diff --git a/tests/functional/filter-source.sh b/tests/functional/filter-source.sh
index c5e10be93..b32f5b59d 100755
--- a/tests/functional/filter-source.sh
+++ b/tests/functional/filter-source.sh
@@ -2,26 +2,26 @@
 
 source common.sh
 
-rm -rf $TEST_ROOT/filterin
-mkdir $TEST_ROOT/filterin
-mkdir $TEST_ROOT/filterin/foo
-touch $TEST_ROOT/filterin/foo/bar
-touch $TEST_ROOT/filterin/xyzzy
-touch $TEST_ROOT/filterin/b
-touch $TEST_ROOT/filterin/bak
-touch $TEST_ROOT/filterin/bla.c.bak
-ln -s xyzzy $TEST_ROOT/filterin/link
+rm -rf "$TEST_ROOT/filterin"
+mkdir "$TEST_ROOT/filterin"
+mkdir "$TEST_ROOT/filterin/foo"
+touch "$TEST_ROOT/filterin/foo/bar"
+touch "$TEST_ROOT/filterin/xyzzy"
+touch "$TEST_ROOT/filterin/b"
+touch "$TEST_ROOT/filterin/bak"
+touch "$TEST_ROOT"/filterin/bla.c.bak
+ln -s xyzzy "$TEST_ROOT/filterin/link"
 
 checkFilter() {
-    test ! -e $1/foo/bar
-    test -e $1/xyzzy
-    test -e $1/bak
-    test ! -e $1/bla.c.bak
-    test ! -L $1/link
+    test ! -e "$1/foo/bar"
+    test -e "$1/xyzzy"
+    test -e "$1/bak"
+    test ! -e "$1"/bla.c.bak
+    test ! -L "$1/link"
 }
 
-nix-build ./filter-source.nix -o $TEST_ROOT/filterout1
-checkFilter $TEST_ROOT/filterout1
+nix-build ./filter-source.nix -o "$TEST_ROOT/filterout1"
+checkFilter "$TEST_ROOT/filterout1"
 
-nix-build ./path.nix -o $TEST_ROOT/filterout2
-checkFilter $TEST_ROOT/filterout2
+nix-build ./path.nix -o "$TEST_ROOT/filterout2"
+checkFilter "$TEST_ROOT/filterout2"