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title: "Git Introduction"
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date: 2021-04-14T18:54:16+02:00
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<!-- This: is way too large -->
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<!-- image: /images/git-logo.png -->
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categories:
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- misc
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- git
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- software
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tags:
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- english
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- software engineering
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- tools
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summary: This post will help you to get started with using and understanding git!
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showToc: false
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draft: true
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---
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Currently, I'm working on my slides for this semester's tutorials for our Software Engineering course.
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I aim to give the students a good introduction to git, as it is an integral part of our toolchain.
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So I decided to share my thoughts on what a beginner should know about git here.
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I'm probably going to expand on this post in later posts.
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---
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<center><i>Source: <a href="https://xkcd.com/1597/">xkcd.com</a></i></center>
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## What is it?
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So, what is Git? The xkcd already implies that git is a collaborative, distributed source control system.
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But that is a rather abstract definition. Let's simplify a bit for now. Git tracks changes to your files.
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Essentially Git keeps different versions of your files around, so you can compare them and refer to old versions.
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This is really useful, as it makes it hard for you to lose working code, and easy to compare a broken version to a
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working one, to find the culprit.
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## So how do we use it?
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The comic above is not wrong. A lot of git comes down to remembering different commands (or button locations for that matter).
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The first thing one needs to do, is to prepare a Repository. A repository encapsulates everything git tracks. So you move to
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your project directory and type:
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```bash
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$ git init
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Initialized empty Git repository in $project_dir/.git
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```
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Git now told you exactly what it did. It prepared a folder for the data it has to track. We now refer to the $project_dir
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as the working directory, and $project_dir/.git as the repository. Let's create some files, track some changes and take a
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look at how we can interact with those.
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```bash
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$ echo "our first change" > file_one.txt
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$ git status
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On branch master
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No commits yet
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Untracked files:
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(use "git add <file>..." to include in what will be committed)
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file_one.txt
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nothing added to commit but untracked files present (use "git add" to track)
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```
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So, we created a new file with the content "our first change". `git status` is used to get information about the current
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state of the working directory and repository. It currently tells us, that we have not committed anything at all, and that
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there is a untracked file. Let's stage it. Staging means that we tell git to *prepare* that file for a snapshot in its current
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version.
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```bash
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$ git add file_one.txt
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$ git status
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On branch master
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No commits yet
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Changes to be committed:
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(use "git rm --cached <file>..." to unstage)
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new file: file_one.txt
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```
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Git now tells us that there are no unstaged chanes, as well as no commits. And that we have prepared a new file for commit.
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So let's create our first commit. A commit basically is a snapshot of our files, frozen in time. We'll not concern ourselves
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with the semantics of a good commit for now.
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A commit consists of a list of files, their contents, author information, and a message describing the commit.
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```diff
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$ git commit -m "This is our first commit"
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$ git show
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commit cf0475067740275c6469836ff743855fcf4e4f85 (HEAD -> master)
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Author: Tobias Manske <tobias.manske@example.com>
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Date: Wed Apr 14 19:37:00 2021 +0200
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This is our first commit
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diff --git a/file_one.txt b/file_one.txt
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new file mode 100644
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index 0000000..103f365
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--- /dev/null
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+++ b/file_one.txt
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@@ -0,0 +1 @@
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+our first change
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```
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`git show` shows us what our last commit did. In my case it is filled in with my email and my name for author information.
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The commit has the id cf04... and is currently the commit the master branch points to (more on that later).
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In the bottom part we see a so called diff of the state before the commit and the new state. We can obtain more information
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about the history with the `git log` command. Without further options it just prints the top part of `git show` for every
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commit in the history. Before we can explore that further, we need some more changes. I'll not go into detail here. Just
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repeat what we did before. Change stuff, add it to git and commit it.
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```diff
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$ git log
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commit 1849c7644425684cea7039e825168d92df4e6936
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Author: Tobias Manske <tobias.manske@example.com>
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Date: Wed Apr 14 19:57:38 2021 +0200
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remove a file
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commit 4db54105f028d23339bbd1eb51fb397ab0d45719
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Author: Tobias Manske <tobias.manske@example.com>
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Date: Wed Apr 14 19:57:23 2021 +0200
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Changes some more stuff
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commit cf0475067740275c6469836ff743855fcf4e4f85
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Author: Tobias Manske <tobias.manske@example.com>
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Date: Wed Apr 14 19:37:00 2021 +0200
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This is our first commit
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```
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As we can see, this is our history. It can be rather cumbersome to look at it this way, let's use some options to make
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it easier to digest:
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```bash
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$ git log --oneline
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1849c76 remove a file
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4db5410 Changes some more stuff
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cf04750 This is our first commit
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```
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This is a lot more concise, but lacks quite a lot of information. I have a shorthand `git lg` in
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[my configuration](https://git.tobiasmanske.de/user/rad4day/public/dotfiles/tree/.gitconfig)
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which would show (colored, not like it's shown here):
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```bash
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$ git lg
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* 1849c76 - (4 minutes ago) remove a file - Tobias Manske (HEAD -> master)
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* 4db5410 - (4 minutes ago) Changes some more stuff - Tobias Manske
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* cf04750 - (25 minutes ago) This is our first commit - Tobias Manske
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```
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The left most column is a abbreviation of the commit hash (that long number we've seen before) that git uses to identify
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and distinguish commits. The length of the abbreviation will adjust if git ever needs more symbols to distinguish two commits.
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## Looks complicated, can you explain this again?
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I will! With a graphic. Below you can see what happened step-by-step, when we committed.
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You can see 4 columns, which are the 4 states our repository was in; every commit changes the state from left to right.
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In the beginning we had no commits, then we created our first commit and git placed the HEAD-Pointer at it (more on that later).
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Each new commit has a parent-pointer to its ancestor and is now the location of our HEAD. This pattern continues till we made
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our last commit.
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## So how do I access my data?
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Until now, we just stored data into git. That's nice to know, but how could we recover code we deleted later on?
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For example: How do we get back our initial version of file_one.txt, how do we look at it? How can we replace our current
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version with it?
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