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 hardwired
and for dial-in lines:
• 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
backspace (erase); carriage return and line feed (end of line). See
also the --erase-chars and --kill-chars options.
• Optionally deduces the baud rate from the CONNECT messages produced
by Hayes(tm)-compatible modems.
• Optionally does not hang up when it is given an already opened line
(useful for call-back applications).
• Optionally does not display the contents of the /etc/issue file.
• Optionally displays an alternative issue files or directories
instead of /etc/issue or /etc/issue.d.
• Optionally does not ask for a login name.
• Optionally invokes a non-standard login program instead of
/bin/login.
• Optionally turns on hardware flow control.
• 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 keep the current baud rate (see
--keep-baud) and if unsuccessful then default to '9600'.
term
The value to be used for the TERM environment variable. This
overrides whatever init(1) may have set, and is inherited by login
and the shell.
The default is 'vt100', or 'linux' for Linux on a virtual terminal,
or 'hurd' for GNU Hurd on a virtual terminal.
OPTIONS
-8, --8bits
Assume that the tty is 8-bit clean, hence disable parity detection.
-a, --autologin username
Automatically log in the specified user without asking for a
username or password. Using this option causes an -f username
option and argument to be added to the /bin/login command line. See
--login-options, which can be used to modify this option’s
behavior.
Note that --autologin may affect the way in which getty 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
Do not reset terminal cflags (control modes). See termios(3) for
more details.
-E, --remote
Typically the login(1) command is given a remote hostname when
called by something such as telnetd(8). This option allows agetty
to pass what it is using for a hostname to login(1) for use in
utmp(5). See --host, login(1), and utmp(5).
If the --host fakehost option is given, then an -h fakehost option
and argument are added to the /bin/login command line.
If the --nohostname option is given, then an -H option is added to
the /bin/login command line.
See --login-options.
-f, --issue-file path
Specifies a ":" delimited list of files and directories to be
displayed instead of /etc/issue (or other). All specified files and
directories are displayed, missing or empty files are silently
ignored. If the specified path is a directory then display all
files with .issue file extension in version-sort order from the
directory. This allows custom messages to be displayed on different
terminals. The --noissue option will override this option.
--show-issue
Display the current issue file (or other) on the current terminal
and exit. Use this option to review the current setting, it is not
designed for any other purpose. Note that output may use some
default or incomplete information as proper output depends on
terminal and agetty command line.
-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 fakehost
Write the specified fakehost 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 linefeed
character (ASCII 10, octal 012), write \12.
-J, --noclear
Do not clear the screen before prompting for the login name. By
default the screen is cleared.
-l, --login-program login_program
Invoke the specified login_program instead of /bin/login. This
allows the use of a non-standard login program. Such a program
could, for example, ask for a dial-up password or use a different
password file. See --login-options.
-L, --local-line[=mode]
Control the CLOCAL line flag. The optional mode argument is auto,
always or never. If the mode argument is omitted, then the default
is always. If the --local-line option is not given at all, then the
default is auto.
always
Forces the line to be a local line with no need for carrier
detect. This can be useful when you have a locally attached
terminal where the serial line does not set the carrier-detect
signal.
never
Explicitly clears the CLOCAL flag from the line setting and the
carrier-detect signal is expected on the line.
auto
The agetty default. Does not modify the CLOCAL setting and
follows the setting enabled by the kernel.
-m, --extract-baud
Try to extract the baud rate from 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 --extract-baud feature may fail on heavily-loaded
systems, you still should enable BREAK processing by enumerating
all expected baud rates on the command line.
--list-speeds
Display supported baud rates. These are determined at compilation
time.
-n, --skip-login
Do not prompt the user for a login name. This can be used in
connection with the --login-program option to invoke a non-standard
login process such as a BBS system. Note that with the --skip-login
option, agetty gets no input from the user who logs in and
therefore will not 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.
-N, --nonewline
Do not print a newline before writing out /etc/issue.
-o, --login-options login_options
Options and arguments that are passed to login(1). Where \u is
replaced by the login name. For example:
--login-options '-h darkstar -- \u'
See --autologin, --login-program and --remote.
Please read the SECURITY NOTICE below before using this option.
-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
Call vhangup(2) to do a virtual 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. If another
baud rates specified then the original baud rate is also saved to
the end of the wanted baud rates list. This can be used to return
to the original baud rate after unexpected BREAKs.
