diff --git a/shell.nix b/shell.nix
index 2a52153..2583f15 100644
--- a/shell.nix
+++ b/shell.nix
@@ -10,12 +10,23 @@ let
       nixpkgs-fmt.enable = true;
     };
   };
+
+  update-sources = pkgs.writeShellScriptBin "update-sources" ''
+    set -e
+    git diff --exit-code -s nix/sources.json || (echo "File nix/sources.json has unstaged changes, refusing to update." >&2 && exit 1)
+    git diff --cached --exit-code -s nix/sources.json || (echo "File nix/sources.json has staged changes, refusing to update." >&2 && exit 1)
+    niv update
+    git diff --exit-code -s nix/sources.json && echo "Already up to date." && exit 0
+    git commit -m "Update sources" nix/sources.json
+  '';
 in
 pkgs.mkShell {
-  nativeBuildInputs = with pkgs; [
+  nativeBuildInputs = (with pkgs; [
     git
     niv
     nixpkgs-fmt
+  ]) ++ [
+    update-sources
   ];
   shellHook = ''
     ${pre-commit-check.shellHook}