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Tips & Tricks with the kernel cmdline
=====================================
This document summarizes the different boot parameters that can be
passed on the Linux kernel command line. Not limited to Finit.
> [!IMPORTANT]
> Remember to use `--` to separate kernel parameters from parameters to
> init. E.g., `init=/sbin/finit -- finit.debug rescue`
The `bool` setting is one of `on, off, true false, 1, 0`.
* `debug`: Enable kernel debug. Debug messages are printed to the
console until Finit starts up, unless `loglevel=7` (below) is used.
* `fsck.mode=<auto,force,skip>`, default: `auto`, unless built with
`configure --enable-fastboot`, in which case the default is `skip`
* `fsck.repair=<preen,yes,no>`, default: `preen`, unless built with
`configure --enable-fsckfix`, in which case the default is `yes`.
This configure option also sets `fsck.mode=force`, unless fastboot
is selected at the same time. I.e., the `--enable-fastboot` build
option overrides the `fsck.mode` default value.
* `finit.cond=foo[,bar[,baz]]`: set `<boot/foo>` condition, optionally
multiple conditions can be set using the same option, separated with a
comma. Alternatively, multiple `foo.cond=arg` can be given. Each will
result in a `<boot/arg>` condition being set to control the rest of the
system bootstrap.
Very useful for selecting different boot modes, e.g. manufacturing test,
firmware upgrade, or [rescue mode][rescue].
> [!NOTE]
> `<boot/...>` conditions cannot be cleared with `initctl`!
* `finit.config=/path/to/alternative/finit.conf`: override the
compile-time bootstrap config file, default:
./configure --with-config=/etc/finit.conf
Useful when starting up in various [rescue mode][rescue], factory, or
production test setups. Use the top-level configuration file
directive `rcsd /path/to/finit.d` to override the default rcS.d
directory.
* `finit.debug[=bool]`: Enable finit debug. This is operated
independently of the kernel `debug` setting. New as of Finit v4.
* `finit.fstab=</path/to/etc/fstab.alternative>`: Tell Finit to use an
alternate `fstab` to mount the file system from. Remember, this file
must be on the `root=...` file system provided to Finit from the
kernel. By default the built-in fstab is used, which itself defaults
to `/etc/fstab`, but can be changed at build time with:
./configure --with-fstab=/path/to/fstab
It is even possible to disable a built-in default using:
./configure --without-fstab
Making `finit.fstab=/path/to/fstab` a *mandatory* command line option.
Note, if the command line fstab is missing, Finit falls back to the
built-in fstab, and if both are missing, the system treats this as a
bad `fsck` and thus calls `sulogin`. If, in turn, `sulogin` is not
available on the system, Finit calls reboot, which is also what will
happen when a user exits from `sulogin`.
* `finit.status[=bool]`: Control finit boot progress, including banner.
(Used to be `finit.show_status`, which works but is deprecated.)
* `finit.status_style=<classic,modern>`: Set Finit boot progress style,
when enabled.
* `init=/bin/sh`: Bypass system default init and tell kernel to start a
shell. Note, this shell is very limited and does not support
signals and has no job control. Recommend using, and modifying,
[`rescue` mode][rescue] instead.
* `loglevel=<0-7>`, sets the kernel's log level, which is more granular
than `debug`. Also, when `loglevel=7`, Finit will *not disable*
kernel logs to the console. This is very useful when debugging the
kernel at system bring-up. Since `loglevel=7` is the same as `debug`
this means you have to use `quiet` for a quiet boot, until sysklogd
takes over logging of kernel events.
* `panic=SEC`: By default the kernel does not reboot after a kernel
panic. This setting will cause a kernel reboot after SEC seconds.
* `quiet`: Suppress kernel logging to console, except for warnings and
errors. Also, see `loglevel` and `quiet` above.
* `rescue`: Start [rescue/maintenance mode][rescue]. If your system
comes with the bundled `sulogin` program (Finit, or from util-linux,
or Busybox), you will be given a root login to a maintenance shell.
However, if `sulogin` is missing, the file `/lib/finit/rescue.conf`
is read and the system booted in a limited fallback mode.
This option can be disabled with `configure --without-rescue`
**Note:** in this mode `initctl` will not work. Correct the problem
and use `reboot -f` to force reboot.
* `single`, or `S`: Overrides the configured runlevel (default: 2) to go
to after bootstrap by forcing it to runlevel 1, this is also known
as single user mode. Useful to debug startup problems. All services
and TTYs in `[1]` will be started, so a `tty [1] @console nologin`
configuration presents you with a root console without login.
* `1-9`, except `6`: override the configured `runlevel`. Like the `S`
and `rescue`, giving a single number on the kernel command line tells
Finit to ignore any `runlevel` in `/etc/finit.conf` as well as the
configure fallback `--with-runlevel=N` setting. Remember, `6` is the
reboot runlevel and is not permitted. Any other values are ignored.
For more on kernel boot parameters, see the man page [bootparam(7)][].
[rescue]: config/rescue.md
[bootparam(7)]: https://www.man7.org/linux/man-pages/man7/bootparam.7.html
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