fetch-commit.sh

#!/bin/bash
set -eo pipefail

this_dir="$( unset CDPATH && cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
: ${GIT_DIR="$this_dir/shared-git-repo"}

remote="$1"
sha="$2"
local_path="$3"

if [[ -z "$remote" || -z "$sha" || -z "$local_path" ]]; then
  echo 'Usage: fetch-commit REMOTE SHA LOCAL_PATH' >&2
  echo >&2
  echo 'Fetch commit referenced by SHA from REMOTE and check it out in LOCAL_PATH' >&2
  exit 1
fi

export GIT_DIR
mkdir -p "$GIT_DIR"
git init --bare --quiet
if ! git cat-file -e "$sha^{commit}" 2>/dev/null; then
  echo "Fetching commit $sha from $remote" >&2
  git fetch --depth 1 --force "$remote" "$sha"
else
  echo "Commit $sha already downloaded" >&2
fi
mkdir -p "$local_path"
git --work-tree="$local_path" checkout --quiet  --force "$sha"