diff --git a/justfile b/justfile
index cfea086..49ca6cd 100644
--- a/justfile
+++ b/justfile
@@ -730,3 +730,27 @@ _packages_json:
 @_fmt:
     just --unstable --fmt
     #sd "\n+(@_[a-zA-Z_-]+:)" "\n" justfile
+
+#sync ./dev.nix
+dev:
+    #!/usr/bin/env -S bash -euo pipefail
+    [[ -f "$invokedir"/flake.nix
+    && -f "$invokedir"/default.nix
+    && -e "$invokedir"/.git
+    && -f "$invokedir"/pkgs/top-level/impure.nix
+    ]] || ! echo >&2 "ERROR: not in nixpkgs root"
+
+    # local .gitignore for dev.nix
+    if ! grep -qxF "/dev.nix" master/.git/info/exclude; then
+        echo "/dev.nix" >> master/.git/info/exclude
+    fi
+
+    if [[ ! -f "$invokedir"/dev.nix ]]; then
+        (set -x; cp dev.nix "$invokedir"/dev.nix)
+    elif [[ dev.nix -nt "$invokedir"/dev.nix ]]; then
+        (set -x; cp -f dev.nix "$invokedir"/dev.nix)
+    elif diff -qs dev.nix "$invokedir"/default.nix; then
+        :
+    else
+        ! echo >&2 "dunnolol"
+    fi