'. See also \S escape code.
S or S{VARIABLE}
Insert the VARIABLE data from /etc/os-release. If the VARIABLE
argument is not specified then use PRETTY_NAME from the file or
the system name (see \s). This escape code allows to keep
/etc/issue distribution and release independent. Note that
\S{ANSI_COLOR} is converted to the real terminal escape
sequence.
l Insert the name of the current tty line.
m Insert the architecture identifier of the machine. Same as
`uname -m'.
n Insert the nodename of the machine, also known as the hostname.
Same as `uname -n'.
o Insert the NIS domainname of the machine. Same as `hostname -d'.
O Insert the DNS domainname of the machine.
r Insert the release number of the OS. Same as `uname -r'.
t Insert the current time.
u Insert the number of current users logged in.
U Insert the string "1 user" or "<n> users" where <n> is the num-
ber of current users logged in.
v Insert the version of the OS, eg. the build-date etc.
Example: On my system, the following /etc/issue file:
This is \n.\o (\s \m \r) \t
displays as
This is thingol.orcan.dk (Linux i386 1.1.9) 18:29:30
FILES
/var/run/utmp
the system status file.
/etc/issue
printed before the login prompt.
/etc/os-release
operating system identification data.
/dev/console
problem reports (if syslog(3) is not used).
/etc/inittab
init(8) configuration file for SysV-style init daemon.
BUGS
The baud-rate detection feature (the -m option) requires that agetty be
scheduled soon enough after completion of a dial-in call (within 30 ms
with modems that talk at 2400 baud). For robustness, always use the -m
option in combination with a multiple baud rate command-line argument,
so that BREAK processing is enabled.
The text in the /etc/issue file (or other) and the login prompt are
always output with 7-bit characters and space parity.
The baud-rate detection feature (the -m option) requires that the modem
emits its status message after raising the DCD line.
DIAGNOSTICS
Depending on how the program was configured, all diagnostics are writ-
ten to the console device or reported via the syslog(3) facility.
Error messages are produced if the port argument does not specify a
terminal device; if there is no utmp entry for the current process
(System V only); and so on.
AUTHORS
Werner Fink <werner AT suse.de>
Karel Zak <kzak AT redhat.com>
The original agetty for serial terminals was written by W.Z. Venema
<wietse AT wzv.nl> and ported to Linux by Peter Orbaek
<poe AT daimi.dk>.
AVAILABILITY
The agetty command is part of the util-linux package and is available
from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
util-linux May 2011 AGETTY(8)
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