Subtree is a way to embed other git repository histories into your own by merging in the subtree history into yours. It also enables you to push patches to the subtree repository. This allows you to grab source change directly via merging histories, as well as pushing them back.

This feature is useful if you decide that for instance a specific folder should be a repository on its own because other repositories/projects would need it.

1. Run ./setup.sh

NOTE: If running setup.sh on windows, you can run into problems by sourcing the setup script. Instead, run ./setup.sh, and the folders would be created correctly.

After running . setup.sh or . ./setup.sh, you'll have two repositories that we will use for this exercise.

INFO: You now have the following under the excercise directory:

• A product repository that contains the "product" history.
• A component repository that contains the "component" history.

Exercise

• Go to the component repository and run git log and notice the history.
• Go to the product repository and run git log and notice the history.
• Add the component repo as a remote reference: git remote add component ../component/.git ( optional, but good practice )
• Check the remotes using git remote -v and it should look like this:

$ git remote -v
component       ../component/.git (fetch)
component       ../component/.git (push)

• Add the component history to the this repository (product) under the directory component: git subtree add --prefix component component master . You should see something like this:

git fetch component master
warning: no common commits
remote: Enumerating objects: 3, done.
remote: Counting objects: 100% (3/3), done.
remote: Total 3 (delta 0), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (3/3), 224 bytes | 18.00 KiB/s, done.
From ../component/
 * branch            master     -> FETCH_HEAD
 * [new branch]      master     -> component/master
Added dir 'component'

• Please explain what happend and what the result is. Consider running this command in both product and component repositories. git log --graph --decorate --oneline --all and explain the common history and the newest commit in product.

We have added the component subtree to the component directory also named a prefixin subtree terms. We now want add changes to the component and integrate them into the product

• Go to the component repository
• Make changes and commit them
• Go to the product repository and pull in the changes with this command: git subtree pull --prefix component component master

remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Total 3 (delta 0), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (3/3), 276 bytes | 5.00 KiB/s, done.
From ../component/
 * branch            master     -> FETCH_HEAD
   99d28dd..a930ba9  master     -> component/master
Merge made by the 'recursive' strategy.
 component/component.h | 1 +
 1 file changed, 1 insertion(+)

• Please explain what happend and what the result is. Consider running this command in both product and component repositories. git log --graph --decorate --oneline --all and explain the common history and the newest commits in product.

We have updated component directory from the component repository. We now want to modify the component directory and deliver the changes to the component repository. NOTE: We push to a new branch.

• Go to the product repository.
• Go to the component directory and make changes and commit them.
• We want to deliver the changes just made to the component repository using this command: git subtree push --prefix component component patch.

git push using:  component patch
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 4 threads
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 357 bytes | 357.00 KiB/s, done.
Total 3 (delta 0), reused 0 (delta 0), pack-reused 0
To ../component/.git
 * [new branch]      e6fb29fe9a6213a9806e86195c609ba0bfd94249 -> patch

• Please explain what happend and what the result is. Consider running this command in both product and component repositories. git log --graph --decorate --oneline --all and explain the common history and the newest commits in product.
• Why do we push to a new branch?
• How would you do it if you not owner nor collaborator on the component repository on for example GitHub?

• git log --graph --decorate --oneline --all
• git subtree add --prefix component component master
• git remote add component ../component/.git
• git subtree push --prefix <prefix/directory> <repo> <branch>

Run all the above commands to see the commands and their results.

replay=true debug=true ./setup.sh

We found some resources that may assist you in understanding this feature of Git better.

• https://github.com/git/git/blob/master/contrib/subtree/git-subtree.txt
• https://blog.developer.atlassian.com/the-power-of-git-subtree/