dev: NixOS 25.11 매뉴얼 로컬 사본 추가

nixpkgs nixos-25.11 브랜치의 nixos/doc/manual/ 디렉토리를 sparse clone으로
가져와 ~/obsidian/dev/nixos-manual/에 복사. _index.md에 구조/갱신 방법 정리.

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dev/nixos-manual/installation/changing-config.chapter.md

@@ -0,0 +1,113 @@
+# Changing the Configuration {#sec-changing-config}
+
+The file `/etc/nixos/configuration.nix` contains the current
+configuration of your machine. Whenever you've [changed
+something](#ch-configuration) in that file, you should do
+
+```ShellSession
+# 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](#opt-systemd.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
+
+```ShellSession
+# 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
+
+```ShellSession
+# 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.
+
+```ShellSession
+# 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
+
+```ShellSession
+# 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](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-repl.html) to learn more.
+
+Finally, you can do
+
+```ShellSession
+$ 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
+
+```ShellSession
+$ 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:
+
+```nix
+{ 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):
+
+```ShellSession
+$ 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):
+
+```ShellSession
+$ 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.