#Bash 的 agetty 命令

AGETTY(8)                    System Administration                   AGETTY(8)

NAME
       agetty - alternative Linux getty

SYNOPSIS
       agetty [options] 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 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
       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
       -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. This setting is also possible to
           able by LOGIN_PLAIN_PROMPT option in the /etc/login.defs
           configuration file (see below for more details).

       --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).

       -h, --help
           Display help text and exit.

       -V, --version
           Print version and exit.

CONFIG FILE ITEMS
       agetty reads the /etc/login.defs configuration file (see
       login.defs(5)). Note that the configuration file could be distributed
       with another package (usually shadow-utils). The following
       configuration items are relevant for agetty:

       LOGIN_PLAIN_PROMPT (boolean)
           Tell agetty that printing the hostname should be suppressed in the
           login: prompt. This is an alternative to the --nohostname command
           line option. The default value is no.

EXAMPLE
       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
       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
       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 no 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
       /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
       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
       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
       Werner Fink <[email protected]>, Karel Zak <[email protected]>

       The original agetty for serial terminals was written by W.Z. Venema
       <[email protected]> and ported to Linux by Peter Orbaek
       <[email protected]>.

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

AVAILABILITY
       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/>.

util-linux 2.39.3                 2023-12-01                         AGETTY(8)