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

@@ -318,27 +318,34 @@ 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:
-        if lines[0].rstrip() != shebang:
-            yield (1, 'script does not have shebang "{}"'.format(shebang))
+        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()
+    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
 
-            # 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.
                 # The shebang, options, etc. must come before this.
                 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 @@ def test_shell(self):
         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):