-t, --timeout timeout
Terminate if no user name could be read within timeout seconds. Use
of this option with hardwired terminal lines is not recommended.
-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 file (or others)
and the login prompt. This is useful with the --init-string option.
--nohints
Do not print hints about Num, Caps and Scroll Locks.
--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(3P) or (if not found) by getaddrinfo(3) 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 characters are
enabled by default.
--chdir directory
Change directory before the login.
--delay number
Sleep seconds before open tty.
--nice number
Run login with this priority.
--reload
Ask all running agetty instances to reload and update their
displayed prompts, if the user has not yet commenced logging in.
After doing so the command will exit. This feature might be
unsupported on systems without Linux inotify(7).
-h, --help
Display help text and exit.
-V, --version
Print version and exit.
EXAMPLE
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 hardwired line or a console tty:
/sbin/agetty 9600 ttyS1
For a directly connected terminal without proper carrier-detect wiring
(try this if your terminal just sleeps instead of giving you a
password: prompt):
/sbin/agetty --local-line 9600 ttyS1 vt100
For an old-style dial-in line with a 9600/2400/1200 baud modem:
/sbin/agetty --extract-baud --timeout 60 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
disconnection, and turns on auto-answer after 1 ring):
/sbin/agetty --wait-cr --init-string '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 cannot be abused
this way.
Some programs use "--" to indicate that the rest of the command line
should not be interpreted as options. Use this feature if available by
passing "--" before the username gets passed by \u.
ISSUE FILES
The default issue file is /etc/issue. If the file exists, then agetty
also checks for /etc/issue.d directory. The directory is optional
extension to the default issue file and content of the directory is
printed after /etc/issue content. If the /etc/issue does not exist,
then the directory is ignored. All files with .issue extension from the
directory are printed in version-sort order. The directory can be used
to maintain 3rd-party messages independently on the primary system
/etc/issue file.
Since version 2.35 additional locations for issue file and directory
are supported. If the default /etc/issue does not exist, then agetty
checks for /run/issue and /run/issue.d, thereafter for /usr/lib/issue
and /usr/lib/issue.d. The directory /etc is expected for host specific
configuration, /run is expected for generated stuff and /usr/lib for
static distribution maintained configuration.
The default path maybe overridden by --issue-file option. In this case
specified path has to be file or directory and all the default issue
file and directory locations are ignored.
The issue file feature can be completely disabled by --noissue option.
It is possible to review the current issue file by agetty --show-issue
on the current terminal.
The issue files may contain certain escape codes to display the system
name, date, time et cetera. All escape codes consist of a backslash (\)
immediately followed by one of the characters listed below.
4 or 4{interface}
Insert the IPv4 address of the specified network interface (for
example: \4{eth0}). If the interface argument is not specified,
then select the first fully configured (UP, non-LOCALBACK, RUNNING)
interface. If no configured interface is found, fall back to the IP
address of the machine’s hostname.
6 or 6{interface}
The same as \4 but for IPv6.
b
Insert the baudrate of the current line.
d
Insert the current date.
e or e{name}
Translate the human-readable name to an escape sequence and insert
it (for example: \e{red}Alert text.\e{reset}). If the name argument
is not specified, then insert \033. The currently supported names
are: black, blink, blue, bold, brown, cyan, darkgray, gray, green,
halfbright, lightblue, lightcyan, lightgray, lightgreen,
lightmagenta, lightred, magenta, red, reset, reverse, yellow and
white. All unknown names are silently ignored.
s
Insert the system name (the name of the operating system). Same as
'uname -s'. See also the \S escape code.
S or S{VARIABLE}
Insert the VARIABLE data from /etc/os-release. If this file does
not exist then fall back to /usr/lib/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 can be used 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, that is, the build-date and such.
An 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 /usr/lib/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 --extract-baud 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 --extract-baud 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 --extract-baud 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@suse.de>, Karel Zak <kzak@redhat.com>
The original agetty for serial terminals was written by W.Z. Venema
<wietse@wzv.win.tue.nl> and ported to Linux by Peter Orbaek
<poe@daimi.aau.dk>.
REPORTING BUGS
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
AVAILABILITY
The agetty command is part of the util-linux package which can be
downloaded from Linux Kernel Archive
<https://www.kernel.org/pub/linux/utils/util-linux/>.
util-linux 2.38.1 2022-08-04 AGETTY(8)
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