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. 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.
CONFIG FILE ITEMS¶
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.
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:
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
/etc/inittab
init(8) configuration file for SysV-style init
daemon.
CREDENTIALS¶
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.
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>.
