Introduction to git

What is Git, and what is a Git repository?

Git is a version control system: can record snapshots and track the content of a folder as it changes over time.
Every time we commit a snapshot, Git records a snapshot of the entire project, saves it, and assigns it a version.
These snapshots are kept inside a sub-folder called .git.
If we remove .git, we remove the repository and history (but keep the working directory!).
.git uses relative paths - you can move the whole thing somewhere else and it will still work
Git doesn’t do anything unless you ask it to (it does not record anything automatically).
Multiple interfaces to Git exist (command line, graphical interfaces, web interfaces).

Recording a snapshot with Git

Git takes snapshots only if we request it.
We will record changes always in two steps (we will later explain why this is a recommended practice):

$ git add somefile.txt
$ git commit

$ git add file.txt anotherfile.txt
$ git commit

We first focus (git add, we “stage” the change), then shoot (git commit):

Git staging

What do you think will be the outcome if you stage a file and then edit it and stage it again, do this several times and at the end perform a commit? (think of focusing several scenes and pressing the shoot button only at the end)

Before we start we need to configure Git

Also see instructions here

$ git config --global user.name "Your Name"
$ git config --global user.email yourname@example.com
$ git config --global core.editor your-editor

your-editor: vim, emacs, nano, any other of your choice

Type-along: Tracking a guacamole recipe with Git

We will learn how to initialize a Git repository, how to track changes, and how to make delicious guacamole!

This example is inspired by Byron Smith.

Let us start. One of the basic principles of Git is that it is easy to create repositories:

$ mkdir recipe
$ cd recipe
$ git init

That’s it! We have now created an empty Git repository.

We will use git status a lot to check out what is going on:

$ git status

On branch master

No commits yet

nothing to commit (create/copy files and use "git add" to track)

We will make sense of this information during this workshop.

Let us now create two files.

One file is called instructions.txt and contains:

* chop avocados
* chop onion
* squeeze lime
* add salt
* and mix well

The second file is called ingredients.txt and contains:

* 2 avocados
* 1 lime
* 2 tsp salt

As mentioned above, in Git you can always check the status of files in your repository using git status. It is always a safe command to run and in general a good idea to do when you are trying to figure out what to do next:

$ git status

On branch master

No commits yet

Untracked files:
  (use "git add <file>..." to include in what will be committed)

	ingredients.txt
	instructions.txt

nothing added to commit but untracked files present (use "git add" to track)

The two files are untracked in the repository (directory). You want to add the files (focus the camera) to the list of files tracked by Git. Git does not track any files automatically and you need make a conscious decision to add a file. Let’s do what Git hints at and add the files:

$ git add ingredients.txt
$ git add instructions.txt
$ git status

On branch master

Initial commit

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)

	new file:   ingredients.txt
	new file:   instructions.txt

Now this change is staged and ready to be committed.

Let us now commit the change to the repository:

$ git commit -m "adding ingredients and instructions"

[master (root-commit) aa243ea] adding ingredients and instructions
 2 files changed, 8 insertions(+)
 create mode 100644 ingredients.txt
 create mode 100644 instructions.txt

Right after we query the status to get this useful command into our muscle memory:

$ git status

What does the -m flag mean? Let us check the help page for that command (to get out of the help mode press ‘q’):

$ git help commit

You should see a very long help page as the tool is very versatile. Do not worry about this now but keep in mind that you can always read the help files when in doubt. Searching online can also be useful, but choosing search terms to find relevant information takes some practice and discussions in some online threads may be confusing. Note that help pages also work when you don’t have a network connection!

Git history and log

Now try git log:

$ git log

commit 49baa1f9a7be6422090c4a8a10694a6708c2ff41 (HEAD -> master)
Author: gosia olejniczak <gosia.olejniczak@gmail.com>
Date:   Tue Jan 28 08:39:07 2020 +0100

    adding ingredients and instructions

We can browse the development and access each state that we have committed.
Output is in reverse chronological order, i.e. newest commits on top.
The long hashes uniquely label a state of the code (here: 470bb4706fafcd9981514db7b09e9147fd20151b).
If the first characters of the hash are unique it is not necessary to type the entire hash.
We will use them when comparing versions and when going back in time.
git log --oneline only shows the first 7 characters of the commit hash and is good to get an overview.
git log --stat is nice to show which files have been modified.

