Git Subtree only one file or directory

Question

I use Git Subtree like below:

git subtree add --prefix=directory_destination_path --squash [e-mail]:kicaj/projectname.git master

But in path: directory_destination_path copy all repo from projectname.git

How to copy to directory_destination_path only subdirectory or only some file from projectname.git ?

EDIT:
One more question:
How to update (automatic) files changes in both repositories were still the same? It is possible?

Answer 1

Suppose you wanted to add the images/ prefix of the octocat git repo at the ref master.

Suppose we want to use the remote hosted at https://github.com/octocat/octocat.github.io.git (Note: GitHub returns error: Server does not allow request for unadvertised object in the following fetch command is you specify a sha1 not associated with named ref)

Starting on the branch you wish to add the subtree to, let's first fetch the desired commit history and checkout our new branch

git fetch https://github.com/octocat/octocat.github.io.git master:tmp_octocat_master
git checkout tmp_octocat_master

Next, let's create a new history where only the files under our desired prefix (images/) exist.

git subtree split --prefix=images -b subtree_split_branch

Finally, let's add this subtree to our desired branch (presumably the last branch you were on, git checkout -)

git checkout -
git subtree add --prefix=public/images subtree_split_branch

Now you should have all the desired files on your current branch with a full git history.

Alt Squashed history

At time you want the commits in our subtree to be squashed into a single commit. This to a certain extent defeats the purposes of adding a subtree but it has it's place. The following is a variation of what is described above to limit the history pulled into your repo

Starting on the branch you wish to add the subtree to:

git fetch --depth=1 https://github.com/octocat/octocat.github.io.git master:tmp_octocat_master
git checkout tmp_octocat_master

Note: we are specifying --depth=1 in the above since we are using the --squash is the following git subtree split command.

git subtree split --squash --prefix=images -b subtree_split_branch
git checkout -
git subtree add --prefix=public/images subtree_split_branch

Answer 2

If I understand, you seem to want to only merge in a certain directory of a different repository, and you want it to be a subtree in your repository. I am going to call the directory of interest in the project.git path_of_interest_in_project and call the destination in your repo directory_desination_path.

Try adding the remote project.git as a remote, then checking out one of its branches locally. Then use git-subtree split to split out just the directory of project.git you are interested in. After that merge it into your repo using subtree merge.

git remote add project [e-mail]:kicaj/projectname.git
git branch project_master project/master

The branch project_master should now store the entire history of your project.git repo.

Then you'll need to use the git-subtrees-split process.

git checkout -f project_master
git subtree split --squash --prefix=path_of_interest_in_project -b temp_branch

There should now be a branch called temp_branch containing just the directory you are interested in. Now you can perform a git-subtree-merge to bring it all into your repo.

git checkout -f master
git subtree merge --allow-unrelated-histories --prefix=directory_destination_path temp_branch

This should merge in the temp_branch into your master branch.