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(8) 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.
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 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
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 \012.
−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() 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).
−−version
Display version information and exit.
−−help
Display help text and exit.
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 15' 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 than
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 than 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 is possible to completely disable
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.
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 not any configured interface is found, fall back to the IP address of the machine's hostname.
The same as \4 but for IPv6.
b
Insert the baudrate of the current line.
d
Insert the current date.
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.
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.
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.