How to combine two unrelated git repositories, preserving history

Question

I don\'t know git very well. :-/

Background

I have two unrelated git-based document repositories that I would like to combine into a single repository. I

Answer 1

You will have to write a script that will do it.

How to do it

1. get a list of all your commits timestamps per branch

   # print out the commits time stamp & sha-1 of each commit
   # do it for all your branches
   git log --oneline --format="%at %H"
   

2. Combine the 2 lists together and sort them by the time stamp using any sort tool (sublime, unix sort etc)

3. Checkout new branch starting the first commit you have in your files

   git checkout <first commit id>
   

4. Create new branch starting from this commit

   git checkout -b <new_branch_name>
   

5. Loop over all the other commits and use cherry-pick to bring them into your branch (script)

   git cherry-pick <next commit id>
   

Answer 2

While @codeWizard's reply was helpful, that approach didn't retain the timestamps the way I wanted. It did lead me down a rabbit hole that helped me find a solution though...

1. Create a new, blank repository

   git init
   

2. Add and fetch the old repositories as remotes

   git remote add -f oldRepoA ../oldRepoA
   git remote add -f oldRepoB ../oldRepoB
   

3. Export the combined commit history by timestamp and hash, pipe the output to sort, discard the timestamps via cut, and then pipe the list of chronologically sorted hashes to xargs, which runs a shell script to export a patch for each individual hash and then immediately apply the patch to the new repo.

   git log --all --oneline --format="%at %H" | sort | cut -c12- | 
       xargs -I {} sh -c 
           'git format-patch -1 {} --stdout | 
            git am --committer-date-is-author-date'
   

The --committer-date-is-author-date is key to keeping the original timestamps. There might be a better way of doing this, but this works well enough for my use case!