Everything You'll Need to Know About Git

Watch on Frontend Masters

You have seen it a bunch of times and probably can guess what it means...


















Throughout this course you may have notice that whenever we call git log there a HEAD within the logs.



What is HEAD?



Lets do some basic experimenting first



➜  hello-git git:(foo-rebase-trunk) git checkout trunk
Switched to branch 'trunk'
➜  hello-git git:(trunk) git log --oneline --decorate --graph

* b23e632 (HEAD -> trunk, bar) Y
...         ^--- we currently have trunk checked out


swapping branches and doing another git log will result in similar results.



➜  hello-git git:(trunk) git checkout foo
Switched to branch 'foo'
➜  hello-git git:(trunk) git log --oneline --decorate --graph

* 16984cb (HEAD -> foo) C
...         ^--- we currently have foo checked out

















Problem

In the .git folder can you tell what HEAD is pointing to?


















Solution

➜  hello-git git:(foo) cat .git/HEAD
ref: refs/heads/foo


That means as foo updates HEAD does not have to!


















Reflog

Its not just a wizards tool



Introducing git reflog. The default command of git reflog allows you to see where head has been.


















Thought Exercise

Give reflog command a try. What do you see. Think about what you have had checked out.


















Answers

➜  hello-git git:(trunk) git reflog

# You should see something similar if you have been following along
16984cb (HEAD -> foo) HEAD@{0}: checkout: moving from trunk to foo
b23e632 (trunk, bar) HEAD@{1}: checkout: moving from foo-rebase-trunk to trunk
3ade655 (foo-rebase-trunk) HEAD@{2}: rebase (finish): returning to refs/heads/foo-rebase-trunk
3ade655 (foo-rebase-trunk) HEAD@{3}: rebase (pick): C
d810248 HEAD@{4}: rebase (pick): B
...


If you look carefully you will see every checkout you have done of any branch is listed, in time based ordering


















Can you find where this information is stored inside .git


















Solution

In reflog command



➜  hello-git git:(foo) git reflog -3
44016b0 (HEAD -> foo) HEAD@{0}: checkout: moving from foo-rebase-trunk to foo
11a3f97 (foo-rebase-trunk) HEAD@{1}: rebase (finish): returning to refs/heads/foo-rebase-trunk
11a3f97 (foo-rebase-trunk) HEAD@{2}: rebase (pick): C


In .git/logs/HEAD



➜  hello-git git:(foo) cat .git/logs/HEAD | tail -3
6fd6e31690d138de9380544d01842841b0d37963 11a3f97202b9f28f5511dbfaa823168756cca03e ThePrimeagen <theprimeagen@gmail.com> 1710875458 -0600    rebase (pick): C
11a3f97202b9f28f5511dbfaa823168756cca03e 11a3f97202b9f28f5511dbfaa823168756cca03e ThePrimeagen <theprimeagen@gmail.com> 1710875458 -0600    rebase (finish): returning to refs/heads/foo-rebase-trunk
11a3f97202b9f28f5511dbfaa823168756cca03e 44016b016fd77883d98b1fce4c3dea2cf877f408 ThePrimeagen <theprimeagen@gmail.com> 1710877063 -0600    checkout: moving from foo-rebase-trunk to foo


Again. Its not magic


















Problem

This will be an interesting set of tasks to do, but can be quite useful in understanding reflog.



  1. create a new branch off of trunk, call it baz.
  2. add one commit to baz. Do it in a new file baz.md
  3. switch back to trunk and delete baz (git branch -D baz from earlier)
  4. can you bring back from the dead the commit sha of baz?

















Solution

First thing we will do is create the baz branch then add a commit to it



➜  hello-git git:(foo) git checkout trunk
Switched to branch 'trunk'
➜  hello-git git:(trunk) git checkout -b baz
Switched to a new branch 'baz'
➜  hello-git git:(baz) echo "baz" > baz.md
➜  hello-git git:(baz) git add baz.md
➜  hello-git git:(baz) git commit -m 'Baz'
[baz f330d23] Baz
 1 file changed, 1 insertion(+)
 create mode 100644 baz.md


Next, lets switch back to trunk and delete the branch baz



➜  hello-git git:(baz) git checkout trunk
Switched to branch 'trunk'
➜  hello-git git:(trunk) git branch -D baz
Deleted branch baz (was f330d23).


Now we recover the branch via reflog (should be obvious because i just talked about it)



➜  hello-git git:(baz) git reflog
b23e632 (HEAD -> trunk, bar) HEAD@{0}: checkout: moving from baz to trunk
f330d23 HEAD@{1}: commit: Baz # <--- there is our target commit
b23e632 (HEAD -> trunk, bar) HEAD@{2}: checkout: moving from trunk to baz
...


Now that we see f330d23 is our target commit, what can we do to recover the work? There are a lot of possibilities.


















Problem

Use our knowledge of how git plumbing works to retrieve the contents of a commit, use this super power to grab out the file baz.md


















Solution

To get the information we need we will have to use the commit to get the tree sha and the tree sha for the blob, file baz.md, contents



# Step 1, get the file tree try cat-file'ing the commit sha
➜  hello-git git:(trunk) git cat-file -p f330d23
tree d2d8e10a88b4e985003930d45c5c488abe712e6b # <-- the tree
parent b23e6320e6fba64d93338543dcbcdcc9caadb71e
author ThePrimeagen <ThePrimeagen@netflix.com> 1707069560 -0700
committer ThePrimeagen <ThePrimeagen@netflix.com> 1707069560 -0700

Baz

# Step 2, print out the trees content.  baz.md is our taget file
➜  hello-git git:(trunk) git cat-file -p d2d8e10
100644 blob d3045e2c2d21fcf774700b3d5fa681cf26b300ad    README.md
100644 blob 16858db7afb62f3e027d8f9379085d3567bcac62    bar.md
100644 blob 76018072e09c5d31c8c6e3113b8aa0fe625195ca    baz.md # <-- target file

# Step 3, print the file and copy the contents!
➜  hello-git git:(trunk) git cat-file -p 7601807 > baz.md


Pure wizardry


















Problem

Lets not use the internals, what is another way, using the commands we have gone over thus far to get the same information. We flexed... but we didn't have to.



State the change, don't perform it


















Solution

Remember a branch is just a pointer to a commit



➜  hello-git git:(trunk) git merge f330d23
Updating b23e632..f330d23
Fast-forward
 baz.md | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 baz.md


I can just merge that change directly into trunk!



Warning

If histories have diverged far enough, this could cause some problems as you wouldn't just be merging the one commit, but all the commits in betwixt f330d23 and trunk


















Cherry Pick

from man git-cherry-pick

Given one or more existing commits, apply the change each one introduces,
recording a new commit for each. This requires your working tree to be clean
(no modifications from the HEAD commit).

















Problem

Git cherry-pick a try and see if you can also get the changes from baz into trunk


















Solution

➜  hello-git git:(trunk) git cherry-pick f330d23
[trunk e6d9d4b] Baz
 Date: Sun Feb 4 10:59:20 2024 -0700
 1 file changed, 1 insertion(+)
 create mode 100644 baz.md


I have used cherry pick a ton of times throughout my career as a software dev



Draw excalidraw the downsides of merge vs cherry-pick