Auto merge of #12736 - aneeshusa:prohibit-backticks-in-shell-scripts, r=Wafflespeanut

Add more shell script lints

<!-- Please describe your changes on the following line: -->

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.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12736)
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ports/geckolib/gecko_bindings/tools/regen.sh

@@ -8,28 +8,28 @@ set -o errexit
 set -o nounset
 set -o pipefail
 
-if [ $# -eq 0 ]; then
-  echo "Usage: $0 /path/to/gecko/objdir [other-regen.py-flags]"
+if [[ ${#} -eq 0 ]]; then
+  echo "Usage: ${0} /path/to/gecko/objdir [other-regen.py-flags]"
   exit 1
 fi
 
 # Check for rust-bindgen
-if [ ! -d rust-bindgen ]; then
+if [[ ! -d rust-bindgen ]]; then
   echo "rust-bindgen not found. Run setup_bindgen.sh first."
   exit 1
 fi
 
 # Check for /usr/include
-if [ ! -d /usr/include ]; then
+if [[ ! -d /usr/include ]]; then
   echo "/usr/include doesn't exist." \
        "Mac users may need to run xcode-select --install."
   exit 1
 fi
 
-if [ "$(uname)" == "Linux" ]; then
-  LIBCLANG_PATH=/usr/lib/llvm-3.8/lib;
+if [[ "$(uname)" == "Linux" ]]; then
+  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 "$@"
+./regen.py --target all "${@}"

ports/geckolib/gecko_bindings/tools/setup_bindgen.sh

@@ -9,33 +9,33 @@ set -o nounset
 set -o pipefail
 
 # Run in the tools directory.
-cd "$(dirname $0)"
+cd "$(dirname ${0})"
 
 # Setup and build bindgen.
-if [ "$(uname)" == "Linux" ]; then
-  export LIBCLANG_PATH=/usr/lib/llvm-3.8/lib;
+if [[ "$(uname)" == "Linux" ]]; then
+  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.
-if [ ! -x "$(command -v clang-3.8)" ]; then
+if [[ ! -x "$(command -v clang-3.8)" ]]; then
     echo "llmv38 must be installed." \
          "Mac users should |brew install llvm38|, Linux varies by distro."
     exit 1
 fi
 
-export LD_LIBRARY_PATH=$LIBCLANG_PATH
-export DYLD_LIBRARY_PATH=$LIBCLANG_PATH
+export LD_LIBRARY_PATH="${LIBCLANG_PATH}"
+export DYLD_LIBRARY_PATH="${LIBCLANG_PATH}"
 
 # Check for multirust
-if [ ! -x "$(command -v multirust)" ]; then
+if [[ ! -x "$(command -v multirust)" ]]; then
     echo "multirust must be installed."
     exit 1
 fi
 
 # Don't try to clone twice.
-if [ ! -d rust-bindgen ]; then
+if [[ ! -d rust-bindgen ]]; then
   git clone https://github.com/servo/rust-bindgen.git
 fi