diff --git a/test/lib/internals-test.sh b/test/lib/internals-test.sh
index a8464c0..3b0ccbd 100755
--- a/test/lib/internals-test.sh
+++ b/test/lib/internals-test.sh
@@ -1,13 +1,7 @@
 #! /usr/bin/env bash
-source "../assert.sh"
-
+source "../test-helper.sh"
 source "../../src/lib/internals.sh"
 
-# silent shortcut to "cd -"
-cd-back() {
-  cd - 1>/dev/null
-}
-
 #
 # locate-dotfile()
 #
diff --git a/test/test-helper.sh b/test/test-helper.sh
new file mode 100644
index 0000000..d3b153c
--- /dev/null
+++ b/test/test-helper.sh
@@ -0,0 +1,41 @@
+# Set testroot variable.
+testroot="$(dirname "$BASH_SOURCE")"
+
+# Include assert.sh testing library.
+source "$testroot/assert.sh"
+
+#
+# Additional test helpers.
+#
+
+# Silent shortcut to "cd -".
+cd-back() {
+  cd - 1>/dev/null
+}
+
+# Stub commands printing it's name and arguments to STDOUT or STDERR.
+stub() {
+  local cmd="$1"
+  if [ "$2" == "STDERR" ]; then local redirect=" 1>&2"; fi
+
+  if [[ "$(type "$cmd" | head -1)" == *"is a function" ]]; then
+    local source="$(type "$cmd" | tail -n +2)"
+    source="${source/$cmd/original_${cmd}}"
+    eval "$source"
+  fi
+  eval "$(echo -e "${1}() {\n  echo \"$1 stub: \$@\"$redirect\n}")"
+}
+
+# Restore the original command/function that was stubbed with stub.
+restore() {
+  local cmd="$1"
+  unset "$cmd"
+  if type "original_${cmd}" &>/dev/null; then
+    if [[ "$(type "original_${cmd}" | head -1)" == *"is a function" ]]; then
+      local source="$(type "original_$cmd" | tail -n +2)"
+      source="${source/original_${cmd}/$cmd}"
+      eval "$source"
+      unset "original_${cmd}"
+    fi
+  fi
+}