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nixpkgs/nixos/doc/manual/administration/boot-problems.section.md

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Boot Problems

If NixOS fails to boot, there are a number of kernel command line parameters that may help you to identify or fix the issue. You can add these parameters in the GRUB boot menu by pressing “e” to modify the selected boot entry and editing the line starting with linux. The following are some useful kernel command line parameters that are recognised by the NixOS boot scripts or by systemd:

boot.shell_on_fail

Allows the user to start a root shell if something goes wrong in stage 1 of the boot process (the initial ramdisk). This is disabled by default because there is no authentication for the root shell.

boot.debug1

Start an interactive shell in stage 1 before anything useful has been done. That is, no modules have been loaded and no file systems have been mounted, except for /proc and /sys.

boot.debug1devices

Like boot.debug1, but runs stage1 until kernel modules are loaded and device nodes are created. This may help with e.g. making the keyboard work.

boot.debug1mounts

Like boot.debug1 or boot.debug1devices, but runs stage1 until all filesystems that are mounted during initrd are mounted (see neededForBoot). As a motivating example, this could be useful if you've forgotten to set neededForBoot on a file system.

boot.trace

Print every shell command executed by the stage 1 and 2 boot scripts.

single

Boot into rescue mode (a.k.a. single user mode). This will cause systemd to start nothing but the unit rescue.target, which runs sulogin to prompt for the root password and start a root login shell. Exiting the shell causes the system to continue with the normal boot process.

systemd.log_level=debug systemd.log_target=console

Make systemd very verbose and send log messages to the console instead of the journal. For more parameters recognised by systemd, see systemd(1).

Notice that for boot.shell_on_fail, boot.debug1, boot.debug1devices, and boot.debug1mounts, if you did not select "start the new shell as pid 1", and you exit from the new shell, boot will proceed normally from the point where it failed, as if you'd chosen "ignore the error and continue".

If no login prompts or X11 login screens appear (e.g. due to hanging dependencies), you can press Alt+ArrowUp. If you’re lucky, this will start rescue mode (described above). (Also note that since most units have a 90-second timeout before systemd gives up on them, the agetty login prompts should appear eventually unless something is very wrong.)