util-linux/term-utils/agetty.8

.TH AGETTY 8 "February 2016" "util-linux" "System Administration"
.SH NAME
agetty \- alternative Linux getty

.SH SYNOPSIS
.B agetty
[options]
.IR port " [" baud_rate "...] [" term ]

.SH DESCRIPTION
.ad
.fi
\fBagetty\fP opens a tty port, prompts for a login name and invokes
the /bin/login command.  It is normally invoked by \fBinit\fP(8).

\fBagetty\fP has several \fInon-standard\fP features that are useful
for hardwired and for dial-in lines:
.IP \(bu
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 \fB\-\-erase\-chars\fP and \fB\-\-kill\-chars\fP options.
.IP \(bu
Optionally deduces the baud rate from the CONNECT messages produced by
Hayes(tm)-compatible modems.
.IP \(bu
Optionally does not hang up when it is given an already opened line
(useful for call-back applications).
.IP \(bu
Optionally does not display the contents of the \fI/etc/issue\fP file.
.IP \(bu
Optionally displays an alternative issue file or directory instead of \fI/etc/issue\fP or \fI/etc/issue.d\fP.
.IP \(bu
Optionally does not ask for a login name.
.IP \(bu
Optionally invokes a non-standard login program instead of
\fI/bin/login\fP.
.IP \(bu
Optionally turns on hardware flow control.
.IP \(bu
Optionally forces the line to be local with no need for carrier detect.
.PP
This program does not use the \fI/etc/gettydefs\fP (System V) or
\fI/etc/gettytab\fP (SunOS 4) files.
.SH ARGUMENTS
.na
.nf
.fi
.ad
.TP
.I port
A path name relative to the \fI/dev\fP directory.  If a "\-" is
specified, \fBagetty\fP assumes that its standard input is
already connected to a tty port and that a connection to a
remote user has already been established.
.sp
Under System V, a "\-" \fIport\fP argument should be preceded
by a "\-\-".
.TP
.IR baud_rate ,...
A comma-separated list of one or more baud rates.  Each time
\fBagetty\fP receives a BREAK character it advances through
the list, which is treated as if it were circular.
.sp
Baud rates should be specified in descending order, so that the
null character (Ctrl\-@) can also be used for baud-rate switching.
.sp
This argument is optional and unnecessary for \fBvirtual terminals\fP.
.sp
The default for \fBserial terminals\fP is keep the current baud rate
(see \fB\-\-keep\-baud\fP) and if unsuccessful then default to '9600'.
.TP
.I 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.
.sp
The default is 'vt100', or 'linux' for Linux on a virtual terminal,
or 'hurd' for GNU Hurd on a virtual terminal.
.SH OPTIONS
.na
.nf
.fi
.ad
.TP
\-8, \-\-8bits
Assume that the tty is 8-bit clean, hence disable parity detection.
.TP
\-a, \-\-autologin \fIusername\fP
Automatically log in the specified user without asking for a username or password.
Using this option causes an \fB\-f \fIusername\fR option and argument to be
added to the \fB/bin/login\fP command line.  See \fB\-\-login\-options\fR, which
can be used to modify this option's behavior.

