Merging

Overview

Teaching: 30 min
Exercises: 15 min

Questions

• How to combine the changes of parallel tracks of work?

Objectives

• Be able to merge branches.

Merging branches

If you got stuck in the branching exercises:

• Skip this unless you got stuck.

  cd ..
  git clone https://github.com/csiro-data-school/git-guacamole-branched
  cd recipe-branching
  git log --all --graph --oneline 
  

It turned out that the experiment with garlic was a good idea. Our goal now is to merge experiment into master.

First we make sure we are on the branch we wish to merge into:

$ git branch

  experiment
  less-salt
* master

Then we merge experiment into master:

git merge experiment

We can verify the result in the terminal:

git log --all --graph --oneline 

*   393dfaf (HEAD -> master) Merge branch 'experiment'
|\
| * 47d0a9d (experiment) A bit less garlic
| * e1e6f7d I wonder if garlic will work
* | 29e2be2 (origin/master) added readme
|/
| * 3db05f9 (less-salt) reduce the salt
|/
* 40a87b4 Revert "Add an onion to the recipe"
* 295b424 Add an onion to the recipe
* 53f42b7 Added salt to ingredients
* 1fea6fd Added instructions file
* 1805665 added txt file extension to ingredients
* db9b3a9 Initial commit - added ingredients

What happens internally when you merge two branches is that git creates a new commit, attempts to incorporate changes from both branches and records the state of all files in the new commit. While a regular commit has one parent, a merge commit has two (or more) parents.

To view the branches that are merged into the current branch we can use the command:

$ git branch --merged

  experiment
* master

We are also happy with the work on the less-salt branch. Let us merge that one, into master as well:

$ git branch  # make sure you are on master
$ git merge less-salt

We can verify the result in the terminal:

$ git log --all --graph --oneline 

*   95a5f4c (HEAD -> master) Merge branch 'less-salt'
|\
| * 3db05f9 (less-salt) reduce the salt
* |   393dfaf Merge branch 'experiment'
|\ \
| * | 47d0a9d (experiment) A bit less garlic
| * | e1e6f7d I wonder if garlic will work
| |/
* | 29e2be2 (origin/master) added readme
|/
* 40a87b4 Revert "Add an onion to the recipe"
* 295b424 Add an onion to the recipe
* 53f42b7 Added salt to ingredients
* 1fea6fd Added instructions file
* 1805665 added txt file extension to ingredients
* db9b3a9 Initial commit - added ingredients

Observe how git nicely merged the changed amount of salt and the new ingredient in the same file without us merging it manually:

$ cat ingredients.txt

* 1/2 clove garlic
* 2 avocados
* 1 lime
* 1 tsp salt

If the same file is changed in both branches, git attempts to incorporate both changes into the merged file. If the changes overlap then the user has to manually settle merge conflicts (we will do that later).

Deleting branches safely

Both feature branches are merged:

$ git branch --merged

  experiment
  less-salt
* master

This means we can delete the branches:

$ git branch -d experiment less-salt

Deleted branch experiment (was 47d0a9d).
Deleted branch less-salt (was 3db05f9).

Discussion

Think about what you expect the graph to look like now. What has been removed?

Solution

This is the result:

Notice all of the history is still there, just the lables (pointers) have been removed.

Compare in the terminal:

$ git log --all --graph --oneline 

*   95a5f4c (HEAD -> master) Merge branch 'less-salt'
|\
| * 3db05f9 reduce the salt
* |   393dfaf Merge branch 'experiment'
|\ \
| * | 47d0a9d A bit less garlic
| * | e1e6f7d I wonder if garlic will work
| |/
* | 29e2be2 (origin/master) added readme
|/
* 40a87b4 Revert "Add an onion to the recipe"
* 295b424 Add an onion to the recipe
* 53f42b7 Added salt to ingredients
* 1fea6fd Added instructions file
* 1805665 added txt file extension to ingredients
* db9b3a9 Initial commit - added ingredients

git will not let you delete a branch which has not been reintegrated unless you insist using git branch -D. Even then your commits will not be lost but you may have a hard time finding them as there is no branch label pointing to them.

Exercise: encounter a fast-forward merge

• Create a new branch from master and switch to it.
• Create a couple of commits on the new branch (for instance edit README.md):

• Now switch to master.
• Merge the new branch to master.
• Examine the result with git graph.
• Have you expected the result? Discuss what you see.

Key Points

• Branches can be brought back together while maintaining their independent history