git-merge-nested-repos.sh

#!/usr/bin/env bash

# https://help.github.com/articles/about-git-subtree-merges/

set -euo pipefail

red='\e[1;31m'
green='\e[1;32m'
yellow='\e[1;33m'
reset='\e[0m'

source="${1?Need source directory (first arg)}"
target="${2?Need target directory (second arg)}"
git=(git -C "$target")

rm -rf -- "$target"
mkdir -p -- "$target"
"${git[@]}" init
printf '# Override core.autocrlf with a sane defaults.\n* text=auto\n' > "$target/.gitattributes"
cp "$source/NuGet.config" "$target"
"${git[@]}" add .
"${git[@]}" commit --message 'Initial commit'

while IFS= read -d $'\0' -r dotgit; do
  dotgit_absolute="$(readlink -f "$dotgit")"
  repo="${dotgit%.git}"

  branch="$(git -C "$repo" symbolic-ref --short HEAD)"

  printf '%b%s%b is checked out at %b%s%b\n' \
         "$red" \
         "$repo" \
         "$reset" \
         "$green" \
         "$branch" \
         "$reset"

  target_dir="${repo#$source}"
  target_dir="${target_dir##/}"

  printf 'Target directory: %b%s%b\n' \
         "$yellow" \
         "$target_dir" \
         "$reset"

  "${git[@]}" remote \
              add \
              --fetch \
              temp \
              "$dotgit_absolute"

  "${git[@]}" merge \
              --strategy ours \
              --no-commit \
              --allow-unrelated-histories \
              "temp/$branch"

  "${git[@]}" read-tree \
              --prefix "$target_dir" \
              -u \
              "temp/$branch"

  "${git[@]}" commit \
              --message "Subtree merged in $target_dir"

  "${git[@]}" remote rm temp

done < <(find "$source" -mindepth 1 -type d -name .git -print0 | sort --zero-terminated)