Note that \fB\-\-autologin\fP 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.
.TP
\-c, \-\-noreset
Do not reset terminal cflags (control modes).  See \fBtermios\fP(3) for more
details.
.TP
\-E, \-\-remote
Typically the \fBlogin\fP(1) command is given a remote hostname when
called by something such as \fBtelnetd\fP(8).  This option allows \fBagetty\fP
to pass what it is using for a hostname to \fBlogin\fP(1) for use
in \fButmp\fP(5).  See \fB\-\-host\fP, \fBlogin\fP(1), and \fButmp\fP(5).
.IP
If the \fB\-\-host\fP \fIfakehost\fP option is given, then an \fB\-h\fP
\fIfakehost\fP option and argument are added to the \fB/bin/login\fP
command line.
.IP
If the \fB\-\-nohostname\fR option is given, then an \fB\-H\fP option
is added to the \fB/bin/login\fP command line.
.IP
See \fB\-\-login\-options\fR.
.TP
\-f, \-\-issue\-file \fIfile|directory\fP
Display the contents of \fIfile\fP instead of \fI/etc/issue\fP.  If the
specified path is a \fIdirectory\fP 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.
.TP
\-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.
.TP
\-H, \-\-host \fIfakehost\fP
Write the specified \fIfakehost\fP into the utmp file.  Normally,
no login host is given, since \fBagetty\fP is used for local hardwired
connections and consoles.  However, this option can be useful for
identifying terminal concentrators and the like.
.TP
\-i, \-\-noissue
Do not display the contents of \fI/etc/issue\fP (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.
.TP
\-I, \-\-init\-string \fIinitstring\fP
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.
.TP
\-J, \-\-noclear
Do not clear the screen before prompting for the login name.
By default the screen is cleared.
.TP
\-l, \-\-login\-program \fIlogin_program\fP
Invoke the specified \fIlogin_program\fP 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
\fB\-\-login\-options\fP.
.TP
\-L, \-\-local\-line[=\fImode\fP]
Control the CLOCAL line flag.  The optional \fImode\fP argument is 'auto', 'always' or 'never'.
If the \fImode\fP argument is omitted, then the default is 'always'.  If the
\-\-local\-line option is not given at all, then the default is 'auto'.
.PP
.RS
.PD 1
.TP
\fIalways\fR
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.
.TP
\fInever\fR
Explicitly clears the CLOCAL flag from the line setting and the
carrier-detect signal is expected on the line.
.TP
\fIauto\fR
The \fBagetty\fR default.  Does not modify the CLOCAL setting and follows
the setting enabled by the kernel.
.PD
.RE
.TP
\-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>".
\fBagetty\fP assumes that the modem emits its status message at
the same speed as specified with (the first) \fIbaud_rate\fP value
on the command line.
.sp
Since the \fB\-\-extract\-baud\fP feature may fail on heavily-loaded
systems, you still should enable BREAK processing by enumerating all
expected baud rates on the command line.
.TP
\-\-list\-speeds
Display supported baud rates.  These are determined at compilation time.
.TP
\-n, \-\-skip\-login
Do not prompt the user for a login name.  This can be used in connection
with the \fB\-\-login\-program\fP option to invoke a non-standard login
process such as a BBS system.  Note that with the \fB\-\-skip\-login\fR
option, \fBagetty\fR 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 \fBagetty\fR starts (usually /bin/login) is run as root.
.TP
\-N, \-\-nonewline
Do not print a newline before writing out /etc/issue.
.TP
\-o, \-\-login\-options "\fIlogin_options\fP"
Options  and arguments that  are passed to \fBlogin\fP(1). Where \\u is
replaced by the login name. For example:
.RS
.IP "" 4
.B "\-\-login\-options '-h darkstar -- \\\u'"
.PP
See \fB\-\-autologin\fR, \fB\-\-login\-program\fR and \fB\-\-remote\fR.
.PP
Please read the SECURITY NOTICE below before using this option.
.RE
.TP
\-p, \-\-login\-pause
Wait for any key before dropping to the login prompt.  Can be combined
with \fB\-\-autologin\fP to save memory by lazily spawning shells.
.TP
\-r, \-\-chroot \fIdirectory\fP
Change root to the specified directory.
.TP
\-R, \-\-hangup
Call vhangup() to do a virtual hangup of the specified terminal.
.TP
\-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.
.TP
\-t, \-\-timeout \fItimeout\fP
Terminate if no user name could be read within \fItimeout\fP seconds.
Use of this option with hardwired terminal lines is not recommended.
.TP
\-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.
.TP
\-w, \-\-wait\-cr
Wait for the user or the modem to send a carriage-return or a
linefeed character before sending the \fI/etc/issue\fP file (or others)
and the login prompt.  This is useful with the \fB\-\-init\-string\fP
option.
.TP
\-\-nohints
Do not print hints about Num, Caps and Scroll Locks.
.TP
\-\-nohostname
By default the hostname will be printed.  With this option enabled,
no hostname at all will be shown.
.TP
\-\-long\-hostname
By default the hostname is only printed until the first dot.  With
this option enabled, the fully qualified hostname by \fBgethostname\fR(3P)
or (if not found) by \fBgetaddrinfo\fR(3) is shown.
.TP
\-\-erase\-chars \fIstring\fP
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.
.TP
\-\-kill\-chars \fIstring\fP
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.
.TP
\-\-chdir \fIdirectory\fP
Change directory before the login.
.TP
\-\-delay \fInumber\fP
Sleep seconds before open tty.
.TP
\-\-nice \fInumber\fP
Run login with this priority.
.TP
\-\-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
.BR inotify (7).
.TP
\-\-version
Display version information and exit.
.TP
\-\-help
Display help text and exit.
.PP
.SH EXAMPLES
This section shows examples for the process field of an entry in the
\fI/etc/inittab\fP file.  You'll have to prepend appropriate values
for the other fields.  See \fIinittab(5)\fP for more details.

