git-split: Script to duplicate a file while preserving git line history

git-split

#!/bin/sh

# Script to duplicate a file while preserving git line history
#
#   This could be useful if you want two copies of a file,
#   say, one where you are doing a bunch of disruptive work,
#   and another that remains largely unchanged. The project continues to use the old,
#   stable version, but there’s a feature flag to switch to the new one.
#   Eventually, you’ll make the new one the default version.
#
# NOTE: Based on https://devblogs.microsoft.com/oldnewthing/20190919-00/?p=102904

set -euo pipefail

usage() {
    echo "Usage: git-split <originalFile> <copyName> [customCommitMessage]"
    echo "Duplicate a file while preserving git line history"
    echo "Optionally, provide custom commit message as explanation."
    exit 1
}

if [[ "$#" -lt 2 ]] || [[ "$#" -gt 3 ]]; then
    usage
fi

originalFile=$1
copyName=$2

defaultCommitMsg="to preserve original file history (see: script/git-split)."
explanationCommitMsg=${3:-$defaultCommitMsg}

branchName=copy-with-history-$1

# Method: Create a branch where the desired new file appears
#   to have been created via a rename of the original file.
#   And then restore the original file.

commit_message() {
  if [ "$1" ]; then
    echo "${1} $2"
  else
    echo "$2"
  fi
}

# Extract XX-##### tag for commit messages
BRANCH_TAG=$(git rev-parse --abbrev-ref HEAD | grep -Eo '[A-Z]+-[0-9]+' || true)

# Create and switch to a branch
git checkout -b $branchName

# Make the duplicate of the original file
git mv $originalFile $copyName
summaryCommitMsg=$(commit_message "$BRANCH_TAG" "Duplicate $originalFile to $copyName")
git commit -m "$summaryCommitMsg" -m "$explanationCommitMsg"

# Bring back the original file
git checkout HEAD~ $originalFile
summaryCommitMsg=$(commit_message "$BRANCH_TAG" "Restore duplicated $originalFile")
git commit -m "$summaryCommitMsg" -m "$explanationCommitMsg"

# Switch back to the previous branch
#  `-` (dash) is a special symbol that Git interprets
#  as a reference to the previous branch you were working on.
git checkout -

# Merge created branch back into source
summaryCommitMsg=$(commit_message "$BRANCH_TAG" "Merge branch $branchName")
git merge --no-ff $branchName \
          -m "$summaryCommitMsg" -m "$explanationCommitMsg"

# Delete the branch
git branch -D $branchName