nixpkgs/nixos/doc/manual/installation/changing-config.chapter.md
Robert Hensing 0e062cb692 nixos-rebuild: Add nixos-rebuild repl
Apologies to the non-flake users; your repl isn't quite as fancy,
but at least evaluates your config exactly as you would expect,
unlike flakes which are only evaluated impurely for now.
2023-12-10 06:36:02 +01:00

3.6 KiB

Changing the Configuration

The file /etc/nixos/configuration.nix contains the current configuration of your machine. Whenever you've changed something in that file, you should do

# nixos-rebuild switch

to build the new configuration, make it the default configuration for booting, and try to realise the configuration in the running system (e.g., by restarting system services).

::: {.warning} This command doesn't start/stop user services automatically. nixos-rebuild only runs a daemon-reload for each user with running user services. :::

::: {.warning} These commands must be executed as root, so you should either run them from a root shell or by prefixing them with sudo -i. :::

You can also do

# nixos-rebuild test

to build the configuration and switch the running system to it, but without making it the boot default. So if (say) the configuration locks up your machine, you can just reboot to get back to a working configuration.

There is also

# nixos-rebuild boot

to build the configuration and make it the boot default, but not switch to it now (so it will only take effect after the next reboot).

You can make your configuration show up in a different submenu of the GRUB 2 boot screen by giving it a different profile name, e.g.

# nixos-rebuild switch -p test

which causes the new configuration (and previous ones created using -p test) to show up in the GRUB submenu "NixOS - Profile 'test'". This can be useful to separate test configurations from "stable" configurations.

A repl, or read-eval-print loop, is also available. You can inspect your configuration and use the Nix language with

# nixos-rebuild repl

Your configuration is loaded into the config variable. Use tab for autocompletion, use the :r command to reload the configuration files. See :? or nix repl in the Nix manual to learn more.

Finally, you can do

$ nixos-rebuild build

to build the configuration but nothing more. This is useful to see whether everything compiles cleanly.

If you have a machine that supports hardware virtualisation, you can also test the new configuration in a sandbox by building and running a QEMU virtual machine that contains the desired configuration. Just do

$ nixos-rebuild build-vm
$ ./result/bin/run-*-vm

The VM does not have any data from your host system, so your existing user accounts and home directories will not be available unless you have set mutableUsers = false. Another way is to temporarily add the following to your configuration:

users.users.your-user.initialHashedPassword = "test";

Important: delete the $hostname.qcow2 file if you have started the virtual machine at least once without the right users, otherwise the changes will not get picked up. You can forward ports on the host to the guest. For instance, the following will forward host port 2222 to guest port 22 (SSH):

$ QEMU_NET_OPTS="hostfwd=tcp:127.0.0.1:2222-:22" ./result/bin/run-*-vm

allowing you to log in via SSH (assuming you have set the appropriate passwords or SSH authorized keys):

$ ssh -p 2222 localhost

Such port forwardings connect via the VM's virtual network interface. Thus they cannot connect to ports that are only bound to the VM's loopback interface (127.0.0.1), and the VM's NixOS firewall must be configured to allow these connections.