agetty(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | ARGUMENTS | OPTIONS | EXAMPLE | SECURITY NOTICE | ISSUE FILES | FILES | BUGS | DIAGNOSTICS | AUTHORS | REPORTING BUGS | AVAILABILITY

AGETTY(8)                 System Administration                AGETTY(8)

NAME         top

       agetty - alternative Linux getty

SYNOPSIS         top

       agetty [options] port [baud_rate...] [term]

DESCRIPTION         top

       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 files or directories
           instead of /etc/issue or /etc/issue.d.

       •   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         top

       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(1) 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         top

       -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.

           Note that --autologin may affect the way in which getty
           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
           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 path
           Specifies a ":" delimited list of files and directories to be
           displayed instead of /etc/issue (or other). All specified
           files and directories are displayed, missing or empty files
           are silently ignored. If the specified path is a directory
           then display 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.

       --show-issue
           Display the current issue file (or other) on the current
           terminal and exit. Use this option to review the current
           setting, it is not designed for any other purpose. Note that
           output may use some default or incomplete information as
           proper output depends on terminal and agetty command line.

       -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 \12.

       -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 — \u'

           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(2) 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.
           If another baud rates specified then the original baud rate
           is also saved to the end of the wanted baud rates list. This
           can be used to return to the original baud rate after
           unexpected BREAKs.

       -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.

EXAMPLE         top

       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         top

       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 command
       line should not be interpreted as options. Use this feature if
       available by passing "--" before the username gets passed by \u.

ISSUE FILES         top

       The default issue file is /etc/issue. If the file exists, then
       agetty also checks for /etc/issue.d directory. The directory is
       optional extension to the default issue file and content of the
       directory is printed after /etc/issue content. If the /etc/issue
       does not exist, then the directory is ignored. All files with
       .issue extension from the directory are printed in version-sort
       order. The directory can be used to maintain 3rd-party messages
       independently on the primary system /etc/issue file.

       Since version 2.35 additional locations for issue file and
       directory are supported. If the default /etc/issue does not
       exist, then agetty checks for /run/issue and /run/issue.d,
       thereafter for /usr/lib/issue and /usr/lib/issue.d. The directory
       /etc is expected for host specific configuration, /run is
       expected for generated stuff and /usr/lib for static distribution
       maintained configuration.

       The default path maybe overridden by --issue-file option. In this
       case specified path has to be file or directory and all the
       default issue file and directory locations are ignored.

       The issue file feature can be completely disabled by --noissue
       option.

       It is possible to review the current issue file by agetty
       --show-issue on the current terminal.

       The issue files may contain certain escape codes to display the
       system name, date, time et cetera. 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, yellow and white. 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 can be
           used 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         top

       /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         top

       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         top

       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         top

       Werner Fink <werner@suse.de>, Karel Zak <kzak@redhat.com>

       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>.

REPORTING BUGS         top

       For bug reports, use the issue tracker at
       https://github.com/karelzak/util-linux/issues.

AVAILABILITY         top

       The agetty command is part of the util-linux package which can be
       downloaded from Linux Kernel Archive
       <https://www.kernel.org/pub/linux/utils/util-linux/>. This page
       is part of the util-linux (a random collection of Linux
       utilities) project. Information about the project can be found at
       ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have
       a bug report for this manual page, send it to
       util-linux@vger.kernel.org. This page was obtained from the
       project's upstream Git repository
       ⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
       2021-06-20. (At that time, the date of the most recent commit
       that was found in the repository was 2021-06-18.) If you discover
       any rendering problems in this HTML version of the page, or you
       believe there is a better or more up-to-date source for the page,
       or you have corrections or improvements to the information in
       this COLOPHON (which is not part of the original manual page),
       send a mail to man-pages@man7.org

util-linux 2.37.85-637cc       2021-04-02                      AGETTY(8)

Pages that refer to this page: tty(4)ttyS(4)issue(5)systemd.exec(5)ttytype(5)utmp(5)systemd-getty-generator(8)