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NAME | SYNOPSIS | DESCRIPTION | ARGUMENTS | OPTIONS | CONFIG FILE ITEMS | EXAMPLE | SECURITY NOTICE | ISSUE FILES | FILES | CREDENTIALS | BUGS | DIAGNOSTICS | AUTHORS | REPORTING BUGS | AVAILABILITY |
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AGETTY(8) System Administration AGETTY(8)
agetty - alternative Linux getty
agetty [options] port [baud_rate...] [term]
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.
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.
-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. This setting is
also possible to able by LOGIN_PLAIN_PROMPT option in the
/etc/login.defs configuration file (see below for more
details).
--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.
agetty reads the /etc/login.defs configuration file (see
login.defs(5)). Note that the configuration file could be
distributed with another package (usually shadow-utils). The
following configuration items are relevant for agetty:
LOGIN_PLAIN_PROMPT (boolean)
Tell agetty that printing the hostname should be suppressed
in the login: prompt. This is an alternative to the
--nohostname command line option. The default value is no.
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
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.
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
/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.
agetty supports configuration via systemd credentials (see
https://systemd.io/CREDENTIALS/). agetty reads the following
systemd credentials:
agetty.autologin (string)
If set, configures agetty to automatically log in the
specified user without asking for a username or password,
similarly to the --autologin option.
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.
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.
Werner Fink <[email protected]>, Karel Zak <[email protected]>
The original agetty for serial terminals was written by W.Z.
Venema <[email protected]> and ported to Linux by Peter
Orbaek <[email protected]>.
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
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/>. This page
is part of the util-linux (a random collection of Linux
utilities) project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have
a bug report for this manual page, send it to
[email protected]. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-10.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in
this COLOPHON (which is not part of the original manual page),
send a mail to [email protected]
util-linux 2.39.594-1e0ad 2023-07-19 AGETTY(8)
Pages that refer to this page: tty(4), ttyS(4), issue(5), systemd.exec(5), ttytype(5), utmp(5), systemd.system-credentials(7), systemd-getty-generator(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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