diff --git a/tools/env.sh b/tools/env.sh
index 55736a21..f2de6462 100755
--- a/tools/env.sh
+++ b/tools/env.sh
@@ -10,7 +10,9 @@ export AR=x86_64-luna-ar
 export ASM=nasm
 export STRIP=x86_64-luna-strip
 
-unset -f filter-lines
+if ! command -v filter-lines >/dev/null
+then unset filter-lines
+fi
 filter-lines()
 {
     sed $'s|^|\x1b[32m('"$1/$2"$')\x1b[39m |'