Exercise: record changes

Add 1/2 onion to ingredients.txt and also the instruction to “enjoy!” to instructions.txt. Do not stage the changes yet.

When you are done editing the files, try git diff:
$ git diff
diff --git a/ingredients.txt b/ingredients.txt
index 2607525..ec0abc6 100644
--- a/ingredients.txt
+++ b/ingredients.txt
@@ -1,3 +1,4 @@
 * 2 avocados
 * 1 lime
 * 2 tsp salt
+* 1/2 onion
diff --git a/instructions.txt b/instructions.txt
index 3eda565..f7dd63a 100644
--- a/instructions.txt
+++ b/instructions.txt
@@ -3,4 +3,4 @@
 * squeeze lime
 * add salt
 * and mix well
+* enjoy!
Now first stage and commit each change separately (what happens when we leave out the -m flag?):
$ git add ingredients.txt
$ git commit -m "add half an onion"
$ git add instructions.txt
$ git commit                   # <-- we have left out -m "..."
When you leave out the -m flag, Git should open an editor where you can edit your commit message. This message will be associated and stored with the changes you made. This message is your chance to explain what you’ve done and convince others (and your future self) that the changes you made were justified. Write a message and save and close the file.

When you are done committing the changes, experiment with these commands:
$ git log    # show commit logs
$ git show   # show various types of objects
$ git diff   # show changes

Writing useful commit messages

Using git log --oneline we understand that the first line of the commit message is very important.

Good example:

increase threshold alpha to 2.0

the motivation for this change is
to enable ...
...

Convention: one line summarizing the commit, then one empty line, then paragraph(s) with more details in free form, if necessary.

Bad commit messages: “fix”, “oops”, “save work”, “foobar”, “toto”, “qppjdfjd”, “”.
http://whatthecommit.com
Write commit messages in English that will be understood 15 years from now by someone else than you.
Many projects start out as projects “just for me” and end up to be successful projects that are developed by 50 people over decades.
Commits with multiple authors

Good references:

Ignoring files and paths with .gitignore

Should we add and track all files in a project?
How about generated files?
Why is it considered a bad idea to commit compiled binaries to version control?
What types of generated files do you know?

As a general rule compiled files are not committed to version control. There are many reasons for this:

Your code could be run on different platforms.
These files are automatically generated and thus do not contribute in any meaningful way.
The number of changes to track per source code change can increase quickly.
When tracking generated files you could see differences in the code although you haven’t touched the code.

For this we use .gitignore files. Example:

# ignore compiled LaTeX files
*.log *.out *.pdf 
# ignore compiled python 3 files
__pycache__
# ignore everything under build directory
build/

More about the syntax used to determine files to ignore is in the documentation.

You can have .gitignore files in lower level directories and they affect the paths relatively.

.gitignore should be part of the repository (why?).

Good practice: maintain clean working area

Use git status a lot.
Untracked files belong to .gitignore.
All files should be either tracked or ignored.

Summary

Now we know how to save snapshots:

$ git add <file(s)>
$ git commit

Every state is then saved and later we will learn how to go back to these “checkpoints” and how to undo things.

$ git init    # initialize new repository
$ git add     # add files or stage file(s)
$ git commit  # commit staged file(s)
$ git status  # see what is going on
$ git log     # see history
$ git diff    # show unstaged/uncommitted modifications
$ git show    # show the change for a specific commit
$ git mv      # move tracked files
$ git rm      # remove tracked files

Git is not ideal for large binary files (for this consider http://git-annex.branchable.com).

Test your understanding

Which command(s) below would save the changes of myfile.txt to my local Git repository?
$ git commit -m "my recent changes"
$ git init myfile.txt
$ git commit -m "my recent changes"
$ git add myfile.txt
$ git commit -m "my recent changes"
$ git commit -m myfile.txt "my recent changes"
Solution

Would only create a commit if files have already been staged.

Would try to create a new repository.

Is correct: first add the file to the staging area, then commit.

Would try to commit a file “my recent changes” with the message myfile.txt.

Graphical user interfaces

Here we worked via command line, but it is also possible to work from within a Git graphical user interface (GUI):

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