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Git Branches and Merging

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Branches Behind the Scenes

5:58 with Jay McGavren

In this video, we'll be looking inside the hidden `.git` directory to see how branches work behind the scenes.

A quick warning: we are not supposed to be poking around in the .git directory! The file and folder structure you see here might change in different versions of Git. And you mustn't change the file contents directly, or you could render your repo unusable! We're just going to peek at the file contents, though, so we'll be OK.

  • The HEAD file in the .git directory is used to track which branch we're on.
    • Let's use a program that's on Mac, Linux, and some Windows computers called cat to look at the file's contents.
    • To view the contents of a file, you just type cat followed by the path to the file: cat .git/HEAD
    • On Windows, you can use the type program instead: type .git/HEAD
    • There's not much to the file. It just contains a single line, saying ref: refs/heads/master.
    • Let's try checking out a different branch: git checkout testing
    • If we look at the HEAD file's contents again now, we'll see that they've changed: ref: refs/heads/testing
  • The refs/heads/ subdirectory within the .git directory holds the branches for our repo.
    • We can list its contents with the ls command: ls .git/refs/heads
    • On Windows, replace ls with dir: dir .git/refs/heads
  • The refs/heads/ directory contains one file for each branch in your repo.
    • Let's look at the file for the testing branch: cat .git/refs/heads/testing
    • Again, it's just one line, but this time it's a SHA checksum.
    • If we run git log we'll see that it matches the checksum of the most recent commit on this branch.
    • If we look at the file for the add-letters branch, we'll see the checksum of the most recent commit on that branch: cat .git/refs/heads/add-letters
    • Like we said, a branch is just a pointer to a particular commit. We weren't kidding.
    • There's no other data files for our branches or anything. Those one-line files in the refs/heads directory are the branches.

We can also leave branches behind and check out an individual commit.

  • You can run git log and note the SHA checksum for a particular commit you want to check out.
  • Then you can run git checkout again, but instead of a branch name, give it the SHA for a specific commit: git checkout <partial SHA>
  • The output will say "You are in 'detached HEAD' state".
  • Now that we're not on a branch, the .git/HEAD file contains the specific SHA that's checked out, not a branch name.

We can get back onto a branch by simply checking one out: git checkout testing

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    In this video we'll be looking inside the hidden .git 0:00
    directory to see how branches work behind the scenes. 0:03
    A quick warning, we are not supposed to be poking around in this directory. 0:06
    The file and folder structures you see here might change in different versions of 0:10
    Git, and you mustn't change the file contents directly or 0:14
    you could render your repo unusable. 0:17
    We're just going to peak at the file contents though, so we should be okay. 0:20
    When we ran git log in previous videos, we saw a head marker pointing to the name 0:24
    of the branch that's currently checked out. 0:28
    So what is this head? 0:33
    The head file in the .git directory is used to track which branch we're on. 0:35
    Let's use a program that's on Mac, Linux and 0:39
    some Windows computers called cat to look at the file's contents. 0:42
    If cat isn't available on your system, see the teacher's notes for 0:46
    an alternate way to read the file. 0:49
    To view the contents of a file, you just type cat followed by the path to the file, 0:50
    so cat .git/HEAD. 0:55
    There's not much in the file, it just contains a single line saying, 0:59
    ref: refs/heads/master. 1:02
    Now, let's try checking out a different branch, git checkout testing. 1:05
    If we look at the HEAD file's contents again now, we'll see that they've changed. 1:10
    The file now reads, ref: refs/heads/testing. 1:14
    The refs/heads subdirectory within the .git directory holds the branches for 1:17
    our repo, we can list its contents with the ls command. 1:22
    Again, if the ls command is unavailable on your system, see the teacher's notes for 1:29
    an alternative. 1:33
    The refs/heads directory contains one file for each branch in your repo. 1:34
    The contents of these files are really simple, too. 1:40
    Let's look at the file for the testing branch, cat .git/refs/heads/testing. 1:43
    Again, it's just one line, but this time it's a SHA checksum. 1:50
    if we run git log, 1:54
    we'll see that it matches the checksum of the most recent commit on this branch. 1:55
    If we look at the file for the add-letters branch, 2:00
    we'll see the checksum of the most recent commit on that branch. 2:03
    Like we said, a branch is just a pointer to a particular commit, 2:09
    we weren't kidding. 2:12
    There's no other data files for our branches or anything. 2:14
    Those one line files in the refs/heads directory are the branches. 2:17
    We can also leave branches behind and check out an individual commit. 2:22
    Let's display our history and find a commit to check out. 2:26
    I'll make a note of the SHA for the commit that I want. 2:30
    Now I'll run git checkout again, but instead of a branch name, 2:34
    I'll give it the SHA for a specific commit. 2:37
    Git says, you are in detached HEAD state, now let's run git status. 2:40
    Git status also shows HEAD detached, detached HEAD, 2:47
    normally the git/HEAD file refers to a branch. 2:52
    Let's see what it contains now that we're not on a branch, cat .git/HEAD. 2:55
    It shows the specific SHA that's checked out, not a branch name. 3:01
    If I run git log, the history will start at the checked out commit, and 3:05
    follow its parents all the way back to the first commit for the repo. 3:08
    We can get back onto a branch by simply checking one out. 3:14
    Let's get the list of our branches, git branch, and 3:17
    let's check the testing branch back out again, git checkout testing. 3:21
    Commands like git status will show us that everything's back to normal and 3:30
    we're back on a branch. 3:33
    Using this technique, we can create a new branch based off of any commit we want to. 3:35
    First, we check out the commit we want, 3:41
    I'll just check out the same commit we did before. 3:43
    The hint says, if you want to create a new branch to retain commits you create, 3:46
    you can do so now or later, by using -b with the checkout command again. 3:51
    Let's try the command it suggests, git checkout -b and 3:56
    I'll use the branch name of testing-temp. 3:59
    If we run git log, we'll see that the history starts at the same commit 4:03
    as before, but we'll also see that we're on a branch now. 4:07
    Git status shows the branch that we're on as well. 4:12
    And git branch shows our new branch, along with all the others. 4:19
    We can also make commits to this new branch, just like any other. 4:25
    I'll edit a file. 4:29
    Hi there. 4:35
    I'll save it, and then I'll stage it with git add test_decoder.rb. 4:37
    Then I'll commit it, git commit -m, 4:43
    I'm just playing around so I'll use a temporary commit message. 4:45
    The new commit will appear in the new branch's history. 4:51
    Now let's check out the testing branch, 4:55
    so we can compare it to the testing-temp branch, git checkout testing. 4:57
    Let's look at its history git log. 5:02
    We based the testing-temp branch off the second to last commit on the testing 5:06
    branch, so the latest commit doesn't appear on testing-temp, but 5:09
    it does here on testing. 5:13
    Add test with more letters. 5:15
    There's also no sign of our playing around commit, 5:18
    that's on the testing-temp branch but not here. 5:20
    We aren't going to need the testing-temp branch any further, so 5:24
    I'm going to delete it. 5:27
    Git branch -D testing-temp. 5:28
    Hopefully now it's a little clearer that a branch is just a pointer to a commit. 5:33
    Don't worry if you don't fully understand right now, 5:38
    you'll need some time playing around with branches to get the hang of them. 5:40
    But now you have the info you need to get started. 5:44
    Branches keep changes in your code on separate timelines, but eventually, 5:46
    you're going to want to bring all that code back together. 5:50
    In the next stage, 5:53
    we'll show you how to do that by merging branches, I'll see you there. 5:54

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