Git 101

My early professional career required that I knew how to do six things in git: branch, stage, commit, merge, push and pull. Beyond that there was always google. And of course that stack overflow page that everyone stumbles on eventually: if I effed something up there was git reset --hard HEAD, and if I really effed it up I could do git reset --hard HEAD~. Or was it the other way round?

To my surprise now, I got a lot of leverage out of just those six (or seven) commands. But that was probably because no-one else really minded what I was doing. We committed to master and dealt with problems as they came up. No-one read the history. We pushed to a gitolite server, which, as great as that is, is so far away from the world of GitHub that to any novice it was something of a black box. Code got committed and pushed. Who knows what happened after that? If something broke, it meant doing more committing and pushing.

Fortunately for me this didn’t last for too long. I decided at some point that I needed to understand git a little better.

Now, I still don’t consider myself an expert in any way. I did give a talk on the subject at work recently which I enjoyed, and wanted to summarize more formally the contents of that here. So here it is. Something like the guide I wish I had read a couple years ago to get me through the git 101 blues. It will cover:

• Some standard and some not-so-standard terms
• How to write a better commit message (that old chestnut)
• How to make better commits
• Some ways to configure git to make your life easier
• What the hell rebasing is
• A few odd parts of git’s syntax
• Some lesser-used tools that you might like

Terms!

First of all, let’s define a few terms. I won’t define every term, just a few that are either vague or that I will use frequently throughout.

Private branch

A branch that is used by just you. Pushing it to a remote does not necessarily make it public.

Public branch

A branch that is shared (read: committed to) by many.

HEAD

I always wondered if you were supposed to scream this. I might less formally refer to it as simply as the 'head' or 'tip'. It is simply the current revision of a given branch.

The graph

A lot can be said about the graph, and it's probably beyond the scope of this article to talk about this in any detail. Let's just say that a requirement for understanding git's internals is some rudimentary knowledge about graph theory. I really do mean rudimentary, so don't let that put you off. There is a great resource on explaining git in terms of graph theory here, which I would highly recommend.

In terms of graph theory, your git history is essentially a graph composed of commit 'nodes'. The commits at the HEAD of branches are your 'leaf' nodes. Your current revision in this sense refers to the series of changes (i.e. Commit nodes) that are 'reachable' (i.e. Pointed to by HEAD, or pointed to by commits that are pointed to by HEAD, and on and on).

Merge bubble

When you merge two branches, you will get a merge 'bubble' by creating a new commit in the target branch that retains the integrity of both branches. This is a special 'merge commit', and it's special because it points to two different commits in the history - the tip of the target (typically `master`) branch, and the tip of the topic branch. You wouldn't create this commit by hand, it will happen automatically depending on how you've set up your `.gitconfig`. Typically, if you're working on a team and you haven't configured git at all, or if you're using the github web interface to merge branches, you will end up with lots of merge bubbles.

Fast-forward

This is what happens when you merge without creating a merge bubble. Git will merge your changes in at the top of your target branch as if you had just been committing to it all along. No merge commit is created.

Squashing

This is a technique used for combining commits that have already been made into bigger, more consolidated ones.

Some committing anti-patterns

Here are some things that can generally go wrong:

• Putting everything into one big commit
• Writing an incomplete commit message
• Breaking something. Committing. Fixing it later.
• (More advanced) rebasing or committing in hunks without checking the state of each commit

One thing I learned early on was that it is a good idea to commit frequently. Unfortunately that’s not the whole story. Although it does address anti-pattern #1, it will often mean trading it for #2 or #3. Practicing TDD is actually conducive to making frequent, small commits because you’re concentrating on either getting to green (a requirement for a good commit) without getting distracted or writing more code than is needed, or refactoring in small steps. Essentially, it’s OK to do #2 or #3 as long as you’re working in a private branch and you squash or rewrite your commits before merging by performing an interactive rebase (more on this later).

Squashing everything isn’t necessarily a good idea either. The goal should be to be left with a small number of commits that each mark a distinct progression toward some goal (adding a new feature, refactoring, etc.). As you become more savvy with rebasing interactively you may fall prey to antipattern #4. In other words, when you’re rewriting history it’s important to check the integrity of each commit that you’re creating after the fact. If you really care about your history, and not just your HEAD, you’ll want every commit to be green and deployable.

There are actually a few reasons why you might want to take such care of your history. The first that comes to mind is being able to use git’s bisect feature with more confidence. bisect is a tool used for examing a portion of your history, typically for locating a commit that introduced some regression. It is a very powerful and useful tool that I’ve personally seen rendered completely useless by careless committing. More on bisect later.

Another reason might be being able to generate metrics for your application across a range of commits.

Another is simply being able to read your history with relative ease. This is more a comment on composing good commits with good commit messages. (Occasionally, for inspiration, I’ll go spelunking through the history of some open source software that I love, go right back to the first commit and rediscover the steps of creating its first complete feature.)

