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AGETTY(8) |
System Administration |
AGETTY(8) |
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 hard-wired 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 back space (erase); carriage return
and line feed (end of line). See also --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 hard-ware 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 '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 virtual
terminal, or 'hurd' for GNU Hurd on virtual terminal.
- -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.
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
- Don't reset terminal cflags (control modes). See termios(3) for
more details.
- -E, --remote
- If -H fakehost option is given then -r fakehost options 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.
- -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, --chroot "directory"
- Change root to the specified directory.
- -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).
- --nohints
- Do not print hints about Num, Caps and Scroll Locks.
- --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 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.
- --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 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
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.
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.
- 4 or 4{interface}
- Insert the IPv4 address of the machine hostname or IPv4 address the
configured network interface if the interface argument is specified (e.g.
\4{eth0}).
- 6 or 6{interface}
- Insert the IPv6 address of the machine hostname or IPv6 address the
configured network interface if the interface argument is specified (e.g.
\6{eth0}}
- 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'. See also \S escape code.
- S or S{VARIABLE}
- Insert the VARIABLE data from /etc/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, 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
- /var/run/utmp
- the system status file.
- /etc/issue
- printed before the login prompt.
- /etc/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 -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.
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.
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>.
The agetty command is part of the util-linux package and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
Visit the GSP FreeBSD Man Page Interface. Output converted with ManDoc.
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