demo.md

A Simple Git Demo

Let's start with a simple demo. Begin by cloning a repository from GitHub. Just for grins, check out the source code for git itself.

time git clone https://github.com/git/git.git
cd git
du -sh .
ls

It's about 100 megabytes, with over 2500 files, so a reasonable-sized project. The clone it takes under half a minute. That seems fast enough.

But unlike earlier, Centralized Version Control Systems (CVCSs), we didn't check out a version of the code, we cloned the entire repository.

To emphasize that, let's first turn off networking, so we no longer have access to GitHub, where we cloned from.

sudo ifconfig en0 down # on a mac
ping -n github.com # no DNS
ping -n 4.2.2.2  # okay, it's down

Now let's try a few things. What's the history look like?

git log
git log | tail

That last one is the first version of git. Ever. We can tag it.

git tag initial e83c5163316f89bfbde7d9ab23ca2e25604af290
git checkout initial
ls

Eleven files.

git was created by Linus Torvalds to maintain the Linux kernel, so the earliest version is quite easy to build even a decade later, on modern Linuxes.

Here's what you need to change for Ubuntu 14.04

echo 'LIBS=-lcrypto -lz' >> Makefile
sudo apt-get install libssl-dev # in case it's not there
make

And, since it builds with that change, make a branch and commit the change on that branch

git checkout -b devonian initial
git status
git commit -am'Port to Ubuntu 14.04'

All this is local. You can checkout, paw through history, branch, tag, commit, and anything else you'd want to do with a version-control system, all on an airplane, in a coffee shop, or in your home while still in your pajamas.

Note: For a short talk, the intro can probably stop here.

So, if you want to jump in now, click here.

We can look for a version a week later and tag it.

git rev-list master -n 1 $(git rev-parse --before=2005-04-14)
git tag week-1 $(git rev-list master -n 1 $(git rev-parse --before=2005-04-14))
git rev-list --count week-1 # about 10 commits a day

He's doing about 10 commits a day. Over a decade later, that's still about the rate.

Merges? If we can do branches, we can do merges. For fun, let's look at the very first merge commit.

git show $(git rev-list --merges master | tail -1)
git log --oneline --graph --decorate $(git rev-list --merges master | tail -1)

All this completely disconnected from the net. Fast. Local. But how?

No reason to put understanding off any longer, so let's begin.