Lesson 2 - Using git for version control

Learning Objectives

Concepts - After completing this lesson, students will be able to:

• Explain the use of 'version control' in the context of a coding project

Skills - After completing this lesson, students will be able to:

• Clone, commit to, and push from a git repository

• Use the julia REPL to do basic arithmatic

Tasks - This lesson is complete when students have:

• Read Chapter: 1 from Think Julia

• Completed Assignment01

• Run all code examples from Lesson 2 on their own computers

If you've ever worked on an assignment and ended up with a list of files like

• assignment1.docx

• assignment1_v2.docx

• assignment1_v2_kevins_comments.docx

• assignment1_v3_fix_final.docx

• assignment1_v3_fix_final_for_real_this_time.docx

... you'll understand the importance of version control.

It goes well beyond naming of course. How can you tell what changed between version 1 and version2? Does version 3 take the comments Kevin made on v2 into account? Is ...fix_final_for_real_this_timereally the last version?

It's even worse if multiple people are working on the same document. If you and your lab partner are editing a document at the same time, How can you gracefully merge the changes? What if the changes you make and the changes she makes are incompatible?

Software like Google Docs can address some of these issues, but incompatible changes can still occur. Imagine you're writing an essay about a dog. At the beginning of the essay, you've written

The quick brown fox jumped over the lazy dog

You and your partner are both refining this epic story at the same time, and further on you write,

Because of how lazy the dog was, she didn't chase the fox.

But your partner decided the first line needed some more detail and changes it to

The quick brown fox jumped over the lazy male dog.

so your pronouns are out of step.

In writing, a mistake like this might just look silly, but in programming, it can mean your code doesn't run or generates the wrong answer. Even more critically, code often involves many files working together, and keeping track of the versions of multiple files at the same time is necessary.

git is a program for version control

git is a distributed version control system (DVCS). That is, it helps one keep track of one's code, and the information about versions is distributed among many systems.

You can think of a git "repository" (usually shortened to "repo") as a directory with super powers. If you're looking at the directory using Finder or Explorer, it might not look any different, but it's much more powerful. Before we get into that, though, we need to get git installed.

Installing git

If you are using Windows Subsystem for linux, or a linux operation system, git should already be installed.

But lets check - if git is installed, you can execute git --help in the terminal.

$ git --help
usage: git [--version] [--help] [-C <path>] [-c <name>=<value>]
        [--exec-path[=<path>]] [--html-path] [--man-path] [--info-path]
        [-p | --paginate | -P | --no-pager] [--no-replace-objects] [--bare]
        [--git-dir=<path>] [--work-tree=<path>] [--namespace=<name>]
        <command> [<args>]

These are common Git commands used in various situations:

start a working area (see also: git help tutorial)
clone      Clone a repository into a new directory
init       Create an empty Git repository or reinitialize an existing one
# ... output truncated

If you're using a Mac, git might not be installed. Executing the command above will probably result in an error message:

bash: git: command not found...

So you need to install it. If the git help message appeared, you can skip to here).

The easiest way to install git on a mac is using homebrew.

If you're using a mac and don't have git installed, enter the following commands into your terminal (excluding the $), then press enter to execute.

$ xcode-select --install

This may prompt you to download and install "command line developer tools" from the app store. If it does, click install and follow the prompts. When that's finished, and you see the command prompt ($) again, run the following command. Note: this is a case when you should probably use copy/paste.

$ /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
==> This script will install:
/usr/local/bin/brew
/usr/local/share/doc/homebrew
/usr/local/share/man/man1/brew.1
/usr/local/share/zsh/site-functions/_brew
/usr/local/etc/bash_completion.d/brew
/usr/local/Homebrew
==> The following new directories will be created:
/usr/local/sbin
# ...

And follow the prompts. If you are asked for your password, use the one you use to log into your computer. Note that you will not see anything appear as you type, just type the password and hit enter.

Configuring git

The next step is to tell git your name and email address, so that you are credited with the changes you make to repositories. Enter the following commands, changing the name and e-mail address to yours.

$ git config --global user.name "Kevin Bonham, PhD"
$ git config --global user.email kbonham@wellesley.edu

Practice: Create your first git repository

Step 1: In your terminal, change your working directory to your Documents folder (~/Documents)

Step 2: Next, create a new directory called my_repo.

