https://gist.github.com/d3d-z7n/1af74ae78e0b8c4828a6c5c760ff665a, but for the inferior shell

git-prune-local.sh

#!/bin/bash

set -euo pipefail

reset=$'\033[0;0m'
cyan=$'\033[0;36m'
bred=$'\033[1;31m'

# Remove no longer existing remote branches.
git fetch --prune

# Find all branches with a deleted remote branch.
IFS=$'\n'
branches=($(git branch -vv | grep ": gone]" | awk '{ print $1 }'))

if [[ $#branches == 0 ]]; then
  echo "No pruneable local branches found."
  exit 0
fi

echo -e "${cyan}The following local branches will be deleted:${reset}"
for branch in "${branches[@]}"; do
    echo -e "${bred}$branch${reset}"
done

echo
read -p "${cyan}Continue? (y/n)${reset} " confirmation

if [[ $confirmation == "y" ]]; then
  echo
  git branch -D "${branches[@]}"
fi