agetty(8) — Linux manual page

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

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 \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 -- \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() 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 15' 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>.

AVAILABILITY         top

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

COLOPHON         top

       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
       2021-03-21.  (At that time, the date of the most recent commit
       that was found in the repository was 2021-03-19.)  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                    February 2016                    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)