diff --git a/.gitignore b/.gitignore
index 325d16a..e10b0a5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,8 +1,3 @@
 # Local configuration
 local/
 *.local.nix
-
-# Package updates
-/packages/*.cmp
-/*.log
-/update-git-commits.txt
diff --git a/default.nix b/default.nix
deleted file mode 100644
index 851ad1b..0000000
--- a/default.nix
+++ /dev/null
@@ -1,2 +0,0 @@
-# Used by ./tasks/update
-{}: import <nixpkgs> { overlays = [ (import ./common/packages.nix) ]; }
diff --git a/tasks/update b/tasks/update
index 1200937..8a9fffe 100755
--- a/tasks/update
+++ b/tasks/update
@@ -1,12 +1,24 @@
 #!/usr/bin/env bash
 # See https://discourse.nixos.org/t/25274
+set -Eeu
 
 root="$(readlink --canonicalize -- "$(dirname -- "$0")/..")"
-nixpkgs="$(nix-instantiate --eval --expr '<nixpkgs>')"
 
-exec nix-shell "$nixpkgs/maintainers/scripts/update.nix" \
+# Mock nixpkgs
+trap 'rm -f "$root/default.nix"' EXIT; cat > "$root/default.nix" << NIX
+{}: import <nixpkgs> { overlays = [ (import ./common/packages.nix) ]; }
+NIX
+
+# Run update scripts
+nixpkgs="$(nix-instantiate --eval --expr '<nixpkgs>')"
+nix-shell "$nixpkgs/maintainers/scripts/update.nix" \
   --arg include-overlays "(import $root { }).overlays" \
   --arg predicate "(
     let prefix = \"$root/packages/\"; prefixLen = builtins.stringLength prefix;
     in (_: p: (builtins.substring 0 prefixLen p.meta.position) == prefix)
   )"
+
+# Clean up
+if [[ -f "$root/update-git-commits.txt" ]]; then
+  cat "$root/update-git-commits.txt" && rm "$root/update-git-commits.txt"
+fi