check latest version before updating

desktop.nix

@@ -16,6 +16,7 @@ in
     ./backgrounds.nix
     ./skel.nix
     ./fonts.nix
+    ./version.nix
   ];
 
   config = {
@@ -58,7 +59,6 @@ in
       firefox
       flameshot
       tmux
-      (pkgs.callPackage ./update.nix {})
     ];
   };
 }

update.nix

-{pkgs, ...}:
-pkgs.writeShellScriptBin "liucs-update" ''
-  # run as root
-  [[ $(id -un) == root ]] || {
-      exec sudo -H $0 "$@"
-  }
-  set -o xtrace
-  nix-channel --update liucs
-  nixos-rebuild switch
-''

version.nix

-{
+{pkgs, ...}:
+let
   hash = "$Format:%H$";
+  date = "$Format:%ci$";
+
+  repo = "https://gitlab.liu.edu/league/nixvm.git";
+  ref = "master";
+
+  showVersion = ''
+    echo '${hash} (${date})'
+    latest=$(${pkgs.git}/bin/git ls-remote ${repo} ${ref}|cut -f1)
+    if [[ "${hash}" == "$latest" ]]; then
+      echo Your system is up to date.
+    else
+      echo An update is available.
+    fi
+  '';
+
+  doUpdate = ''
+    [[ $(id -un) == root ]] || {
+        exec sudo -H $0 "$@"
+    }
+    set -o xtrace
+    latest=$(${pkgs.git}/bin/git ls-remote ${repo} ${ref}|cut -f1)
+    if [[ "${hash}" == "$latest" ]]; then
+      echo Already up to date.
+    else
+      nix-channel --update liucs
+      nixos-rebuild switch
+    fi
+  '';
+in
+{
+  environment.systemPackages = [
+    (pkgs.writeShellScriptBin "liucs-version" showVersion)
+    (pkgs.writeShellScriptBin "liucs-update" doUpdate)
+  ];
 }