For a hardwired line or a console tty:

.RS
.B /sbin/agetty\ 9600\ ttyS1
.RE

For a directly connected terminal without proper carrier-detect wiring
(try this if your terminal just sleeps instead of giving you a password:
prompt):

.RS
.B /sbin/agetty\ \-\-local\-line\ 9600\ ttyS1\ vt100
.RE

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

.RS
.B /sbin/agetty\ \-\-extract\-baud\ \-\-timeout\ 60\ ttyS1\ 9600,2400,1200
.RE

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):

.ie n .RS 0
.el .RS
.B /sbin/agetty\ \-\-wait\-cr\ \-\-init\-string\ 'ATE0Q1&D2&C1S0=1\\015'\ 115200\ ttyS1
.RE

.SH SECURITY NOTICE
If you use the \fB\-\-login\-program\fP and \fB\-\-login\-options\fP 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.
.PP
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.

.SH ISSUE FILES
The default issue file is \fI/etc/issue\fP. If the file exists then agetty also
checks for \fI/etc/issue.d\fP directory. The directory is optional extension to
the default issue file and content of the directory is printed after
\fI/etc/issue\fP content. If the \fI/etc/issue\fP 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 \fI/etc/issue\fP file.

The default path maybe overridden by \fB\-\-issue\-file\fP option. In this case
specified path has to be file or directory and the default \fI/etc/issue\fP as
well as \fI/etc/issue.d\fP 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.

.TP
4 or 4{\fIinterface\fR}
Insert the IPv4 address of the specified network interface (for example: \\4{eth0}).
If the \fIinterface\fR 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.
.TP
6 or 6{\fIinterface\fR}
The same as \\4 but for IPv6.
.TP
b
Insert the baudrate of the current line.
.TP
d
Insert the current date.
.TP
e or e{\fIname\fR}
Translate the human-readable \fIname\fP to an escape sequence and insert it
(for example: \\e{red}Alert text.\\e{reset}).  If the \fIname\fR 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.
.TP
s
Insert the system name (the name of the operating system).  Same as 'uname \-s'.
See also the \\S escape code.
.TP
S or S{VARIABLE}
Insert the VARIABLE data from \fI/etc/os-release\fP.  If this file does not exist
then fall back to \fI/usr/lib/os-release\fP.  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 \fI/etc/issue\fP distribution and release
independent.  Note that \\S{ANSI_COLOR} is converted to the real terminal
escape sequence.
.TP
l
Insert the name of the current tty line.
.TP
m
Insert the architecture identifier of the machine.  Same as 'uname \-m'.
.TP
n
Insert the nodename of the machine, also known as the hostname.  Same as 'uname \-n'.
.TP
o
Insert the NIS domainname of the machine.  Same as 'hostname \-d'.
.TP
O
Insert the DNS domainname of the machine.
.TP
r
Insert the release number of the OS.  Same as 'uname \-r'.
.TP
t
Insert the current time.
.TP
u
Insert the number of current users logged in.
.TP
U
Insert the string "1 user" or "<n> users" where <n> is the number of current
users logged in.
.TP
v
Insert the version of the OS, that is, the build-date and such.
.PP
An example.  On my system, the following \fI/etc/issue\fP file:
.sp
.na
.RS
.nf
This is \\n.\\o (\\s \\m \\r) \\t
.fi
.RE
.PP
displays as:
.sp
.RS
.nf
This is thingol.orcan.dk (Linux i386 1.1.9) 18:29:30
.fi
.RE

.SH FILES
.na
.TP
.I /var/run/utmp
the system status file.
.TP
.I /etc/issue
printed before the login prompt.
.TP
.I /etc/os-release /usr/lib/os-release
operating system identification data.
.TP
.I /dev/console
problem reports (if syslog(3) is not used).
.TP
.I /etc/inittab
\fIinit\fP(8) configuration file for SysV-style init daemon.
.SH BUGS
.ad
.fi
The baud-rate detection feature (the \fB\-\-extract\-baud\fP option) requires that
\fBagetty\fP 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 \fB\-\-extract\-baud\fP option in combination with a multiple baud
rate command-line argument, so that BREAK processing is enabled.

The text in the \fI/etc/issue\fP file (or other) and the login prompt
are always output with 7-bit characters and space parity.

The baud-rate detection feature (the \fB\-\-extract\-baud\fP option) requires that
the modem emits its status message \fIafter\fP raising the DCD line.
.SH DIAGNOSTICS
.ad
.fi
Depending on how the program was configured, all diagnostics are
written to the console device or reported via the \fBsyslog\fR(3) facility.
Error messages are produced if the \fIport\fP argument does not
specify a terminal device; if there is no utmp entry for the
current process (System V only); and so on.
.SH AUTHORS
.UR werner@suse.de
Werner Fink
.UE
.br
.UR kzak@redhat.com
Karel Zak
.UE
.sp
The original
.B 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>.

.SH AVAILABILITY
The agetty command is part of the util-linux package and is available from
https://www.kernel.org/pub/linux/utils/util\-linux/.