diff --git a/install b/install
index 940cf3f5d..425995648 100755
--- a/install
+++ b/install
@@ -208,11 +208,27 @@ backup_if_exists() {
   fi
 }
 
+# diff_if_exists <src> <dest>
+diff_if_exists() {
+  src="$1"
+  dest="$2"
+  shift 2
+
+  if [[ -f "$dest" ]]; then
+    src_explicit_filename="$(dirname "$src")/$(basename "$dest").new"
+    cp "$src" "$src_explicit_filename"
+    # print what the existing file $dest would become if $src is copied
+    diff -u --color=always "$dest" "$src_explicit_filename" || :
+    rm -f "$src_explicit_filename"
+  fi
+}
+
 # cp_if_different <src> <dest>
 cp_if_different() {
   if [[ -f "$2" ]] && diff -q "$1" "$2" > /dev/null; then
     return
   fi
+  diff_if_exists "$1" "$2"
   backup_if_exists "$2"
   cp "$1" "$2"
 }