1.1 about this slideshow

If you’re viewing this on “gitolite.com”, please be aware that’s only an artifact of some very lazy DNS and hosting decisions I made. Just think of it as “sitaramc.github.com” instead, because these pages really have nothing to do with gitolite per se. They’re about git in general.

1.2 license

3. Copyright Sitaram Chamarty, [email protected]. This documentation is provided under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

1.3 viewing This slideshow

This presentation uses HTML Slidy, a simple presentation software from the W3C. Although there’s a help button in the footer of each presentation, it’s missing some important stuff, so here’s a smaller but more complete summary of the keyboard controls.

<To search> for stuff in the full document using your browser’s Ctrl-F, first view all slides (press the “A” key).

1.4 acknowledgements

this document

This document is vaguely inspired by https://eagain.net/articles/git-for-computer-scientists, except this page is not just for CS folks. And it’s a lot more detailed. Oh, and it’s actively maintained, meaning I will respond to feedback ;-)

this slide show

This slide show was all done with pandoc’s slidy output mode, and a few little pre-processing tweaks. Pandoc is absolutely fantastic and bloody powerful! I wish I’d discovered it sooner but now (Dec 2013) that I have, it’s eliminating a lot of my home-grown scripts to do various things like this!

The bulk of my tweaks are in terms of embedding graphviz diagrams inside the main document, so there’s only one file to maintain!

2.1 what is a SHA

A commit is uniquely identified by a 160-bit hex value (the ‘SHA’). This is computed from the following pieces of information:

• the SHA of the “tree” of files and directories as they exist in that commit (details on how that SHA is computed is not relevant here)
• the SHA of the parent commit(s) – every commit except the very first one in the repo has at least one parent commit that the change is based upon.
• the commit message – what you type in the editor when you commit
• the author name/email/timestamp
• the committer name/email/timestamp

In the end, as I said, it’s just a large, apparently random looking, number, which is actually a cryptographically-strong checksum. It’s usually written out as 40 hex digits.

Humans are not expected to remember this number. For the purposes of this discussion, think of it as something similar to a memory address returned by malloc().

It is also GLOBALLY unique! No commit in any repo anywhere in the world will have the same SHA. (It’s not a mathematical impossibility, but just so extremely improbable that we take it as fact. If you didn’t understand that, just take it on faith).

An example SHA: a30236028b7ddd65f01321af42f904479eaff549

2.2 what is a “ref”

A ref is just a name for a SHA. Branches or tags (described later) are refs.

There are also some special refs (the most common being HEAD), that are usually symbolic refs (i.e., they usually point to a ref instead of directly pointing to a SHA).

Anyway, for now just think of them as a human-readable name you gave to a SHA.

2.3 what is a repo

A repository (‘repo’) is a graph of commits. In our figures, we represent SHAs with numbers for convenience. We also represent time going upward (bottom to top).

a simple chain of commits

3.1 branch

Traditionally, the top of a linked list has a name. That name is a BRANCH name. We show branch names in green circles.

a branch

3.2 more than one branch

(a.k.a “more than one child commit”)

Remember we said a repo is a GRAPH? Specifically, more than one child node may be pointing at the same parent node. In this case, each ‘leaf node’ is a branch, and will have a name.

three branches

3.3 more than one parent commit

Well we can’t keep creating more branches without eventually merging them back. So let’s say “feature X” is now tested enough to be merged into the main branch, so you git merge feature_X. Here’s what you get: Notice that commit 8 now has 2 parents, showing that it is a “merge commit”.

a merge commit

3.4 current branch/checked out branch

There is a notion of a ‘currently checked out’ branch. This is denoted by a special ref called HEAD. HEAD is a symbolic ref, which points to the ‘current branch’.

HEAD

3.5 committing

When you make a new commit, the current branch moves. Technically, whatever branch HEAD is pointing to will move.

committing

3.6 naming non-leaf nodes

It’s not just ‘leaf’ nodes, but inner nodes can also have names. Recall the result of merging feature_X earlier (see the “more than one parent commit” section):

non-leaf