Only lint files added to repo after a certain date

eslint.sh

#  adapted from: https://gist.github.com/jhartikainen/36a955f3bfe06557e16e
#!/bin/bash

# Find all js/jsx files staged to commit
staged_files=$(git diff --staged --diff-filter=ACMTUXB --name-only | grep '\.jsx\?$')
files_to_lint=()

# Find all staged js/jsx files created since 2017-01-13
for file in ${staged_files}; do
  # if the file has previously been  committed, get author date of file:
  author_date="$(git log --format=%at --date=short "$file" | tail -1)"

  # format date:
  if [[ $author_date ]] ; then
    created_at=$(date -r $author_date '+%Y%m%d%H%M%S')
  else
    created_at=0
  fi

  # if the file was created after 2017-01-13 OR has been newly created in this branch:
  if [[ $created_at -gt "20170113000000" || $created_at = 0 ]] ; then
    files_to_lint+=("$file")
  fi
done;

# Prevent ESLint help message if no files matched
if [[ $files_to_lint = "" ]] ; then
  exit 0
fi

failed=0
for file in ${files_to_lint}; do
  echo "*** Linting: ${file} ***"

  git show :$file | eslint --stdin --stdin-filename $file

  if [[ $? != 0 ]] ; then
    failed=1
  fi
done;

if [[ $failed != 0 ]] ; then
  echo "ESLint check failed, commit denied. Fix those errors!"
  exit $failed
fi