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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 upper-
case characters when it reads a login name. The program can handle 7-bit charac-
ters 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 produced 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/gettytab (SunOS 4) files.
ARGUMENTS
port A path name relative to the /dev directory. If a "-" is specified, agetty assumes
that its standard input is already connected 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 switching.
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 terminal, or 'hurd' for GNU
Hurd on virtual terminal.
OPTIONS
-8, --8bits
Assume that the tty is 8-bit clean, hence disable parity detection.
-a, --autologin username
Log the specified user automatically in without asking for a login name and pass-
word. 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 mes-
sages 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 preceded 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 line-
feed 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 password file).
-L, --local-line
Force the line to be a local line with no need for carrier detect. This can be use-
ful when you have a locally attached terminal where the serial line does not set
the carrier detect signal.
-m, --extract-baud
Try to extract the baud rate the CONNECT status message produced by Hayes(tm)-com-
patible 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 speci-
fied 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 connec-
tion. 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 terminal.
-s, --keep-baud
Try to keep the existing baud rate. The baud rates from the command 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 gethostname() or (if not found) by getad-
drinfo() 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 addi-
tional 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 depending 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 inter-
preted 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 contain certain escape
codes to display the system name, date and time etc. All escape codes consist of a back-
slash (\) 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 net-
work 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 net-
work 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 number 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 written 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.ker-
nel.org/pub/linux/utils/util-linux/.
util-linux May 2011 AGETTY(8)
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