Behind the scene: git clone

The classic usage of git is to work with a remote repository.

Let’s consider Adele and Blair, 2 developers working on the project paris, which is also the name of the repository they are both working on.

Blair starts by cloning the repo.

$ git clone ssh://remote.mygit.com/git-server/repos/paris.git

Cloning into 'paris'...
remote: Counting objects: 3, done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Receiving objects:  33% (1/3)
Receiving objects:  66% (2/3)
Receiving objects: 100% (3/3)
Receiving objects: 100% (3/3), done.

Adele is now the proud of owner of a brand new commit on her machine:

Adele starts to work on that repo and eventually has a commit ready to be pushed.

$ git push

Enumerating objects: 4, done.
Counting objects:  25% (1/4)
Counting objects:  50% (2/4)
Counting objects:  75% (3/4)
Counting objects: 100% (4/4)
Counting objects: 100% (4/4), done.
Delta compression using up to 4 threads
Compressing objects:  50% (1/2)
Compressing objects: 100% (2/2)
Compressing objects: 100% (2/2), done.
Writing objects:  33% (1/3)
Writing objects:  66% (2/3)
Writing objects: 100% (3/3)
Writing objects: 100% (3/3), 321 bytes | 321.00 KiB/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To ssh://remote.mygit.com/git-server/repos/paris.git
   848af7d..33b6807  master -> master

Adele’s commit tree is now:

Adele continues her development and has a new commit ready:

But this time, when she tries to push to her origin server:

$ git push

To ssh://remote.mygit.com/git-server/repos/paris.git
 ! [rejected]        master -> master (fetch first)
error: failed to push some refs to 'ssh://remote.mygit.com/git-server/repos/paris.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

The dreaded git error message.

The git status command doesn’t give a lot of information… besides the fact that Adele must push her commit:

$ git status

On branch master
Your branch is ahead of 'origin/master' by 1 commit.
  (use "git push" to publish your local commits)

nothing to commit, working tree clean

In that specific scenario, the reason for the rejection is easy to guess: her friend Blair has already pushed a new commit to master.

You can vizualize the problem by looking at the commit tree on the server.

On the paris remote repository, the commit tree is:

There is now a new commit (2251) that Adele doesn’t have locally, but Adele has a commit (7111) that the remote doesn’t have… Bummer.

One way to solve that situation is to follow git’s recommendations and perform a git pull:

$ git pull

remote: Counting objects: 3, done.
remote: Compressing objects:  50% (1/2)   remote: Compressing objects: 100% (2/2)   remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects:  33% (1/3)
Unpacking objects:  66% (2/3)
Unpacking objects: 100% (3/3)
Unpacking objects: 100% (3/3), done.
From ssh://remote.mygit.com/git-server/repos/paris
   33b6807..2251a9d  master     -> origin/master
Merge made by the 'recursive' strategy.
 README.txt | 1 +
 1 file changed, 1 insertion(+)

The commit tree on Adele’s machine looks like this now:

The git pull command has created a merge commit (e449) to reconciliate the local commit (7111) and the remote commit (2251).

Forget the wars vi vs emacs or space vs tab : some developers have an allergic reaction when they encounter merge commits and they would get crazy if they see a commit tree like above.

We’ll see at another time how to avoid this merge commit and, by doing so, to prevent the end of the civilized world.

After this git pull, Adele has now 2 commits that the remote doesn’t have:

$ git status

On branch master
Your branch is ahead of 'origin/master' by 2 commits.
  (use "git push" to publish your local commits)

nothing to commit, working tree clean

Time for another push to sync up the remote and Adele’s machine:

$ git push

Enumerating objects: 7, done.
Counting objects: 100% (7/7), done.
Delta compression using up to 4 threads
Compressing objects: 100% (4/4), done.
Writing objects: 100% (5/5), 680 bytes | 680.00 KiB/s, done.
Total 5 (delta 0), reused 0 (delta 0)
To ssh://remote.mygit.com/git-server/repos/paris.git
   2251a9d..e4490f9  master -> master

And now, the remote has the same commit tree as Adele’s machine: