Add lint for [ instead of [[ in shells scripts

python/tidy/servo_tidy/tidy.py

@@ -347,6 +347,9 @@ def check_shell(file_name, lines):
         if "`" in stripped:
             yield (idx + 1, "script should not use backticks for command substitution")
 
+        if " [ " in stripped or stripped.startswith("[ "):
+            yield (idx + 1, "script should use `[[` instead of `[` for conditional testing")
+
         for dollar in re.finditer('\$', stripped):
             next_idx = dollar.end()
             if next_idx < len(stripped):

python/tidy/servo_tidy_tests/shell_tidy.sh

@@ -8,3 +8,7 @@ set -o nounset
 echo "hello world"
 some_var=`echo "command substitution"`
 another_var="$some_var"
+if [ -z "${some_var}" ]; then
+  echo "should have used [["
+fi
+[ -z "${another_var}" ]

python/tidy/servo_tidy_tests/test_tidy.py

@@ -54,6 +54,8 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('script is missing options "set -o errexit", "set -o pipefail"', errors.next()[2])
         self.assertEqual('script should not use backticks for command substitution', errors.next()[2])
         self.assertEqual('variable substitutions should use the full \"${VAR}\" form', errors.next()[2])
+        self.assertEqual('script should use `[[` instead of `[` for conditional testing', errors.next()[2])
+        self.assertEqual('script should use `[[` instead of `[` for conditional testing', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
     def test_rust(self):