Skip to content

Merging

Learning objectives

  • practice fixing merge conflicts on GitHub
  • practice merging branches using the command-line interface
  • practice fixing merge conflicts on local computer, using the command-line interface
For teachers

Teaching goals are:

  • Learners have practiced fixing merge conflicts on GitHub
  • Learners have practiced merging branches using the command-line interface
  • Learners have practiced fixing merge conflicts on local computer, using the command-line interface
gantt
  title Lesson plan apply merge
  dateFormat X
  axisFormat %s
  Introduction: intro, 0, 5s
  Theory 1: theory_1, after intro, 5s
  Exercise 1: crit, exercise_1, after theory_1, 40s
  Feedback 1: feedback_1, after exercise_1, 10s

Prior questions

  • What does a merge do?
  • Do we need merging? When? Why?
  • When does a merge give a merge conflict?
  • Can a git commit result in a merge conflict? Why?
  • Can a git push result in a merge conflict? Why?

  • Can a git pull result in a merge conflict? Why?

  • Isolated tracks

Workflow of merge

  • Once a feature is ready, switch to main!
    • This is the Branch we want to merge to
  • You may want to double check with git branch
git switch main    # switch to main branch
git branch           # check that we are on main branch
git merge <feature branch>

Merge conflicts

  • However, when we merge branches, it may result in a merge conflict.
  • A merge conflict occurs when git is unsure how to merge branches and asks a human for help.
  • Here we create merge conflicts on trivial code.

Two branches to be merged

Based on: https://coderefinery.github.io/git-intro/conflicts/

License: Creative Commons Attribution 4.0 International
  • Permits almost any use subject to providing credit and license notice.
  • Frequently used for media assets and educational materials.
  • The most common license for Open Access scientific publications.
  • Not recommended for software.
  • Once all features are ready, switch to main!
  • This is the Branch we want to merge to

The first merge will work

$ git switch main    # switch to main branch
$ git branch           # check that we are on main branch
$ git merge like-cilantro

Updating 4e03d4b..3caa632
Fast-forward
 ingredients.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
  • But the second will fail:
$ git merge dislike-cilantro

Auto-merging ingredients.txt
CONFLICT (content): Merge conflict in ingredients.txt
Automatic merge failed; fix conflicts and then commit the result.
  • Without conflict Git would have automatically created a merge commit, but since there is a conflict, Git did not commit:
$ git status

You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
    both modified:   ingredients.txt

no changes added to commit (use "git add" and/or "git commit -a")
  • Git won’t decide which to take and we need to decide. Observe how Git gives us clear instructions on how to move forward.
$ git diff

diff --cc ingredients.txt
index 6cacd50,6484462..0000000
--- a/ingredients.txt
+++ b/ingredients.txt
@@@ -1,4 -1,4 +1,10 @@@
++<<<<<<< HEAD
 +* 2 tbsp cilantro
++=======
+ * 1/2 tbsp cilantro
++>>>>>>> dislike-cilantro
  * 2 avocados
  * 1 chili
  * 1 lime
  • Check status with git status and git diff.
  • Decide what you keep (the one, the other, or both or something else). Edit the file to do this.
    • Remove the resolution markers, if not already done.
    • The file(s) should now look exactly how you want them.
  • Check status with git status and git diff.
  • Tell Git that you have resolved the conflict with git add ingredients.txt (if you use the Emacs editor with a certain plugin the editor may stage the change for you after you have removed the conflict markers).
  • Verify the result with git status.
  • Finally commit the merge with only git commit. Everything is pre-filled.

Another example of merged branches

$ git graph
*   1b29a8f (HEAD -> main, origin/main) Merge branch 'modularity'
|\
| * 4d4acaf (modularity) 4 modular files
* | 000b440 rm print
|/
| * 2d4e252 (jupiter) add jupiter
|/
* b9465e4 planet.py documentation
* 6a416b5 add folders and planet code

On GitHub

  • Let's view the branches on Github!
  • Go to Insights in the top menu and then go to Network in side-bar

  • If we did this after the merging the branches do not show up.

  • Conflicts may show up like this:

conflict-edit-2

  • We merge together one of the pull requests (this will work)
  • Then we try to merge the other and we see a conflict:

conflict-pull-request

  • We try to resolve the conflict via web
  • Choose the version that you wish to keep, remove conflict markers, "Mark as resolved" and commit the change

