Add lint for backticks in shell scripts

The "$(some_command arg1 arg2)" form is preferred to the
`some_command arg1 arg2` form because it nests unambiguously.
Add a lint for this to tidy.

ports/geckolib/gecko_bindings/tools/regen.sh

@@ -27,9 +27,9 @@ if [ ! -d /usr/include ]; then
 fi
 
 if [ "$(uname)" == "Linux" ]; then
-  LIBCLANG_PATH=/usr/lib/llvm-3.8/lib;
+  LIBCLANG_PATH=/usr/lib/llvm-3.8/lib
 else
-  LIBCLANG_PATH=`brew --prefix llvm38`/lib/llvm-3.8/lib;
+  LIBCLANG_PATH="$(brew --prefix llvm38)/lib/llvm-3.8/lib"
 fi
 
 ./regen.py --target all "$@"

ports/geckolib/gecko_bindings/tools/setup_bindgen.sh

@@ -13,9 +13,9 @@ cd "$(dirname $0)"
 
 # Setup and build bindgen.
 if [ "$(uname)" == "Linux" ]; then
-  export LIBCLANG_PATH=/usr/lib/llvm-3.8/lib;
+  export LIBCLANG_PATH=/usr/lib/llvm-3.8/lib
 else
-  export LIBCLANG_PATH=`brew --prefix llvm38`/lib/llvm-3.8/lib;
+  export LIBCLANG_PATH="$(brew --prefix llvm38)/lib/llvm-3.8/lib"
 fi
 
 # Make sure we have llvm38.

python/tidy/servo_tidy/tidy.py

@@ -318,19 +318,23 @@ def check_shell(file_name, lines):
     shebang = "#!/usr/bin/env bash"
     required_options = {"set -o errexit", "set -o nounset", "set -o pipefail"}
 
+    did_shebang_check = False
+
     if len(lines) == 0:
         yield (0, 'script is an empty file')
-    else:
+        return
+
     if lines[0].rstrip() != shebang:
         yield (1, 'script does not have shebang "{}"'.format(shebang))
 
     for idx in range(1, len(lines)):
         stripped = lines[idx].rstrip()
-
         # Comments or blank lines are ignored. (Trailing whitespace is caught with a separate linter.)
         if lines[idx].startswith("#") or stripped == "":
             continue
-            elif stripped in required_options:
+
+        if not did_shebang_check:
+            if stripped in required_options:
                 required_options.remove(stripped)
             else:
                 # The first non-comment, non-whitespace, non-option line is the first "real" line of the script.
@@ -338,7 +342,10 @@ def check_shell(file_name, lines):
                 if len(required_options) != 0:
                     formatted = ['"{}"'.format(opt) for opt in required_options]
                     yield (idx + 1, "script is missing options {}".format(", ".join(formatted)))
-                    break
+                did_shebang_check = True
+
+        if "`" in stripped:
+            yield (idx + 1, "script should not use backticks for command substitution")
 
 
 def check_rust(file_name, lines):

python/tidy/servo_tidy_tests/shell_tidy.sh

@@ -4,4 +4,6 @@
 
 set -o nounset
 
+# Talking about some `concept in backticks` # shouldn't trigger
 echo "hello world"
+some_var=`echo "command substitution"`

python/tidy/servo_tidy_tests/test_tidy.py

@@ -52,6 +52,7 @@ class CheckTidiness(unittest.TestCase):
         errors = tidy.collect_errors_for_files(iterFile('shell_tidy.sh'), [], [tidy.check_shell], print_text=False)
         self.assertEqual('script does not have shebang "#!/usr/bin/env bash"', errors.next()[2])
         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.assertNoMoreErrors(errors)
 
     def test_rust(self):