shell deploy: add --diff

shell.nix

@@ -7,6 +7,7 @@ let
   #
 
   cmds.deploy = pkgs.withGetopt {
+    diff = { default = /* sh */ "false"; switch = true; };
     force-populate = { default = /* sh */ "false"; switch = true; };
     quiet = { default = /* sh */ "false"; switch = true; };
     source_file = {
@@ -22,6 +23,65 @@ let
     . ${init.env}
     . ${init.proxy "deploy" opts}
 
+    if \test ${opts.diff.ref} = true; then
+
+      system_profile=/nix/var/nix/profiles/system
+      system_drv_cur=/etc/system.drv
+
+      system_drv_new=$(
+        ${pkgs.nix}/bin/nix-instantiate \
+            -Q \
+            -I "$target_path" \
+            -E '
+              (import <nixpkgs/nixos/lib/eval-config.nix> {
+                modules = [ <nixos-config> ];
+              }).config.system.build.toplevel
+            '
+      )
+
+      if \test -e "$system_drv_cur"; then
+
+        system_drv_cur_c=$(${pkgs.coreutils}/bin/readlink -f "$system_drv_cur")
+        system_drv_new_c=$(${pkgs.coreutils}/bin/readlink -f "$system_drv_new")
+
+        if \test "$system_drv_cur_c" = "$system_drv_new_c"; then
+          echo "$0: system up to date" >&2
+          exit 0
+        fi
+
+        system_drv_cur=$system_drv_cur_c \
+        system_drv_new=$system_drv_new_c \
+        ${pkgs.utillinux}/bin/script \
+            --command '
+              ${pkgs.haskellPackages.nix-diff}/bin/nix-diff \
+                  "$system_drv_cur" "$system_drv_new"
+            ' \
+            --quiet \
+            --return \
+            /dev/null
+
+        printf 'deploy? [N/y] ' >&2
+        read -r REPLY
+        if \test "$REPLY" != y; then
+          echo "$0: abort!" >&2
+          exit 1
+        fi
+      else
+        echo "$0: --${opts.diff.long} has no effect because "$system_drv_cur" doesn't exist" >&2
+      fi
+
+      new_system=$(${pkgs.nix}/bin/nix-store --realize "$system_drv_new")
+
+      ${pkgs.nix}/bin/nix-env -p "$system_profile" --set "$new_system"
+      PATH=${lib.makeBinPath [
+        pkgs.systemd
+      ]} \
+      "$system_profile"/bin/switch-to-configuration switch
+
+      ${pkgs.coreutils}/bin/ln -fns "$system_drv_new" "$system_drv_cur"
+      exit
+    fi
+
     # Use system's nixos-rebuild, which is not self-contained
     export PATH=/run/current-system/sw/bin
     exec ${utils.with-whatsupnix} \