diff --git a/.travis.yml b/.travis.yml
index 30f5fa2..7abf0e3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,6 +2,12 @@ os:
   - linux
   - osx
 
+env:
+  - TEST_SHELLS="zsh bash"
+  - TEST_SHELLS=bash
+  - TEST_SHELLS=zsh
+  - HI=mom
+
 install:
   - ./test/support/travisci_deps.sh
 
diff --git a/run_tests.sh b/run_tests.sh
index fff4438..877f9ed 100755
--- a/run_tests.sh
+++ b/run_tests.sh
@@ -3,8 +3,15 @@
 
 failed=false
 
+# allow list of shells to run tests in to be overriden by environment variable
+# if empty or null, use defaults
+if [ -z "$TEST_SHELLS" ]; then
+  TEST_SHELLS="bash zsh"
+fi
+
+echo "== Will run all tests with following shells: ${TEST_SHELLS}"
 for test in $(find test/lib -name *_test.sh); do
-  for shell in bash zsh; do
+  for shell in $TEST_SHELLS; do
     echo "== Running tests with [$shell]: $test"
     $shell $test || failed=true
   done
@@ -16,4 +23,4 @@ if [ "$failed" = "true" ]; then
 else
   echo "All tests passed!"
   return 0;
-fi
\ No newline at end of file
+fi