Lesson 2 of 9 · about 15 minutes

Create, switch, compare

Five commands that let you move between branches without fear.

The five commands

Lesson 1 happened in a simulator. This time you'll use your own terminal: Git Bash on Windows, or Terminal on macOS/Linux. Here is everything you need:

The docs describe git switch -c as "the transactional equivalent of git branch <new-branch> and git switch <new-branch>". It is lesson 1's two steps in one.2

Older tutorials (and Claude, sometimes) use git checkout -b add-cleaning. It does the same thing. git switch arrived in Git 2.23.1 It was labelled experimental until Git 2.51 declared it "no longer experimental."5 This course uses switch because it does one job: changing branches.

Do it: your practice repository

You'll keep using penguins-lab for the rest of the course, so set it up properly. Tick each step as you go. Your progress is saved in this browser.

1. Create the repo on GitHub and clone it (this uses the gh CLI you already logged in with):

   $ gh repo create penguins-lab --public --clone --add-readme
   $ cd penguins-lab

2. Create and move onto a feature branch:

   $ git switch -c add-cleaning
   Switched to a new branch 'add-cleaning'

3. Create clean.py in your editor with this content:

   import pandas as pd
   
   def drop_missing(df: pd.DataFrame, cols: list[str]) -> pd.DataFrame:
       """Return a copy of df without rows that have NaN in any of cols."""
       return df.dropna(subset=cols).reset_index(drop=True)

4. Commit it. The commit lands on add-cleaning, not main:

   $ git add clean.py
   $ git commit -m "Add drop_missing cleaning function"
   [add-cleaning 3ef57a2] Add drop_missing cleaning function
    1 file changed, 5 insertions(+)

5. Ask Git where you are:

   $ git branch -vv
   * add-cleaning 3ef57a2 Add drop_missing cleaning function
     main         1d28f0e [origin/main] Initial commit
   $ git status
   On branch add-cleaning

   Your hashes will differ. main shows [origin/main] because it is linked to GitHub. Lesson 4 explains that link.

6. Switch to main and list the folder. clean.py is gone. Switch back and it returns:

   $ git switch main
   $ ls
   README.md
   $ git switch -
   $ ls
   README.md  clean.py

   Nothing was deleted. main's commit simply doesn't contain clean.py, and switching makes your files match the branch's commit.2

7. Draw the graph:

   $ git log --oneline --graph --all
   * 3ef57a2 (HEAD -> add-cleaning) Add drop_missing cleaning function
   * 1d28f0e (origin/main, origin/HEAD, main) Initial commit

   Same picture as lesson 1: add-cleaning is one commit ahead of main.

8. See only what the branch changed, with a three-dot diff:

   $ git diff main...add-cleaning
   diff --git a/clean.py b/clean.py
   new file mode 100644
   --- /dev/null
   +++ b/clean.py
   @@ -0,0 +1,5 @@
   +import pandas as pd
   ...

   main...add-cleaning (three dots) compares the branch with the commit where it split from main. You see the branch's own changes and nothing else.6 GitHub pull requests use the same three-dot view.7 Later, this is how you'll review everything an agent did on its branch.

Leave add-cleaning unmerged. Lesson 3 brings it into main.

On Windows you may see LF will be replaced by CRLF warnings. They are about line endings, and you can ignore them for this course.

Recall the commands

Type each command from memory. Don't scroll up. The effort of remembering is the point.

Check yourself