diff --git a/contrib/check-code.py b/contrib/check-code.py
--- a/contrib/check-code.py
+++ b/contrib/check-code.py
@@ -116,6 +116,7 @@ testpats = [
     (r'printf.*[^\\]\\x', "don't use printf \\x, use Python"),
     (r'\$\(.*\)', "don't use $(expr), use `expr`"),
     (r'rm -rf \*', "don't use naked rm -rf, target a directory"),
+    (r'\[[^\]]+==', '[ foo == bar ] is a bashism, use [ foo = bar ] instead'),
     (r'(^|\|\s*)grep (-\w\s+)*[^|]*[(|]\w',
      "use egrep for extended grep syntax"),
     (r'/bin/', "don't use explicit paths for tools"),
diff --git a/tests/test-contrib-check-code.t b/tests/test-contrib-check-code.t
--- a/tests/test-contrib-check-code.t
+++ b/tests/test-contrib-check-code.t
@@ -152,6 +152,15 @@
    >   $ function onwarn {}
    warning: don't use 'function', use old style
   [1]
+  $ cat > error.t <<EOF
+  >   $ [ foo == bar ]
+  > EOF
+  $ "$check_code" error.t
+  error.t:1:
+   >   $ [ foo == bar ]
+   [ foo == bar ] is a bashism, use [ foo = bar ] instead
+  [1]
+  $ rm error.t
   $ cat > raise-format.py <<EOF
   > raise SomeException, message
   > # this next line is okay