agetty man page

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 instead of /etc/issue.
·
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(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.

Options

-8, --8bits
Assume that the tty is 8-bit clean, hence disable parity detection.
-a, --autologin username
Log the specified user automatically in without asking for a login name and password. The -f username option is added to the /bin/login command line by default. The --login-options option changes this default behavior and then only \u is replaced by the username and no other option is added to the login command line.
-c, --noreset
Don't reset terminal cflags (control modes). See termios(3) for more details.
-E, --remote
If an -H fakehost option is given, then an -r fakehost option is added to the /bin/login command line.
-f, --issue-file issue_file
Display the contents of issue_file instead of /etc/issue. This allows custom messages to be displayed on different terminals. The -i 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 login_host
Write the specified login_host 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 (the screen is normally 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 (for example, one that asks for a dial-up password or that uses a different password file).
-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'.

The mode '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.

The mode 'never' explicitly clears the CLOCAL flag from the line setting and the carrier-detect signal is expected on the line.

The mode 'auto' (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 -m feature may fail on heavily-loaded systems, you still should enable BREAK processing by enumerating all expected baud rates on the command line.
-n, --skip-login
Do not prompt the user for a login name. This can be used in connection with the -l option to invoke a non-standard login process such as a BBS system. Note that with the -n option, agetty gets no input from the user who logs in and therefore won't 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 that are passed to the login program. \u is replaced by the login name. The default /bin/login command line is "/bin/login -- <username>".

Please read the Security Notice below if you want to use this.

-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. This option should probably not be used with hardwired lines.
-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 (or other) file and the login prompt. Very useful in connection with the -I 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() or (if not found) by getaddrinfo() 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.

Examples

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 -L 9600 ttyS1 vt100

For an old-style dial-in line with a 9600/2400/1200 baud modem:

/sbin/agetty -mt60 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 -w -I '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 can not 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 Escapes

The issue-file (/etc/issue, or the file set with the -f option) 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'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, and yellow. 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 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 `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, e.g. the build-date etc.

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 -m 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 -m 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 -m 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
Karel Zak

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>.

Availability

The agetty command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/ut….

Referenced By

ipmi-config.conf(5), issue(5), mingetty(8), systemd.exec(5), systemd-getty-generator(8), tty(4), ttyS(4), ttytype(5), utmp(5).

February 2016 util-linux System Administration