chore: print the last modified date of "flake.lock"

Author: whisperpine

flake.nix

@@ -21,13 +21,20 @@
         { pkgs }:
         {
           default = pkgs.mkShell {
-            # The Nix packages provided in the environment.
+            # The Nix packages installed in the dev environment.
             packages = with pkgs; [
               opentofu # infrastructure as code
               sops # simple tool for managing secrets
               git-cliff # changelog generator
               trivy # scan security issues
             ];
+            # The shell script executed when the environment is activated.
+            shellHook = ''
+              # Print the last modified date of "flake.lock".
+              stat flake.lock | grep "Modify" |
+                awk '{printf "\"flake.lock\" last modified on: %s", $2}' &&
+                echo " ($((($(date +%s) - $(stat -c %Y flake.lock)) / 86400)) days ago)"
+            '';
           };
         }
       );