Filter a command like rubocop to look for issues only on modified lines.

diff_filter.sh

#!/bin/bash

##
# Filter a command like rubocop to look for issues only on modified lines.
#
# Inspired by http://takemikami.com/2018/01/30/RubocopPullRequestCI.html
#

BASE_REMOTE=origin
BASE_BRANCH=master

diff_list=()
commit_list=$(git --no-pager log --no-merges $BASE_REMOTE/$BASE_BRANCH...HEAD | grep -e '^commit' | sed -e "s/^commit \(.\{8\}\).*/\1/")
for f in $(git --no-pager diff $BASE_REMOTE/$BASE_BRANCH...HEAD --name-only --diff-filter=AM); do
  for c in $commit_list; do
    diffs=$(git --no-pager blame --show-name -s $f | grep $c | sed -e "s/^[^ ]* *\([^ ]*\) *\([0-9]*\)*).*$/\1:\2/")
    for ln in $diffs; do
      diff_list+=( $ln )
    done
  done
done

err_count=0
while read -r ln; do
  for m in ${diff_list[@]}; do
    if [[ ${ln} =~ ^$m ]]; then
      echo $ln
      err_count=$((err_count+1))
      break
    fi
  done
done < /dev/stdin

if [ $err_count -ne 0 ]; then
  echo -e "\033[0;31mERROR FOUND, check above messages.\033[0;39m"
  exit 1
fi

echo "OK"