diff --git a/format b/format
index 4f46c7b9..2eb16d9a 100755
--- a/format
+++ b/format
@@ -1,6 +1,11 @@
 #! /usr/bin/env nix-shell
 #! nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/6616de389ed55fba6eeba60377fc04732d5a207c.tar.gz -i bash -p git gnugrep gnused findutils nixfmt
 
+# Avoid being affected by system and user git config.
+export GIT_CONFIG_NOSYSTEM=1
+export HOME=
+export XDG_CONFIG_HOME=
+
 nixfmt_args=()
 files=()