git-sync

#!/usr/bin/env bash

# If you save this as `git-sync` in your PATH, then you can run `git sync`

# shellcheck disable=SC2034
USAGE="Synchronize the local and origin \`master\` branch with \`upstream\` remote's"

# shellcheck source=/dev/null
source "$(git --exec-path)/git-sh-setup"

# TODO: I ought to detect the default branch instead of hardcoding master
BRANCH="master"

if [ $# -ge 2 ]; then
  echo "Too many arguments. Specify one and only one upstream branch to sync from."
  exit 1
fi

if [ $# -eq 1 ]; then
  BRANCH=$1
fi

ORIGIN="origin"
UPSTREAM="upstream"

ORIGIN_REF="origin/$BRANCH"
UPSTREAM_REF="$UPSTREAM/$BRANCH"

echo "Starting to sync from '$UPSTREAM_REF' to '$ORIGIN_REF'."

require_work_tree
require_clean_work_tree

if ! git ls-remote --exit-code --heads "$UPSTREAM" "$BRANCH" >/dev/null; then
  die "error: cannot sync without '$UPSTREAM_REF'."
fi

if ! git ls-remote --exit-code --heads "$ORIGIN" "$BRANCH" >/dev/null; then
  die "error: cannot sync without '$ORIGIN_REF'."
fi

if ! git show-ref --verify --quiet "refs/heads/$BRANCH"; then
  die "error: cannot sync without '$BRANCH'."
fi

if ! git show-ref --verify --quiet "refs/remotes/$ORIGIN_REF"; then
  die "error: cannot sync without '$ORIGIN_REF'."
fi

if ! git show-ref --verify --quiet "refs/remotes/$UPSTREAM_REF"; then
  die "error: cannot sync without '$UPSTREAM_REF'."
fi

git switch --quiet "$BRANCH" &&
  git fetch "$UPSTREAM" &&
  git rebase "$UPSTREAM_REF" &&
  git push "$ORIGIN" "$BRANCH" &&
  git switch --quiet -

echo "Finished syncing from '$UPSTREAM_REF' to '$ORIGIN_REF'."