Import an Existing Git Repository into Another

Before: Folder Structure (Two Separate Repositories)

XXX
 |- .git
 |- (project files)
YYY
 |- .git
 |- (project files)

After: Folder Structure (XXX Imported into YYY as a Subdirectory)

YYY
 |- .git  <-- This now contains the full change history from XXX
 |- ZZZ   <-- This was originally XXX, now a subdirectory of YYY
    |- (project files)
 |- (project files from YYY)

Steps to Import

Run the following commands inside YYY to import XXX as a subtree:

git remote add XXX_remote <path-or-url-to-XXX-repo> # Add XXX as a remote
git fetch XXX_remote # Fetch history from XXX
git merge -s ours --no-commit XXX_remote/master # Merge without modifying YYY's files
git read-tree --prefix=ZZZ/ -u XXX_remote/master # Move XXX into subdirectory ZZZ
git commit -m "Imported XXX as a subtree." # Commit the changes
git remote rm XXX_remote # Remove temporary remote
git push # Push changes to the repository

Explanation

• Preserves full commit history of XXX.
• Places XXX’s files inside ZZZ/ within YYY.
• Does not modify YYY’s existing files during the merge.

Source: Stack Overflow