conflict-resolution

Exercises

Exercise 1: practice merging git branches using the GitHub interface

Learning objectives

  • practice merging git branches without a merge conflict
gitGraph
    commit id: "Stuff on main"
    branch develop
    checkout develop
    commit id: "Stuff on develop"
    branch anna
    checkout anna
    commit id: "Some work"
    commit id: "Branching version"
    branch bertil
    checkout bertil
    commit id: "Modify my file"
    checkout anna
    merge bertil
    commit id: "Another commit"
    checkout develop
    merge anna
  • You work in a pair or trio
  • On GitHub, create a branch for person A, e.g. anna that branches off from develop
  • On GitHub, use the branch of person A and create a new commit. Use the web interface or command-line.
  • On GitHub, create a branch for person B, e.g. bertil that branches off from anna
  • On GitHub, use the branch of person B and create a new commit. Use the web interface or command-line.
  • On GitHub, use web interface to create a Pull Request from bertil to anna. The person that does this requests a reviewer.
  • On GitHub, the other person approves the Pull Request and merges
  • On GitHub, use web interface to create a Pull Request from anna to develop. The person that does this requests a reviewer. If there is a merge conflict, either stop (you've done the exercise, well done!) or fix the merge conflict
  • On GitHub, the other person approves the Pull Request and merges

Extra exercise 2: practice merging git branches using the command-line

Learning objectives

  • practice merging git branches without a merge conflict

Here we use the main branch for now

Instead of updating this exercise, its answer and video to use a proper branching workflow, we branch from main in this exercise

gitGraph
    commit id: "Before start"
    commit id: "Branching version"
    branch sven
    checkout sven
    commit id: "Modify my file"
    checkout main
    merge sven
    commit id: "Another commit"
  • For our GitHub repo, create a branch with your first name that is unique, e.g. sven, sven_svensson or sven_svensson_314. You may branch of from main or develop (if it exists). You may use the web interface (easiest!) or use the command line
  • On your local computer:
    • update your repository
    • switch to that branch
    • change the repo
    • push your changes online
  • Verify the changes are online
  • On your local computer
    • switch to the main branch
    • merge your topic branch to main
    • upload your changes
  • Delete your topic branch (i.e. the one with the unique name). You may use the web interface (easiest!) or use the command line
  • On your local computer, update your code
Answers

github_create_branch_annotated

Click on 1, type your branch name at 2 (in this case, richel), then click 3. Done!

  • On your local computer:
    • update the repository

On your local computer, navigate to the folder of the shared project and update:

git pull
  • On your local computer:
    • switch to the new branch

Switch to the new branch, for example, richel, by doing:

git switch richel
  • On your local computer:
    • change the content of the repository, for example, by creating a file in learners/[your_name]/[your_name]_is_on_[your_branch_name]

This can be any change you'd like. To create a file under Linux (and maybe this works on other operating systems too), one can do:

touch learners/richel/richel_is_on_richel.txt

After the change, commit these:

git add .
git commit -m "Richel is on richel"
  • On your local computer:
    • push your changes online.

Do:

git push

And your code may end up online.

If that does not work, do:

git pull

and try pushing again, maybe multiple times, as many people are pushing to the shared repo.

  • On GitHub, verify that your changes on your branch can be found online

github_pushed_to_branch

Make sure you look at the correct branch, as displayed at 1. Then your commit message shows up at 2.

  • On your local computer
    • switch to the main branch
git switch main
  • On your local computer
    • merge your topic branch to main
git merge richel
  • On your local computer
    • upload your changes
git push
  • Delete your branch (i.e. the one with the unique name). You may use the web interface (easiest!) or use the command line

github_view_branches_annotated

Click on 'Branches', as shown in the image above.

github_view_all_branches_annotated

Click on garbage bin, as shown in the image above.

github_view_all_branches_just_deleted_annotated

The branch will now be deleted, as shown in the image above.

  • On your local computer, update your code

Do:

git pull
Prefer a video?

You can find a video here

Extra exercise 3: practice merge conflicts between branches using the command line

Learning objectives

  • experience merge conflicts between branches
  • fix merge conflicts between branches

Here we use the main branch for now

Instead of updating this exercise, its answer and video to use a proper branching workflow, we branch from main in this exercise

gitGraph
    commit id: "Before start"
    commit id: "Branching version"
    branch anna
    checkout anna
    commit id: "Modify the file"
    checkout main
    checkout main
    commit id: "Modify the file too"
    checkout main
    merge anna
    commit id: "End"

An example picture of how to create a merge conflict.

  • Create a merge conflict between two branches, e.g. a topic branch and the main branch. You can do so by creating random commits on both branches and merge. Alternatively, the figure above shows the minimal git branching history to do so.
Answers

Here, I will replay the figure above

  • in GitHub, create a branch called anna
  • on your local computer, git pull, then git switch anna
  • on your local computer, modify a file, e.g. add the line Anna was here at the bottom of README.md. Then do git add ., git commit -m "Anna was here" and git push. Do not merge braches yet, else there will be no merge conflicts!
  • on your local computer, git switch main
  • on your local computer, modify a file, e.g. add the line main person was here. Then do git add ., git commit -m "main person was here" and git push.
  • To generate the merge conflict, merge anna into main, using git merge main. You will get a clear error :-)
  • Modify the file to have the texts merged. Then do git add ., git commit -m "Fixed merge conflict" and git push.
Enjoy a video?

You can find a video here