git pre-commit hook to check shell scripts with shellcheck

git-hook-shellcheck.sh

#!/usr/bin/env bash

set -efu -o pipefail

# returns staged files
function staged.files {
  if git rev-parse --verify HEAD &> /dev/null
  then
    against=HEAD
  else
    against=4b825dc642cb6eb9a060e54bf8d69288fbee4904
  fi

  git diff-index --cached --name-only $against |
    grep \
      -e '\.bashrc' \
      -e '\.zshrc' \
      -e '\.bash_profile' \
      -e '\.sh$' || true
}

declare -a paths=()

while read -r file
do
  paths[${#paths[@]}+1]=$file
done < <(staged.files)

if [[ ${#paths[@]} -eq 0 ]]
then
  echo -e '\e[1m\e[32m[info]\e[0m \e[1mshellcheck\e[0m no files in stage'
else
  if ! command -v shellcheck &> /dev/null
  then
    echo -e '\e[1m\e[33m[warn]\e[0m \e[1mshellcheck\e[0m is not installed' >&2
    exit 0
  fi

  stage=$(mktemp -d)
  trap 'rm -r $stage' EXIT INT TERM

  for path in "${paths[@]}"
  do
    file="$stage/$path"

    mkdir -p "$(dirname "$file")"
    git show ":$path" > "$file"
  done

  pushd "$stage" &> /dev/null
  shellcheck "${paths[@]}"
  popd &> /dev/null
fi