There are two rules I like to follow when composing a commit message. The first is to use the present tense imperative in the first line. The reason for this is that this is the tense/mood used in git’s generated messages such as on merge commits. A nice side effect of this is that you will probably find that your messages are shorter and succinter. The second rule is never to use the -m flag. Trying to fit your entire message onto the first line is just way too much pressure! How formal you want to get with your message after that is up to you. Generally it’s a good idea to have a short, descriptive first line, followed by a longer description and a link to an issue number or ticket if one exists. I add thoughtbot’s template to help remind me:

# ~/.gitconfig
[commit]
  template = ~/.gitmessage

# ~/.gitmessage


# 50-character subject line
#
# 72-character wrapped longer description. This should answer:
#
# * Why was this change necessary?
# * How does it address the problem?
# * Are there any side effects?
#
# Include a link to the ticket, if any.

More on your gitconfig

There are a couple more things that you may want to consider adding or tweaking in your gitconfig. Often you’ll see official advice telling you to use the git command line interface to accomplish this, but I prefer to edit my ~/.gitconfig by hand.

Here are a few things I recommend playing with:

[alias]
  a = add
  br = branch
  ci = commit
  co = checkout
  st = status

These are a few simple and common aliases that have become more or less standard (see that kernel wiki article for others). I won’t enumerate all the ones I use here, but feel free to check out my dotfiles. Aliasing is essential to being productive if you’re interacting with git at the command line. Feel free to create aliases in your ~/.bashrc too. Alias git to g, and more common commands such as git status to gs. It might seem trivial at first, but if you type git status about 200 times a day as do I, you are going to be saving quite a few keystrokes by the end of the week. And that’s time you could be spending thinking about your design, or even going for a walk in the park.

[merge]
  ff = only

This is useful if you don’t want git to create a merge bubble unless specifically asked to do so. If your branch can’t be fast-forwarded, it won’t be merged either until you rebase, or you pass a flag overriding the above.

[branch]
  autosetuprebase = always

Useful if you are using a rebase-style workflow (more below). With this set, if you pull from an upstream on a branch where you have revisions that have not yet been pushed, your unpushed revisions will get shoved to the front, and no merge commit is made.

Rebasing

If you only learn one thing beyond the git 101 stage it should probably be this. Never rebase a public branch! Now, I don’t like making hard and fast rules with exclamatory remarks like that, particularly because I think they contribute to the fear and trepidation that surrounds rebasing, and the reluctance to use git’s most powerful feature. Please don’t let that put you off. It really is the only thing you need to remember. Everything else is easy to fix =)

Linus Torvalds has said that all of git can be understood in terms of rebase. But I think there’s another command that helps illuminate even further: the cherry-pick.

This is what a cherry-pick looks like:

$ git cherry-pick <commit>

What it does is apply the changes introduced by a given commit anywhere else in your history to the tip of your current branch. You can tell it to apply it somewhere else if you want, but that’s what it does with no other args. If that sounds confusing, or if you’ve never really thought about git in those terms, go back and read that a couple of times.

cherry-pick is sort of the basic unit of a rebase. The difference is with rebase you’re saying: take this series of commits and replay them all, starting at another point in history.

This is what a rebase looks like:

# rebase against local master
$ git rebase master

# rebase against remote master
$ git fetch origin
$ git rebase origin/master

With interactive rebasing you have even more control over how to rewrite history. You can take commits out, shuffle them around, squash commits into other commits, stop the replay right in the middle and change something and continue where you left off. Powerful stuff.

This is what an interactive rebase looks like:

$ git rebase -i master

There are (at least) two distinct benefits that you get from rebasing. One is that you can introduce any upstream changes into your code, address any breakages or refactoring that can be done, then merge all your changes directly onto the tip of master, without a merge ‘bubble’, as if you had just written them in some kind of coding frenzy. The other is that you can commit however you want while you’re developing, and then go back and recompose your commit history into a string of coding pearls, squashing smaller changes, typos and errors, and writing beautiful commit messages with love and care.

One thing you might notice is that if you were pushing your topic branch before you rebased, when you try to push after the remote will refuse (and complain about it, too). This is normal and to be expected. It just means that you have to ‘force’ push your branch.

The reason for this is that you changed history by rebasing. Now, these words are often thrown around, but you might find that explanation to be a little vague. And rightfully so.

Here’s what’s really going on: when you rebase a branch onto another commit, you take that first commit you made when you first branched off and point it to a different commit. Doing so actually creates a new commit with a distinct SHA1 hash (what a commit points to is an essential part of the ‘content’ of a commit), and points HEAD to it. Your original commit is still there, it’s just not visible in your log any more because it’s not reachable from HEAD.

The next commit in your project branch is now pointing at this ‘ghost’ commit. It needs to be updated to point to its new parent. The process begins again. A new commit is created, HEAD is moved, and on and on. As the rebase replays all your changes, it effectively changes every commit hash in the branch. Your local branch and origin now have two different copies of the same changes but none of the hashes is the same. This is why git gives you the somewhat confusing indication to pull your changes down before trying to push. What you need to do instead is tell the remote to forget everything and just accept your local branch in place of whatever it has. And that looks like this:

$ git push -f origin <branch>

Some useful things to know

Reflog

For the longest time I held the reflog at arm’s length. I knew it existed and that it could be of help if you were in serious trouble. Maybe there was some security in thinking that if I managed never to use it then I could never have done anything that bad.

