diff --git a/tool/ifchange b/tool/ifchange
new file mode 100755
index 0000000000..7a192643f9
--- /dev/null
+++ b/tool/ifchange
@@ -0,0 +1,16 @@
+#!/bin/sh
+# usage: ifchange target temporary
+
+target="$1"
+temp="$2"
+if [ "$temp" = - ]; then
+    temp="tmpdata$$.tmp~"
+    cat > "$temp" || exit $?
+    trap 'rm -f "$temp"' 0
+fi
+if cmp "$target" "$temp" >/dev/null 2>&1; then
+    echo "$target unchanged"
+else
+    echo "$target updated"
+    mv -f "$temp" "$target"
+fi