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 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.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 systemd1. 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.)

You can enter rescue mode by running: $ systemctl rescue This will eventually give you a single-user root shell. Systemd will stop (almost) all system services. To get out of maintenance mode, just exit from the rescue shell.

After running nixos-rebuild to switch to a new configuration, you may find that the new configuration doesn’t work very well. In that case, there are several ways to return to a previous configuration. First, the GRUB boot manager allows you to boot into any previous configuration that hasn’t been garbage-collected. These configurations can be found under the GRUB submenu “NixOS - All configurations”. This is especially useful if the new configuration fails to boot. After the system has booted, you can make the selected configuration the default for subsequent boots: $ /run/current-system/bin/switch-to-configuration boot Second, you can switch to the previous configuration in a running system: $ nixos-rebuild switch --rollback This is equivalent to running: $ /nix/var/nix/profiles/system-N-link/bin/switch-to-configuration switch where N is the number of the NixOS system configuration. To get a list of the available configurations, do: $ ls -l /nix/var/nix/profiles/system-*-link ... lrwxrwxrwx 1 root root 78 Aug 12 13:54 /nix/var/nix/profiles/system-268-link -> /nix/store/202b...-nixos-13.07pre4932_5a676e4-4be1055