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AGETTY

Section: System Administration (8)
Updated: May 2011
Index Return to Main Contents

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NAME

agetty - alternative Linux getty

 

SYNOPSIS

agetty [-8chiLmnsUw] [-a user] [-f issue_file] [-H login_host] [-I init] [-l login_program] [-t timeout] 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 hard-wired and for dial-in lines:

o

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: @ and Control-U (kill); #, DEL and back space (erase); carriage return and line feed (end of line).

o

Optionally deduces the baud rate from the CONNECT messages produced by Hayes(tm)-compatible modems.

o

Optionally does not hang up when it is given an already opened line (useful for call-back applications).

o

Optionally does not display the contents of the /etc/issue file.

o

Optionally displays an alternative issue file instead of /etc/issue.

o

Optionally does not ask for a login name.

o

Optionally invokes a non-standard login program instead of /bin/login.

o

Optionally turns on hard-ware flow control

o

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.

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.

 

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 behaviour 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.

-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.

-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

Force 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.

-m, --extract-baud

Try to extract the baud rate 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 -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 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.

-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, --hangup

Do call vhangup() for a virtually 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 hard-wired 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.

--noclear

Do not clear the screen before prompting for the login name (the screen is normally cleared).

--nonewline

Do not print a newline before writing out /etc/issue.

--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 full qualified hostname by gethostname() or if not found by gethostbyname() is shown.

--version

Output version information and exit.

--help

Output help screen 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 hard-wired line or a console tty:
      /sbin/agetty 9600 ttyS1

For a directly connected terminal without proper carriage detect wiring: (try this if your terminal just sleeps instead of giving you a password: prompt.)
      /sbin/agetty -L 9600 ttyS1 vt100

For a 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 dis-connection and turn 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 and time etc. All escape codes consist of a backslash (\) immediately followed by one of the letters explained below.

b

Insert the baudrate of the current line.

d

Insert the current date.

s

Insert the system name, the name of the operating system. Same as `uname -s'.

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, eg. the build-date etc.

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.
/dev/console, problem reports (if syslog(3) is not used).
/etc/inittab, init(8) configuration file.

 

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.  

AUTHOR(S)

W.Z. Venema <wietse@wzv.win.tue.nl>
Eindhoven University of Technology
Department of Mathematics and Computer Science
Den Dolech 2, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Peter Orbaek <poe@daimi.aau.dk>
Linux port and more options. Still maintains the code.

Eric Rasmussen <ear@usfirst.org>
Added -f option to display custom login messages on different terminals.

 

AVAILABILITY

The agetty command is part of the util-linux package and is available from ftp://ftp.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

AUTHOR(S)

AVAILABILITY

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