lesson ── git-labs ── ~22 мин ── 8 шагов

Branches: create, switch, merge, delete

The goal is to take a feature branch through its full cycle. You create it from main, make a couple of commits, see a fast-forward merge and a three-way merge, try to delete an unmerged branch, and get an honest refusal from Git.

интерактивный sandbox

Поднимется контейнер gitlab/git-base с git, bash, pre-commit. В браузере откроется терминал, можно сразу git init. Каждый шаг проверяется автоматически. Сеть air-gapped, github.com недоступен.

запустить sandbox →

stack ── git · bash · 256 MB RAM · air-gapped · самоуничтожается через 30 мин простоя

Шаги

01
Create a repo with one commit
bash
cd /home/student/work
mkdir -p branches-lab && cd branches-lab
git init -b main
echo "base" > readme.txt
git add . && git commit -m "init"
git branch
There is one branch right now, main. Next you create your own.
✓ A repo with one commit on main. Ready to branch.
02
Create branch feat/quick and switch to it
bash
cd /home/student/work/branches-lab
git switch -c feat/quick # -c = create, same as `checkout -b`
git branch # an asterisk marks the active branch
git log --oneline --graph --all # --graph = ASCII graph, --all = all refs
switch -c (== checkout -b) creates a branch from HEAD and switches to it. git branch now shows both, with an asterisk on the active one.
подсказка
If you get `unknown switch 'switch'`, the git is old. Use `checkout -b feat/quick`.
✓ The branch is created, and you are on it.
03
Make a commit on the branch
bash
cd /home/student/work/branches-lab
echo "quick feature" > quick.txt
git add . && git commit -m "feat: add quick"
git log --oneline --graph --all
feat/quick now has 2 commits, main has 1. The graph does not show any divergence yet, because main has not branched off.
✓ The branch moved one commit ahead.
04
Do a fast-forward merge into main
bash
cd /home/student/work/branches-lab
git switch main
git merge feat/quick # ff = just move the ref forward, no merge commit
git log --oneline --graph --all
The message is Fast-forward. main and feat/quick now point to the same commit. There is no merge commit: main just slid forward, because main and feat/quick had not diverged.
✓ Fast-forward done. The graph stayed linear.

Set up a situation for a three-way merge

For a three-way merge, both branches have to diverge from a common ancestor.

bash

cd /home/student/work/branches-lab

git switch -c feat/slow

echo "slow line" > slow.txt

git add . && git commit -m "feat: add slow"

git switch main

echo "main update" >> readme.txt     # >> = append, does not overwrite

git commit -am "main: update readme"  # -a = stage already-tracked files, -m = message

git log --oneline --graph --all

Now main and feat/slow have diverged. The graph shows a V shape.

✓ The branches have diverged. Ready for the three-way.

06
Do a three-way merge
bash
cd /home/student/work/branches-lab
# --no-edit = take the default merge-commit message, do not open the editor
git merge --no-edit feat/slow
git log --oneline --graph --all
The message is Merge made by the 'ort' strategy. A merge commit with two parents appears in the log. The graph is no longer linear.

Files from both branches end up in the working tree. They touched different files, so there is no conflict.
✓ The merge commit is created. That is the three-way.
07
Delete a merged branch
bash
cd /home/student/work/branches-lab
git branch -d feat/quick # -d = "delete if merged", safe
git branch -d feat/slow
git branch
Both delete without complaint, since they are merged into main. branch -d checks this itself and refuses when they are not.
✓ The merged branches are gone. Only main remains.
08
Create an unmerged branch and try to delete it
bash
cd /home/student/work/branches-lab
git switch -c feat/draft
echo "draft idea" > draft.txt
git add . && git commit -m "draft"
git switch main
git branch -d feat/draft # fails: the branch is not merged into main
You should get the error: error: The branch 'feat/draft' is not fully merged.. This is Git's protection: you could lose commits by deleting a branch that is not in main.

To force the delete, use a capital -D:
bash
git branch -D feat/draft # -D (capital) = forced delete
git branch
Now it really is deleted. If the commit is used nowhere else, git gc sweeps it up when it runs (by hand or as gc --auto). The reflog keeps a record of the branch for about 30 days by default, after which the commit becomes a candidate for collection.
✓ You saw Git's protection, and the forced delete worked.

Что ты узнал

A branch is a pointer to a commit. You create it with git branch or switch -c, and switch with switch. A merge can be fast-forward or three-way, depending on whether the target moved. You delete with branch -d (safe) or -D (forced).

команды

git switch -c feat/xcreate and switch
git merge feat/xmerge feat/x into the current branch (ff or 3-way)
git merge --no-ff feat/xforce a merge commit even when ff is possible
git branch -d feat/xdelete if the branch is merged
git branch -D feat/xdelete by force

концепции

· fast-forward: the target branch did not move, the ref just slides over
· three-way: both branches moved, you need a merge commit with two parents
· branch -d refuses when the branch is not merged, which is protection, not a bug