Step 3: Now, change your working directory into the newly created my_repo/ and initialize a git repository using the command git init

my_repo $ git init
Initialized empty Git repository in /home/kevin/Documents/my_repo/.git/

Step 4: Open the folder in your operating system's file system navigator (Finder on a Mac, Explorer in Windows). This folder appears empty right now, but in fact, there's a hidden .git folder that will include all of the version information for all of the files that you track.

Let's see how this works.

Step 5: Open the VS Code text editor and create a new file, then save it in your repository directory as fox.txt.

Step 6: In your terminal, list the contents of the directory to be sure the file was created.

my_repo $ ls
fox.txt

When you create new files, git does not track them automatically. Let's see what git sees at the moment:

$ git status
On branch master

No commits yet

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

    fox.txt

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

So git sees the file exists, but it tells you it's not being tracked.

Step 7: Let's fix that (the status message helpfully tells you how):

my_repo $ git add fox.txt
my_repo $ git status
On branch master

No commits yet

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

    new file:   fox.txt

The file is now "staged"^[1] - that is ready to be "committed." In git, a "commit"^[2] is used to register a specific version of a repository. The current state of all of the tracked files in the repository will be recorded.

We don't really need to track an empty file, let's add some text to it.

Step 8: In VS Code, add the following line to fox.txtand save.

The quick fox jumped.

Now, back in the terminal, what's the status?

$ git status
On branch master

No commits yet

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

    new file:   fox.txt

Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git checkout -- <file>..." to discard changes in working directory)

    modified:   fox.txt

Notice that fox.txt now appears under both "Changes to be committed" and "Changes not staged for commit".

Why? Because you initially staged an empty file, and now there's a modified version of the file that has not been staged. You can see the difference between the current state of the file and what's staged using git diff

$ git diff fox.txt | cat
diff --git a/fox.txt b/fox.txt
index e69de29..395235f 100644
--- a/fox.txt
+++ b/fox.txt
@@ -0,0 +1 @@
+The quick fox jumped.

The syntax of this output is perhaps a bit confusing, but it's saying that a line was added to fox.txt.

Step 9: Let's go ahead and stage this change, and then make our first commit.

$ git add fox.txt
$ git commit -m "my first commit"
[master (root-commit) b183d56] my first commit
1 file changed, 1 insertion(+)
create mode 100644 fox.txt

$ git status
On branch master
nothing to commit, working tree clean

Congratulations! You have a git repository.

Just keep committing

Using git in this course

It might not be clear to you yet why using a version control system is worthwhile. If you don't trust me, the fact that almost every software company uses git (or something similar) should give you some confidence that it's important.

In any case, this entire course will use git and github.com (a website for managing and collaborating on git repositories). In the first assignment, you'll learn how to do this.

Running julia code

For a lot of this course, we will be using the julia programming language rather than the command line.

There are a few different ways to run julia code, and this section will get you acquaninted with a couple of them.

The julia REPL

Open julia, which you should have installed in the first lesson.

Your terminal application should open, running julia:

_
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.6.0 (2021-03-24)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

julia>

Technically speaking, this is the julia "Read, evaluate, print, loop", or "REPL".

When you enter text at the julia> prompt, the REPL reads it, evaluates it as julia code, prints the result, and then loops back to the prompt.

Let's try it! Type println("Hello, World!") at the prompt and hit enter.

println("Hello, World!")

Don't worry if you don't understand all of the components of this command - we'll get there.

println(Hello, World!)

Running julia from the command line

You can also execute short snippets of code from the command line. But first, you need to tell the terminal where to look for the julia program.

Mac users, execute the following in your terminal:

$ echo 'export PATH=$PATH:/Applications/Julia-1.6.app/Contents/Resources/julia/bin/' >> ~/.bash_profile

Then restart the terminal. Don't worry if you don't understand what that command is doing - it's not worth it to understand it at this moment.

$ julia -e 'println("Hello, World!")'
Hello, World!

The -e is a command-line flag that tells julia to just execute the next command as julia code. Note the use of single quotes (') surrounding the command.

Running julia scripts

Our code is often going to be much more complicated than what we've done so far. In those cases, and in order to keep a record of what we're doing, it's useful to put our julia code in a file.

$ julia <path_to_directory>/hello.jl

Hello, World!

When code is saved into a file that can be run from the commandline, it's called a "script." All of your assigments will be julia code written into files and commited to code repositories using git.

But it's important to realize that all of this code is the same; it's just text. That text has specific requirements in order to be parsed by the julia interpreter, but whether you run code in the REPL, from the command line, or in a script, it has the same behavior.