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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.

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, 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 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. 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).