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Git - Master

This guide is intended as a primer for understanding what master is. You should have previously read about commits.

Master

So far we've create a few commits. Let create another.

> echo "goodbye" >> file.txt

# Add the new contents
> git add .

> git commit -m "Third commit"

[master 41ce7fa] Third commit
 1 file changed, 1 insertion(+)

So what is this master exactly? You probably already know that it's a branch, but what is that?

Let's take a detour.

Graph

Git has a very "rich" (read: confusing as hell) command line. In general I want to avoid throwing around 10 different versions of a command, that is probably going to be confusing at this stage. log alone has about 30 options, many of which can be very useful. The ones I want to show now look like this:

> git log --graph --oneline --decorate 41ce7fa

* 41ce7fa (HEAD, master) Third commit
* 672e562 Second commit
* 406bb3b Initial commit

What does each of those options do? Can you use different combinations of them?

So this contains one extra bit of Git terminology we haven't seen before. HEAD.

HEAD

Understanding HEAD is critical to understanding how Git really works, so let's take a second to dig a little deeper.

Way back in the beginning we looked at the contents of the .git directory. Let's do that again:

> ls -F .git

COMMIT_EDITMSG
HEAD
branches/
config
description
hooks/
index
info/
logs/
objects/
refs/

Interesting, there's a file called HEAD. Let's take a look.

# The `cat` command prints the contents of a file on the terminal
> cat .git/HEAD

ref: refs/heads/master

So we should recognise the master part, but what is refs/heads/?

Let's keep digging.

Refs

You'll also notice there is a refs/ directory in .git.

> ls -F .git/refs

heads/
tags/

Ignore tags, let's focus on heads.

> ls -F .git/refs/heads

master

And at the end of the rabbit hole?

> cat .git/refs/heads/master

41ce7faa5b8b2cf480d44c3b12f0545fed9683c3

That's the most recent commit! So HEAD is just a reference to refs/heads/master, which in turn is just a reference to a specific commit, which captures a collection of files.

Something like this:

HEAD -> refs/heads/master -> 41ce7fa -> file.txt

Actually that looks a little like our log --graph command.

> git log --graph --oneline --decorate 41ce7fa

* 41ce7fa (HEAD, master) Third commit
* 672e562 Second commit
* 406bb3b Initial commit

It would appear that refs/heads/master is just the long winded way of saying master. They're the same thing.

Perhaps a slightly more obscure point is that a branch is just a "head" ref. Said another way - everything under refs/heads is a branch. We look at some other types of refs later on, but for now we might just focus on heads.

Can you use HEAD, master or even refs/heads/master in the log command?

What happens if you don't supply log a commit/ref, what does it default to?

If you make another commit what changes? Is it HEAD or the ref?

Next

Ok, that's probably enough for this section. Next we'll see what happens when we start creating multiple branches.