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113 lines
4.5 KiB
Markdown
113 lines
4.5 KiB
Markdown
TTYs and Consoles
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=================
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**Syntax:** `tty [LVLS] <COND> DEV [BAUD] [noclear] [nowait] [nologin] [TERM]`
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`tty [LVLS] <COND> CMD <ARGS> [noclear] [nowait]`
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`tty [LVLS] <COND> [notty] [rescue]`
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The first variant of this option uses the built-in getty on the given
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TTY device DEV, in the given runlevels. DEV may be the special keyword
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`@console`, which is expanded from `/sys/class/tty/console/active`,
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useful on embedded systems.
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The default baud rate is 0, i.e., keep kernel default.
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> The `tty` stanza inherits runlevel, condition (and other feature)
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> parsing from the `service` stanza. So TTYs can run in one or many
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> runlevels and depend on any condition supported by Finit. This is
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> useful e.g. to depend on `<pid/elogind>` before starting a TTY.
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**Example:**
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tty [12345] /dev/ttyAMA0 115200 noclear vt220
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The second `tty` syntax variant is for using an external getty, like
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agetty or the BusyBox getty.
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The third variant is for board bringup and the `rescue` boot mode. No
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device node is required in this variant, the same output that the kernel
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uses is reused for stdio. If the `rescue` option is omitted, a shell is
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started (`nologin`, `noclear`, and `nowait` are implied), if the rescue
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option is set the bundled `/libexec/finit/sulogin` is started to present
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a bare-bones root login prompt. If the root (uid:0, gid:0) user does
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not have a password set, no rescue is possible. For more information,
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see the [Rescue Mode](rescue.md) section.
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By default, the first two syntax variants *clear* the TTY and *wait* for
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the user to press enter before starting getty.
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**Example:**
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tty [12345] /sbin/getty -L 115200 /dev/ttyAMA0 vt100
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tty [12345] /sbin/agetty -L ttyAMA0 115200 vt100 nowait
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The `noclear` option disables clearing the TTY after each session.
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Clearing the TTY when a user logs out is usually preferable.
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The `nowait` option disables the `press Enter to activate console`
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message before actually starting the getty program. On small and
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embedded systems running multiple unused getty wastes both memory
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and CPU cycles, so `wait` is the preferred default.
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The `nologin` option disables getty and `/bin/login`, and gives the
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user a root (login) shell on the given TTY `<DEV>` immediately.
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Needless to say, this is a rather insecure option, but can be very
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useful for developer builds, during board bringup, or similar.
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Notice the ordering, the `TERM` option to the built-in getty must be
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the last argument.
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Embedded systems may want to enable automatic `DEV` by supplying the
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special `@console` device. This works regardless weather the system
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uses `ttyS0`, `ttyAMA0`, `ttyMXC0`, or anything else. Finit figures
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it out by querying sysfs: `/sys/class/tty/console/active`. The speed
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can be omitted to keep the kernel default.
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> Most systems get by fine by just using `console`, which will evaluate
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> to `/dev/console`. If you have to use `@console` to get any output,
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> you may have some issue with your kernel config.
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**Example:**
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tty [12345] @console noclear vt220
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On really bare bones systems, or for board bringup, Finit can give you a
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shell prompt as soon as bootstrap is done, without opening any device
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node:
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tty [12345789] notty
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This should of course not be enabled on production systems. Because it
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may give a user root access without having to log in. However, for
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board bringup and system debugging it can come in handy.
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One can also use the `service` stanza to start a stand-alone shell:
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service [12345] /bin/sh -l
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Controlling TTY for Services
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----------------------------
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The `tty:<dev>` option gives a `run`, `task`, or `service` a controlling
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terminal on the given device. The device is opened, set as the
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controlling terminal for the session (after `setsid()`), and connected to
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the process's stdin, stdout, and stderr. A default `TERM` environment
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variable is set based on the device type: `vt102` for serial lines and
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`linux` for virtual terminals.
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`<dev>` may be a device node like `/dev/ttyS0`, or the special keyword
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`@console` (see above). Note that `@console` expands only to the
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first console, not all.
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When `tty:` is combined with `log:`, stdout and stderr are redirected
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to the log sink instead of the TTY, but stdin remains connected to the
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TTY device.
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> The `tty:<dev>` option is for `run`, `task`, and `service` stanzas only.
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> The `tty` directive itself (for getty/login) has its own syntax, see
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> above.
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**Example:**
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service [2345] tty:/dev/ttyS0 /usr/sbin/foo -- Foo on serial console
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task [S] tty:@console my-setup-script -- Board bringup on console |