Check if source branch has been already merged into destination branch

git-is-merged

#!/usr/bin/env bash

#
# Check if source branch has been already merged into destination branch
# https://stackoverflow.com/questions/226976/how-can-i-know-if-a-branch-has-been-already-merged-into-master
#

git_is_merged () {
  merge_destination_branch=$1
  merge_source_branch=$2

  merge_base=$(git merge-base $merge_destination_branch $merge_source_branch)
  merge_source_current_commit=$(git rev-parse $merge_source_branch)
  if [[ $merge_base = $merge_source_current_commit ]]
  then
    echo $merge_source_branch is merged into $merge_destination_branch
    return 0
  else
    echo $merge_source_branch is not merged into $merge_destination_branch
    return 1
  fi
}

git_is_merged $1 $2

Great to use in Github Actions