diff --git a/lib/files.ncl b/lib/files.ncl
index 6a037fa1..1888a9b1 100644
--- a/lib/files.ncl
+++ b/lib/files.ncl
@@ -83,14 +83,16 @@ let nix = import "./nix-interop/nix.ncl" in
               file_descr.materialisation_method
           in
           nix-s%"
-            rm -f %{target}
-            echo "Regenerating %{target}"
-            target_dir=$(dirname %{target})
-            test "${target_dir}" != "." && mkdir -p "${target_dir}"
-            # XXX: If `source.file` is set explicitely to a relative path
-            # and `materialisation_method` is `'Symlink`, this will link to the
-            # original file, not one in the store. Not sure that's what we want.
-            %{copy_command} %{file_descr.file} %{target}
+            if [[ ! -f "%{target}" ]] || [[ $(cat "%{target}") != $(cat "%{file_descr.file}") ]]; then
+              echo "Regenerating %{target}"
+              rm -f %{target}
+              target_dir=$(dirname "%{target}")
+              test "${target_dir}" != "." && mkdir -p "${target_dir}"
+              # XXX: If `source.file` is set explicitely to a relative path
+              # and `materialisation_method` is `'Symlink`, this will link to the
+              # original file, not one in the store. Not sure that's what we want.
+              %{copy_command} "%{file_descr.file}" "%{target}"
+            fi
           "%
         in
       {