Unit 6: Merging

A merge with actual work to do

Last unit, main sat still while you worked. This time it moved too — someone added a file to main while your branch added another.

Now git cannot just slide a label. It makes a merge commit: a commit with two parents, joining both lines together.

Terminal

$ git merge hats -m "Merge the hats"
Merge made by the 'ort' strategy.
 hats.txt | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 hats.txt
$ ls
hats.txt
shoes.txt
shop.txt

Seeing the join

git log --oneline --graph draws the shape. The lines split apart where the branch began and come back together at the merge commit.

Terminal

$ git log --oneline --graph
*   da0f4bd (HEAD -> main) Merge the hats
|\
| * 21ee6a9 (hats) Sell hats
* | 1628a1d Sell shoes
|/
* 432f8ac Open the shop

The same line, two ways

Git is good at combining edits — as long as they are in different places.

If you changed line 2 and your friend changed line 2, git has no way to know which is right. It stops, and asks you. That is a conflict. It is not an error, and nothing is broken.

Different lines: no problem at all

Watch this. Two branches both edit poem.txt — one changes the first line, one changes the last. Git merges them without a word.

Terminal

$ git merge top -m "Merge top"
Auto-merging poem.txt
Merge made by the 'ort' strategy.
 poem.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
$ cat poem.txt
ONE
two
THREE

The same line: a conflict

Now both branches change the *same* line. Git stops and writes both versions into the file, fenced off with markers.

Between <<<<<<< and ======= is yours. Between ======= and >>>>>>> is theirs.

Terminal

$ git merge sam
Auto-merging list.txt
CONFLICT (content): Merge conflict in list.txt
Automatic merge failed; fix conflicts and then commit the result.
$ cat list.txt
apples
<<<<<<< HEAD
cherries
=======
bananas
>>>>>>> sam

Three steps, every time

1. Edit the file so it says what you actually want, and delete all three marker lines.
2. Run git add on it — that is how you tell git "sorted".
3. Run git commit to finish the merge.

You are the one who decides. Git only refused to guess.

Terminal

$ printf "apples\nbananas\ncherries\n" > list.txt
$ git add list.txt
$ git commit -m "Keep both fruits"
[main 4964701] Keep both fruits
$ cat list.txt
apples
bananas
cherries

Status walks you through it

Stuck in the middle of a conflict? git status names the files it needs you to fix, under Unmerged paths, and tells you the next command.

Terminal

$ git status
On branch main
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:   list.txt

no changes added to commit (use "git add" and/or "git commit -a")

The escape hatch

Panicking? git merge --abort puts everything back exactly as it was before you typed merge. Nothing is lost, and you can try again when you are ready.