Merging

Merging

One use-case for cloning a repository is so that you can work on the same files from multiple different computers. Another, and indeed defining use-case of Git, is so that multiple different people can clone a single repository, and all make contributions to it together.

Other people can only push changes to your cloud repository if you give them permission. You can give other GitHub users permission to push to your cloud repository by editing the “Settings” of your repository. Click on the “Settings” link (normally on the right), and then navigate to “Collaborators”. GitHub provides a search box to find people, based either on their email address or GitHub username. Find the person you want to add, and then click “Add collaborator”.

Conflict

A major problem of allowing multiple people to share and contribute to a single repository is that it increases the probability of conflict. Conflict occurs when two people are trying to make different changes to the same line of a file.

We are going to simulate a conflict by making conflicting changes to our “original” and “cloned” versioned_dir repositories.

First, change into the “original” versioned_dir

cd 
cd versioned_dir

Edit README.MD and set its content equal to

# Hello Useful GitHub

This is a README.MD file that will be used to describe this
repository on GitHub

This is an extra line of text added to the copy
of README.MD in the cloned repository

(we have added the word Useful to the title)

Commit this change and then push it to the cloud.

git commit -a
git push

Ensure that you use an informative commit message.

Now, change into the “cloned” repository

cd
cd tmpdir/versioned_dir

Now edit README.MD and set this equal to

# Hello Brilliant GitHub

This is a README.MD file that will be used to describe this
repository on GitHub

This is an extra line of text added to the copy
of README.MD in the cloned repository

(We have added the word Brilliant to the title)

Commit this change using git commit -a, making sure you use a good commit message. However, now, run

git pull

to pull changes from the cloud. You should see an error message, that will look something like this

remote: Counting objects: 3, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 3 (delta 1), reused 3 (delta 1), pack-reused 0
Unpacking objects: 100% (3/3), done.
From https://github.com/chryswoods/versioned_dir
   46fefdd..6b653ec  master     -> origin/master
Auto-merging README.MD
CONFLICT (content): Merge conflict in README.MD
Automatic merge failed; fix conflicts and then commit the result.

Git has seen that the changes to the local repository conflict with the changes in the cloud repository. Git reports this in the CONFLICT line, with Git reporting that the conflict is in README.MD. Where possible, Git will try to resolve conflicts automatically, by merging changes where possible. However, if the changes occur on the same line, then automatic conflict resolution is not possible, and you will need to fix the conflict manually.

You can see this in more detail by typing git status. This should print out something like

# On branch master
# Your branch and 'origin/master' have diverged,
# and have 1 and 1 different commit each, respectively.
#   (use "git pull" to merge the remote branch into yours)
#
# You have unmerged paths.
#   (fix conflicts and run "git commit")
#
# Unmerged paths:
#   (use "git add <file>..." to mark resolution)
#
#	both modified:      README.MD
#
no changes added to commit (use "git add" and/or "git commit -a")

This tells you that you have a conflict in README.MD that needs to be fixed. Edit this file. When you open it, you will see this

<<<<<<< HEAD
# Hello Brilliant GitHub
=======
# Hello Useful GitHub
>>>>>>> 6b653ec0a49ce966dd3aab3174db5930988b7ae4

This is a README.MD file that will be used to describe this
repository on GitHub

This is an extra line of text added to the copy
of README.MD in the cloned repository

The line that has a conflict is written twice. The first is the version of the line in HEAD. The second is the version of the line on the cloud repository, with the specfied Git commit ID.

To resolve the conflict you need to choose which of the two lines you want to use (or indeed, if you want to use both lines). Edit the file and merge the lines together, set the contents equal to

# Hello Brilliant, Useful GitHub

This is a README.MD file that will be used to describe this
repository on GitHub

This is an extra line of text added to the copy 
of README.MD in the cloned repository

Once you have fixed the conflict, you need to tell Git that you have manually merged the files. You do this by using git add to re-add the file back to the repository, e.g.

git add README.MD

Now, run git status. You should see

# On branch master
# Your branch and 'origin/master' have diverged,
# and have 1 and 1 different commit each, respectively.
#   (use "git pull" to merge the remote branch into yours)
#
# All conflicts fixed but you are still merging.
#   (use "git commit" to conclude merge)
#
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#	modified:   README.MD
#
no changes added to commit (use "git add" and/or "git commit -a")

You should see that Git recognises that you have fixed the conflict, and is happy for you to commit the change.

git commit -a

Make sure that your commit message contains some information about how you fixed and conflict, and why you merged in the way you did, e.g.

Merged both adjectives as GitHub is both brilliant and useful

Note that the below lines are automatically added by Git

Merge branch 'master' of https://github.com/chryswoods/versioned_dir

Conflicts:
	README.MD
#
# It looks like you may be committing a merge.
# If this is not correct, please remove the file
#	.git/MERGE_HEAD
# and try again.


# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Your branch and 'origin/master' have diverged,
# and have 1 and 1 different commit each, respectively.
#   (use "git pull" to merge the remote branch into yours)
#
# All conflicts fixed but you are still merging.
#   (use "git commit" to conclude merge)
#
# Changes to be committed:
#
#	modified:   README.MD
#

Now, run git pull again to make sure that nothing else has changed while we were fixing the conflict.

git pull

You should see that everything is ok, and see something like

Already up-to-date.

Now everything is ok, you can git push the merge up to the cloud repository.

git push

Finally, change back into your “original” versioned_dir, and use git pull to check that you can pull in the new, merged, README.MD, e.g.

cd
cd versioned_dir
git pull

You can follow the merge by taking a look at the commit history on GitHub (visiting https://github.com/USERNAME/versioned_dir/commits/master, where USERNAME is your GitHub username). For my repository, this looks like

Image of commits from a merge

Workflow (multi-user)

A good workflow when working on a shared GitHub repository is

  • git status to ensure your working directory is clean. If it isn’t, then clean it up and git commit -a any changes.
  • git pull the latest changes from other people. If there are any conflicts, then merge them now, then git commit -a and git push the merges to the cloud.
  • Make changes to your files as you want.
  • git commit -a your changes, and use git status to then ensure that your working directory is clean.
  • git pull to pull in any changes that anyone else has made while you were working. If there are any conflicts, then resolve those conflicts and git commit -a until your working directory is clean.
  • When git pull causes no changes and git status shows your working directory is clean, then use git push to push your work to the cloud repository.

Exercise

Make some conflicting changes in your “original” and “cloned” copies of versioned_dir. Try to commit and push both to the cloud. What happens? Use git pull and then resolve the conflict. Once you have resolved, use git add to signal that the conflicted file has been resolved, and then git commit -a and git push the merged file back up to the cloud.

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