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Notes from Git for Ages Four and Up

Here are my notes from watching the video Git for Ages Four and Up. It contains a really good expanation of how Git works under the hood. The speaker (Michael G. Schwern <schwern@pobox.com>) created a new repository, and then created a new file, added some text to it, saved it, closed it, then added it to the Staging Area, and then committed it.

git add

  • Adds a file your working directory, to the staging area (the Index), as a BLOB.

git commit

  • Creates a “Commit” object from that BLOB.
    • That somehow points to the files that are stored in Git.
      • The commit contains all of the files in the repot – not just the ones that were just staged.
  • And attaches two labels (“References”) to it.
    • HEAD
      • The name of the most recent Commit.
      • Always points to the tip of the branch that you currently have checked-out.
    • MASTER
      • The name of the currently checked-out Branch.
      • The one that advances upon “git commit”.

When you create a branch

  • All Git does is attach a new branch label to the Commit currently pointed to by HEAD.

When you perform a “git checkout”

  • Git merely switches which branch it’s going to advance when you perform a “git commit”.
    • We say the branch becomes active.
  • And it moves HEAD to the commit object that has that branch label.
  • Git populates your Working Directory with a snapshot of the branches tip commit..

Commit ID

  • Content + Author + Date + Log + Previous Commit ID.
  • A hexadecimal UUID (Universally Unique Identifier).
  • Is a checksum (Hash)
  • Git calculates this value by running the bits in the BLOB through a function, which produces a thirty eight digit hexadecimal value.

Staging Area (Index)

  • Allows you to build up a commit.
  • You can add multiple files to the index, and then commit them (i.e., all files as one commit object).

The basic Git workflow

  • You isolate your work from everyone else’s (spawn a new branch to work in).
  • Do some work (modify one or more files).
  • Update from everyone else (pull from origin).
  • Commit your work to your local enlistment (commit).
  • Finally, you share your work with everyone else (push to origin).

Merge

  • git checkout your local master (feature/api_doc).
    • You’ll be bringing your work back into this branch.
  • Git performed a Fast Forward.
    • Since no merging of content (all modifications added new content).
      • A merge object wasn’t created.
    • All Git did was move the branch label to the commit from the ‘from’ branch.
    • Git also moved the HEAD label, because you have that branch checked out.

git reset –hard HEAD^

  • This undoes the last commit.
  • But all it does is move reference labels!
  • Moves the current branch label (Master), back one commit to HEAD-1.
  • –hard does a checkout, so that you end up where you want to be.

git log –graph –decorate –all

git commit does not mean share (inflict).

  • Commit is a local operation.

Merge

  • Git can merge in many different ways.
  • It picks the most convenient way.
  • Fast Forward is the simplest.

Working with others

  • This is where remote repositories comes in.
    • remote commands.
      • Push, Fetch, and Pull.

Clone

  • You normally start working on a clone of an existing repository.
  • When you clone a repository, git attaches labels to the objects in your local repository to mark the tracking branches.
  • origin/MASTER
  • Git clones only origin/MASTER (the default branch).

Directed Acyclic Graph

  • Graph – a collection of nodes and edges.
  • Directed – the edges all point to the previous node.
  • Acyclic – there are no cycles. I.e., the path from one commit to any other is unique.

Pull

  • Is actually a Fetch followed by a Merge.
  • When you’re starting out, it’s often easier to do a pull as two steps (a fetch followed by a merge).

We’re going to create a third repository

  • So we can have someone other than us, doing work.

We’ll create a new branch called “bugfix”

  • And we’ll create a new file in it called “baz”.

git push origin bugfix

  • Sometimes you have a space between “origin” and “bugfix”, and sometimes you have a slash between them.
    • The slash version refers to the label (origin/bugfix) on your repository. This is a reference to a tracking branch.
    • The space version refers to the remote (origin) branch (bugfix).

Push

  • Does two things.
    • Git checks to see if the remote repository has the commit object your pushing.
      • If it doesn’t, then it checks to see if it has the ancestor commit object(s).
      • If it does, then
        • Git sends over the commit object (plus a branch label, if it’s on a new branch).
      • If it doesn’t, then
        • Git send all of the ancestors + the commit object.
  • This is where the cool thing about IDs comes in (this is why git is so amazingly fast).
    • Every ID is unique.
    • Every commit is unique.
    • Commits never change.
    • This means that every commit can be uniquely identified by its ID.
    • Since IDs contain the ID of the previous commit,
      • Every commit’s history can be uniquely identified by its ID.
      • Two devs push to the same repo without pulling…
        • If I have ae123, and you have ae123,
          • Then we know everything from ae123 on down is exactly the same.

Tag

  • Tags are good to use when you continually jump back to a particular snapshot.
  • E.g., git tag v1.0 bfce7
  • All Git does is add a label (reference) to the commit object you want to tag.
  • Excep that tag labels never move.

git reflog

  • This is your rescue command.
  • You can use this command when something gets screwed up, to find a particular commit ID.
  • Git shows you the commits you’ve been working on lately.

You can combine git add and git commit into one step

E.g., git commit -a foo.

Rebase

  • This is a local operation.
  • Example use: instead of fixing a typo and then committing the file again, you can perform a rebase instead.
  • Git rebase doesn’t rewrite history. It writes new history (you can’t actually change history in Git).
    • Instead, git creates a whole new line of history.
  • Interactive rebase.
    • Go back two commits.
      • git rebase -i HEAD^^
      • Git says “I’m going to replay these things as if they were patches, one on top of the other.
        • Nothing happened! (because nothing changed).
      • You can “squash” a later commit into a previous commit (so the typo fix you made is written on top of the version with the typo).
        • I.e., you combine two (in this case) commits into a new commit.
        • The original two commits become detached!

Note: Once you have pushed, don’t rebase or you’ll screw up everyone else’s history! So never rebase until after you’ve pulled!

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