Section (8) agetty
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 file or directory 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 _zsingle_quotesz_9600_zsingle_quotesz_. 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 _zsingle_quotesz_vt100_zsingle_quotesz_, or _zsingle_quotesz_linux_zsingle_quotesz_ for Linux on a virtual terminal, or _zsingle_quotesz_hurd_zsingle_quotesz_ 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_zsingle_quotesz_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
file|directory
-
Display the contents of file instead of
/etc/issue
. If the specified path is a directory then displays 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. −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 _zsingle_quotesz_auto_zsingle_quotesz_, _zsingle_quotesz_always_zsingle_quotesz_ or _zsingle_quotesz_never_zsingle_quotesz_. If the mode argument is omitted, then the default is _zsingle_quotesz_always_zsingle_quotesz_. If the −−local−line option is not given at all, then the default is _zsingle_quotesz_auto_zsingle_quotesz_.
- 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 _zsingle_quotesz_−h darkstar −− u_zsingle_quotesz_
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.
−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 _zsingle_quotesz_erase_zsingle_quotesz_ has been _zsingle_quotesz_#_zsingle_quotesz_, 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 _zsingle_quotesz_kill_zsingle_quotesz_ has been [email protected]_zsingle_quotesz_, 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.
EXAMPLES
This section shows examples for the process field of an
entry in the /etc/inittab
file.
You_zsingle_quotesz_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 _zsingle_quotesz_ATE0Q1&D2&C1S0=1 15_zsingle_quotesz_ 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 commandline 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 than
the directory is ignored. All files with .issue extension
from the directory are printed in version-sort order. The
directory allow to maintain 3rd-party messages independently
on the primary system /etc/issue
file.
The default path maybe overridden by −−issue−file
option. In
this case specified path has to be file or directory and the
default /etc/issue
as well as
/etc/issue.d
are ignored.
The issue files may contain certain escape codes to display the system name, date, time etcetera. 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 not any configured interface is found, fall back to the IP address of the machine_zsingle_quotesz_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 33. 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, and yellow. All unknown names are silently ignored.
s
-
Insert the system name (the name of the operating system). Same as _zsingle_quotesz_uname −s_zsingle_quotesz_. 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 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 _zsingle_quotesz_uname −m_zsingle_quotesz_.
n
-
Insert the nodename of the machine, also known as the hostname. Same as _zsingle_quotesz_uname −n_zsingle_quotesz_.
o
-
Insert the NIS domainname of the machine. Same as _zsingle_quotesz_hostname −d_zsingle_quotesz_.
O
-
Insert the DNS domainname of the machine.
r
-
Insert the release number of the OS. Same as _zsingle_quotesz_uname −r_zsingle_quotesz_.
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 .o (s m )
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
The original agetty for serial terminals was written by W.Z. Venema <[email protected]> and ported to Linux by Peter Orbaek <[email protected]>.