update README

.gitignore

@@ -1 +1,2 @@
-/.idea/
+/.idea/
+/result

README.md

@@ -17,36 +17,54 @@ During installation, select manual partitioning:
 * One 512MB (or larger) Fat32 partition, mounted at `/boot`, "boot" flag enabled
 * Another partition (e.g. BTRFS) covering the rest of the drive, mounted at `/`, encryption enabled
 
-## Build and switch
+## Update, build and switch
+
+### Update
+
+* Updating NixOS. https://superuser.com/a/1604695
+
+Update channel and configuration:
+
+```shell
+sudo nix-channel --update && niv update
+```
+
+To apply the updates, continue with "Build and switch".
+
+### Build and switch
 
 * https://nixos.wiki/wiki/Nixos-rebuild
+* https://discourse.nixos.org/t/how-to-get-this-pending-updates-notification-in-gnome/16344/3
+* https://discourse.nixos.org/t/how-to-get-this-pending-updates-notification-in-gnome/16344/6
 
-Build new config, activate it and make it default:
+#### Option I: Build config and view changes:
+
+```shell
+# This leaves a symlink named `result` in the current directory.
+sudo nixos-rebuild -I nixos-config=hosts/$(hostname)/configuration.nix build
+nix --extra-experimental-features nix-command store diff-closures /run/current-system ./result
+```
+
+Depending on if there are large kernel changes, either switch to it directly or wait until next boot:
+
+```shell
+sudo ./result/bin/switch-to-configuration switch
+# or
+sudo ./result/bin/switch-to-configuration boot
+```
+
+#### Option II: Build new config and activate it:
 
 ```shell
 sudo nixos-rebuild -I nixos-config=hosts/$(hostname)/configuration.nix switch
 ```
 
-Or activate during next boot:
+#### Option III: Build new config and activate it during next boot:
 
 ```shell
 sudo nixos-rebuild -I nixos-config=hosts/$(hostname)/configuration.nix boot
 ```
 
-## System updates
-
-* https://superuser.com/a/1604695
-
-```shell
-sudo nix-channel --update
-```
-
-```shell
-niv update
-```
-
-Then run `nixos-rebuild switch`, see above.
-
 ## Garbage collection
 
 * https://nixos.org/manual/nix/stable/package-management/garbage-collection.html
@@ -231,6 +249,34 @@ Excerpt of the result:
 /nix/store/jqvxmx65mfinbsm6db9kmcqmphl44xhp-nano-7.2/share/nano/autoconf.nanorc
 ```
 
+### Compare two versions of NixOS system profile
+
+```shell
+nix --extra-experimental-features nix-command store diff-closures /nix/var/nix/profiles/system-110-link /nix/var/nix/profiles/system-116-link
+```
+```
+cpupower: 6.1.47 → 6.1.51
+element-desktop: 1.11.38 → 1.11.40, +2218.9 KiB
+element-web: 1.11.38 → 1.11.40, -73.1 KiB
+exempi: 2.6.3 → 2.6.4
+firefox: 116.0.3 → 117.0
+firefox-unwrapped: 116.0.3 → 117.0, -292.6 KiB
+gnome-shell-extension-openweather: ∅ → 121, +590.5 KiB
+hm_fontconfigconf.d10hmfonts.conf: ∅ → ε
+initrd: ∅ → ε
+initrd-linux: 6.1.47 → 6.1.51
+libcap: 2.68 → 2.69
+linux: 6.1.47, 6.1.47-modules → 6.1.51, 6.1.51-modules, -11.8 KiB
+meld: ∅ → 3.22.0, +3858.5 KiB
+net-snmp: 5.9.3 → 5.9.4
+nixos-system-yodaTab: 23.05.3085.2ab91c8d65c0 → 23.05.3242.da5adce0ffaf
+openjdk: +19.5 KiB
+python3.10-pygobject: +27.0 KiB
+stage: ∅ → 1-init.sh, +29.5 KiB
+tor-browser-bundle-bin: 12.5.2 → 12.5.3, +18.1 KiB
+user: +2885.0 KiB
+```
+
 ### NixOS configuration debugging
 
 * https://nixos.wiki/wiki/Nix_command/repl