nix: add semi-automatic update process

Justfile

@@ -68,15 +68,32 @@ rebuild-this-device rebuildarg="dry-activate":
 # --argstr moreargs "\'-I nixos-unstable=https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz\'"
 
 
-# Update nix-channels and switch to updated NixOS and home environments
-update-this-device:
+# Update the channel versions and (by default) switch to new environment
+update-this-device rebuild-mode='switch':
 	#!/usr/bin/env bash
-	set -e
-	export SYSREBUILD_LOG=.$(hostname -s)_sysrebuild.log
+	set -xe
 
-	echo Updating system channels...
-	sudo nix-channel --update
-	just -v rebuild-this-device switch
+	template=nix/os/devices/$(hostname -s)/versions.tmpl.nix
+	outfile=nix/os/devices/$(hostname -s)/versions.nix
+
+	esh -o ${outfile} ${template}
+	if ! test "$(git diff ${outfile})"; then
+		echo Already on latest versions
+		exit 0
+	fi
+
+	export SYSREBUILD_LOG=.$(hostname -s)_sysrebuild.log
+	just -v rebuild-this-device dry-activate || {
+		echo ERROR: Update failed, reverting ${outfile}...
+		git checkout ${outfile}
+		exit 1
+	}
+
+	git commit -v ${outfile} -m "nix/os/devices/$(hostname -s): bump versions"
+
+	just -v rebuild-this-device {{rebuild-mode}} || {
+		echo WARNING: Rebuilding in {{rebuild-mode}}-mode failed
+	}
 
 # Iterate on a qtile config by running it inside Xephyr. (un-/grab the mouse with Ctrl + Shift-L)
 hm-iterate-qtile:

nix/os/devices/steveej-t480s-work/versions.tmpl.nix

@@ -0,0 +1,10 @@
+{
+  channelsNixosStable = {
+    ref = "nixos-18.09";
+    rev = "<% git ls-remote https://github.com/nixos/nixpkgs-channels nixos-18.09 | awk '{ print $1 }' | tr -d '\n' -%>";
+  };
+  channelsNixosUnstable = {
+    ref = "nixos-unstable";
+    rev = "<% git ls-remote https://github.com/nixos/nixpkgs-channels nixos-unstable | awk '{ print $1 }' | tr -d '\n' -%>";
+  };
+}

shell.nix

@@ -18,6 +18,8 @@ stdenv.mkDerivation {
     vncdo
     tesseract
     imagemagick
+
+    esh
   ];
 
   # Set Environment Variables