[MAN] agetty

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AGETTY

Section: System Administration (8)
Updated: February 2016
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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 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

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.

-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 issue_file

Display the contents of issue_file instead of /etc/issue. 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 \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 -- \'

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 '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 --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 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 --issue-file 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, 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

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

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 https://www.kernel.org/pub/linux/utils/util-linux/.

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Index

NAME

SYNOPSIS

DESCRIPTION

ARGUMENTS

OPTIONS

EXAMPLES

SECURITY NOTICE

ISSUE ESCAPES

FILES

BUGS

DIAGNOSTICS

AUTHORS

AVAILABILITY

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Time: 04:46:02 GMT, September 16, 2022