Branches

Overview

Teaching: 20 min
Exercises: 0 min

Questions

• What are branches?

• How can I work in parallel using branches?

Objectives

• Understand why branches are useful for:

• working on separate tasks in the same repository concurrently

• trying multiple solutions to a problem

• check-pointing versions of code

• Merge branches back into the main branch

So far we’ve always been working in a straight timeline. However, there are times when we might want to keep our main work safe from experimental changes we are working on. To do this we can use branches to work on separate tasks in parallel without changing our current branch, main.

We didn’t see it before but the first branch made is called main. This is the default branch created when initializing a repository and is often considered to be the “clean” or “working” version of a repository’s code.

We can see what branches exist in a repository by typing

$ git branch

* main

The ‘*’ indicates which branch we are currently on.

In this lesson, Dracula is trying to run an analysis and doesn’t know if it will be faster in bash or python. To keep his main branch safe he will use separate branches for both bash and python analysis. Then he will merge the branch with the faster script into his main branch.

First let’s make the python branch. We use the same git branch command but now add the name we want to give our new branch

$ git branch pythondev

We can now check our work with the git branch command.

$ git branch

* main
  pythondev

We can see that we created the pythondev branch but we are still in the main branch.

We can also see this in the output of the git status command.

$ git status

On branch main
nothing to commit, working directory clean

To switch to our new branch we can use the checkout command we learned earlier and check our work with git branch.

$ git checkout pythondev
$ git branch

  main
* pythondev

Before we used the checkout command to checkout a file from a specific commit using commit hashes or HEAD and the filename (git checkout HEAD <file>). The checkout command can also be used to checkout an entire previous version of the repository, updating all files in the repository to match the state of a desired commit.

Branches allow us to do this using a human-readable name rather than memorizing a commit hash. This name also typically gives purpose to the set of changes in that branch. When we use the command git checkout <branch_name>, we are using a nickname to checkout a version of the repository that matches the most recent commit in that branch (a.k.a. the HEAD of that branch).

Here you can use git log and ls to see that the history and files are the same as our main branch. This will be true until some changes are committed to our new branch.

Now lets make our python script.
For simplicity sake, we will touch the script making an empty file but imagine we spent hours working on this python script for our analysis.

$ touch analysis.py

Now we can add and commit the script to our branch.

$ git add analysis.py
$ git commit -m "Wrote and tested python analysis script"

[pythondev x792csa1] Wrote and tested python analysis script
 1 file changed, 1 insertion(+)
 create mode 100644 analysis.py

Lets check our work!

$ ls
$ git log --oneline

As expected, we see our commit in the log.

Now let’s switch back to the main branch.

$ git checkout main
$ git branch

* main
  pythondev

Let’s explore the repository a bit.

Now that we’ve confirmed we are on the main branch again. Let’s confirm that analysis.py and our last commit aren’t in main.

$ ls
$ git log --oneline

We no longer see the file analysis.py and our latest commit doesn’t appear in this branch’s history. But do not fear! All of our hard work remains in the pythondev branch. We can confirm this by moving back to that branch.

$ git checkout pythondev
$ git branch

  main
* pythondev

$ ls
$ git log --oneline

And we see that our analysis.py file and respective commit have been preserved in the pythondev branch.

Now we can repeat the process for our bash script in a branch called bashdev.

First we must checkout the main branch again. New branches will include the entire history up to the current commit, and we’d like to keep these two tasks separate.

$ git checkout main
$ git branch

* main
  pythondev

This time let’s create and switch two the bashdev branch in one command.

We can do so by adding the -b flag to checkout.

$ git checkout -b bashdev
$ git branch

* bashdev
  main
  pythonndev

We can use ls and git log to see that this branch is the same as our current main branch.

Now we can make analysis.sh and add and commit it. Again imagine instead of touching the file we worked on it for many hours.

$ touch analysis.sh
$ git add analysis.sh
$ git commit –m “Wrote and tested bash analysis script”

[bashdev 2n779ds] Wrote and tested bash analysis script
 1 file changed, 1 insertion(+)
 create mode 100644 analysis.sh

Lets check our work again before we switch back to the main branch.

$ ls
$ git log --oneline

So it turns out the python analysis.py is much faster than analysis.sh.

Let’s merge this version into our main branch so we can use it for our work going forward.

Merging brings the changes from a different branch into the current branch.

First we must switch to the branch we’re merging changes into, main.

$ git checkout main
$ git branch

  bashdev
* main
  pythonndev

Now we can merge the pythondev branch into our current branch (main). In english, this command could be stated as “git, please merge the changes in the pythondev branch into the current branch I’m in”.

$ git merge pythondev

Updating 12687f6..x792csa1
Fast-forward
 analysis.py | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 analysis.py

Now that we’ve merged the pythondev into main, these changes exist in both branches. This could be confusing in the future if we stumble upon the pythondev branch again.

We can delete our old branches so as to avoid this confusion later. We can do so by adding the -d flag to the git branch command.

git branch -d pythondev

Deleted branch pythondev (was x792csa1).

And because we don’t want to keep the changes in the bashdev branch, we can delete the bashdev branch as well

$ git branch -d bashdev

error: The branch 'bashdev' is not fully merged.
If you are sure you want to delete it, run 'git branch -D bashdev'.

Since we’ve never merged the changes from the bashdev branch, git warns us about deleting them and tells us to use the -D flag instead.

Since we really want to delete this branch we will go ahead and do so.

git branch -D bashdev

Deleted branch bashdev (was 2n779ds).

Key Points

• Branches can be useful for developing while keeping the main line static.