Basics#

Objectives

  • Learn to create Git repositories and make commits.

  • Get a grasp of the structure of a repository.

  • Learn how to inspect the project history.

  • Learn how to write useful commit log messages.

Instructor note

  • 35 min teaching/type-along

  • 20 min exercise

What is Git, and what is a Git repository?#

  • Git is a version control system: can record/save 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).

  • Example (we don’t need to type yet):

    $ 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

Git staging and committing.#

Question for the more advanced participants

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 shutter at the end)

Before we start we need to configure Git#

If you haven’t already configured Git, please follow the instructions in the installation instructions.

Instructor note

Instructors might want to show how to set email address and editor:

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

Verify with:

$ git config --list

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 (based on a discussion in the Carpentries mailing list). The motivation for taking a cooking recipe instead of a program is that everybody can relate to cooking but not everybody may be able to relate to a program written in e.g. Python or another language.

Let us start.

Instructor note

Instructors, please encourage now that participants type along.

One of the basic principles of Git is that it is easy to create repositories:

$ mkdir recipe
$ cd recipe
$ git init

That’s it! With git init 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:

$ git help commit

You should see a very long help page as the tool is very versatile (press q to quit). 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!

Exercise: Record changes#

Basic-1: Record changes

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

When you are done editing the files, try git diff:

$ git diff

You will see (can you identify in there the two added lines?):

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 6a8b2af..f7dd63a 100644
--- a/instructions.txt
+++ b/instructions.txt
@@ -3,3 +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
$ git log --stat
$ git log --oneline

Git history and log#

If you haven’t yet, please try now git log:

$ git log

commit ea73a8a9ef9bd5c0aee3f0868d0ebc8457e1f94b
Author: Radovan Bast <bast@users.noreply.github.com>
Date:   Wed Feb 9 10:12:39 2022 +0100

    don't forget to enjoy

commit 6316947beb2b7ffa0cc822370cfdd701050227a4
Author: Radovan Bast <bast@users.noreply.github.com>
Date:   Wed Feb 9 10:12:30 2022 +0100

    add half an onion

commit 03491ed253ffcee16772e35b1de4efd349711f05
Author: Radovan Bast <bast@users.noreply.github.com>
Date:   Wed Feb 9 10:11:30 2022 +0100

    adding ingredients and instructions

  • We can browse the development and access each state that we have committed.

  • The long hashes uniquely label a state of the code.

  • They are not just integers counting 1, 2, 3, 4, … (why?).

  • Output is in reverse chronological order, i.e. newest commits on top.

  • 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.

  • If the first characters of the hash are unique it is not necessary to type the entire hash.

  • git log --stat is nice to show which files have been modified.

Optional exercises: comparing, renaming, and removing#

(optional) Basic-2: Comparing and showing commits

  1. Inspect differences between commit hashes with git diff <hash1> <hash2>.

  2. Have a look at specific commits with git show <hash>.

(optional) Basic-3: Visual diff tools

  • Make further modifications and experiment with git difftool (requires installing one of the visual diff tools):

On Windows or Linux:

$ git difftool --tool=meld <hash>

On macOS:

$ git difftool --tool=opendiff <hash>
Git difftool using meld

Git difftool using meld.#

You probably want to use the same visual diff tool every time and you can configure Git for that:

$ git config --global diff.tool meld

(optional) Basic-4: Renaming and removing files

  1. Create a new file, git add and git commit the file.

  2. Rename the file with git mv (you will need to git commit the rename).

  3. Use git log --oneline and git status.

  4. Remove the file with git rm (again you need to git commit the change).

  5. Inspect the history with git log --stat.

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 ...
...
this is based on a discussion in #123

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

  • Why something was changed is more important than what has changed.

  • Cross-reference to issues and discussions if possible/relevant.

  • Bad commit messages: “fix”, “oops”, “save work”

  • Bad examples: http://whatthecommit.com

  • Write commit messages in English that will be understood 15 years from now by someone else than you. Or by your future 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 are possible.

Good references:

Note

A great way to learn how to write commit messages and to get inspired by their style choices: browse repositories of codes that you use/like:

Some examples (but there are so many good examples):

When designing commit message styles consider also these:

  • How will you easily generate a changelog or release notes?

  • During code review, you can help each other improving commit messages.

But remember: it is better to make any commit, than no commit. Especially in small projects. Let not the perfect be the enemy of the good enough.

Ignoring files and paths with .gitignore#

Discussion

  • 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?

Compiled and generated 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 python 2 files
*.pyc
# ignore compiled python 3 files
__pycache__

An example taken from the official Git documentation:

# ignore objects and archives, anywhere in the tree.
*.[oa]
# ignore generated html files,
*.html
# except foo.html which is maintained by hand
!foo.html
# ignore everything under build directory
build/

  • .gitignore should be part of the repository because we want to make sure that all developers see the same behavior.

  • All files should be either tracked or ignored.

  • .gitignore uses something called a shell glob syntax for determining file patterns to ignore. You can read more about the syntax in the documentation.

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

Graphical user interfaces#

We have seen how to make commits directly via the GitHub website, and also via command line. But it is also possible to work from within a Git graphical user interface (GUI):

Summary#

Now we know how to save snapshots:

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

And this is what we do as we program.

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).

Basic-5: Test your understanding

Which command(s) below would save the changes of myfile.txt to an existing local Git repository?

  1. $ git commit -m "my recent changes"

  2. $ git init myfile.txt
$ git commit -m "my recent changes"

  3. $ git add myfile.txt
$ git commit -m "my recent changes"

  4. $ git commit -m myfile.txt "my recent changes"

Keypoints

  • Initializing a Git repository is simple: git init.

  • Commits should be used to tell a story.

  • Git uses the .git folder to store the snapshots.

  • Don’t be afraid to stage and commit often. Better too often than not often enough.