But I was wrong. The reflog is actually exciting, powerful and pretty straightforward.

$ git reflog
$ git reflog show <branch>

This will show you something that looks like this:

e58096a HEAD@{0}: commit: Really committed now.
5a4acd2 HEAD@{1}: commit: Commitment issues.
6f10f0e HEAD@{2}: commit: Committing some more.
146778b HEAD@{3}: commit: The awkward second commit.
8838e8d HEAD@{4}: commit: Initial commit.

It’s possible that some of the commits the reflog will show you will no longer be reachable on the graph (such as after a rebase). Want to undo a rebase? Just point HEAD to where it was before you started by using reset (more below).

Ranges

Ranges, which is to say the .. and ... syntax, can be pretty confusing because they can mean different things in different contexts. It’s important to know how to use them, though.

In the context of logs:

# git log
# commits that b has that a doesn't have
$ git log <commit a>..<commit b>

# commits in a and b but not both
$ git log <commit a>...<commit b>

# the last n commits
$ git log -<n>

In the context of diffs:

# git diff
# changes between commit a and commit b
$ git diff <commit a> <commit b>

# same
$ git diff <commit a>..<commit b>

# changes that occurred on a's branch since it branched off of b's
$ git diff <commit a>...<commit b>

in the context of checking out:

# git checkout
# checkout the merge base of a and b
$ git checkout <commit a>...<commit b>

Commit Parents

Sometimes it can be easier to refer to commits not by their SHA1 hash but by their relationship with another commit. This is especially so when dealing with recent history and your point of reference is HEAD. There are a number of different ways of saying the same thing, and you can combine them too:

# the current commit
$ HEAD
$ HEAD~0

# the 1st parent of the current commit
$ HEAD~
$ HEAD~1

# the 1st parent of the 1st parent of the current commit
$ HEAD~~
$ HEAD~2
$ HEAD~1~1

# the 2nd parent of the current commit
$ HEAD^2

# uh...
$ HEAD~2^2~5^2

Add

You already know how to do that. But have you tried adding in hunks? It looks like this:

# stage changes in hunks
$ git add -p

This allows you to add interactively. Git will try to present you with smaller ‘hunks’ of your code to stage one by one. If it’s not granular enough for you, you can just tell git to get more granular by splitting it. Here’s what it looks like:

Stage this hunk [y,n,q,a,d,/,j,J,g,s,e,?]?

The most useful options to remember are y for yes, n for no, and s for split.

Bisect

This does a divide-and-conquer approach to locating a commit in your history that introduced some change (typically a regression). It requires only that can identify some point in your history that you know was good, and another point that is bad. Working with bisect will typically look like this:

# start it all off
$ git bisect start

# mark a known good commit
$ git bisect good <commit>

# mark a known bad commit
$ git bisect bad <commit>

# tell bisect the commit it checked out is good
$ git bisect good

# tell bisect the commit it checked out is bad
$ git bisect bad

You then repeat steps 4-5 until you’re down to one commit.

You can even automate the process:

# automate it
$ git bisect run rspec path/to/broken_spec.rb

Great stuff!

Blame

My FAVORITE tool. Mwahaha! In all seriousness though (ahem), this can be useful in situations where you have some code you really don’t understand despite your best efforts, and you need to have a chat with its author. Alternatively, you may want to credit someone for a revision that was really good. It looks like this:

$ git blame path/to/file

Revert

Creates a ‘mirror image’ of another commit that backs out the changes it introduced:

# create a new commit reversing the changes
$ git revert <commit>

You can even revert a merge commit by passing the -m flag and the parent that you want to keep. Typically this will just be 1, indicating master in situations where you merged a topic branch into it. The topic branch would be 2:

# revert a merge
$ git revert -m 1 <merge commit>

Reset

Something you may have used in desperation. Like rebase, reset is a powerful tool and it’s worth knowing what a few of the options do. Something all resets have in common is that they move HEAD to a new, specified commit. Unless you’re resetting to a point way back in history, it’s usually easier to provide a commit relative to HEAD. Here are a few options you want in your tool-belt:

# leave changes not in target in staging area
$ git reset --soft HEAD~

# leave changes not in target in working tree (default)
$ git reset --mixed HEAD~

# destroy all changes not included in target
$ git reset --hard HEAD~

# reset to previous point in the reflog
$ git reset --hard <branch>@{<reflog entry>}

# reset to where you were last week (!!!)
$ git reset --hard <branch>@{one.week.ago}

Conclusion

That’s more or less everything I know about being a git. There are some great resources, included below, that include more advanced topics if you’re interested in learning more. Being an intermediate git only really requires some curiosity and practice using the tools and techniques above. Once you get them, you’ll want to use most of them every day, and you’ll have internalized everything. And being an intermediate git won’t merely bring you up to scratch - it will actually set you apart from the rest (most of the time).