Git and GitHub

Joel Ross

Starting with
Git and GitHub

Joel Ross

Manual Version Control

Version Control Systems

A version control system (VCS) is a tool for managing a collection of program code that provides you with three important capabilities: reversibility, concurrency, and annotation.
— Eric Raymond

A command line application used for software version control. It provides line-by-line change tracking and allows for simultaneous off-line editing.

 

Note that git does not automatically keep track of changes, you will need to manually "snapshot" different versions you wish to save.

xkcd.com/1597/

A web service that stores git-managed projects online. You can use git to upload or download projects from GitHub.

 

git is the application that manages versions of your code. GitHub is just a website where those versions can be stored remotely.

Installing Git

Step 1. Install Git (see textbook)

# configure git to know who you are

git config --global user.email "your_email_address"

git config --global user.name "Your Full Name"
# check on command line that git is installed
git --version

Step 2. Sign up for GitHub

https://github.com/signup

 

Step 3. Configure Git (command line)

Repository

(or "repo")

repo

projectX

projectY

Repository

(or "repo")

projectX

projectY

Repository

(or "repo")

Repos on GitHub

GitHub hosts repositories, each of which can be found at a different address.

 

Example: https://github.com/dpla/black-womens-suffrage​(source code for https://blackwomenssuffrage.dp.la/)

 

Example: https://github.com/joelwross/git-demo

Cloning

You clone (download) a repository to your local machine. This creates a copy of that repo on your computer.

git clone

repo on your
machine

repo on GitHub

Personal Access Tokens

In order to clone repos from Github, you will need to set up an access token (like a password) for authentification. Do this under Github's Settings > Developer Settings

Git Commands

 git clone url

Download repo to local machine. Creates a local copy.

DO NOT PUT ONE REPO INSIDE OF ANOTHER!

Git Commands

 git clone url

Download repo to local machine. Creates a local copy.

 git status

Check status of repo

Branches

Branches allow for non-linear development and for naming different versions of code in the same repo. More on this later!

main

Ada

Remotes

http://github.com/user/repo

origin

A remote is a repository on another machine that a repository can upload and download code from.

Git gives each remote a name (an "alias") to easily refer to it. By convention, the "primary" remote (where you cloned from) is named origin

Tracking Changes with Git

Joel Ross

How do we "save" our changes?

The Staging Area

Put changes in temporary storage before committing.

 git add .

Add everything in directory

Add file to staging area

 git add FILENAME

use this

Committing Changes

Store current snapshot of files in repository!

 git commit -m "message"

Commit changes

If you forget the -m option,
use :q (colon then q) to quit vi

(it happens to everyone)

Commit Message Etiquette

xkcd.com/1296/

Use informative commit messages; they document changes for others. Messages should complete the sentence:

     If applied, this commit will ________________.

Local Process

files

staging area

git add .
git commit -m "message"

repo

Commit Hashes

8ac18e2

a16a4f9

2f7de03

2f7de03

$ git log --oneline
2f7de03 Fourth
8417290 Third
a16a4f9 Second
8ac18e2 First

Each commit has a unique commit hash that refers to it

"First"

"Second"

"Third"

"Fourth"

HEAD

Undoing Changes

View commit history

 git log [--oneline]

Restore previous version of a file

 git checkout COMMIT_HASH FILENAME

Working with GitHub

Joel Ross

A web service that stores git-managed projects online. You can use git to upload or download projects from GitHub.

 

git is the application that manages versions of your code. GitHub is just a website where those versions can be stored remotely.

Repos on GitHub

GitHub hosts repositories, each of which can be found at a different address.

 

Example: https://github.com/dpla/black-womens-suffrage​(source code for https://blackwomenssuffrage.dp.la/)

 

Example: https://github.com/joelwross/git-demo

Forking

Forking creates a copy of a repo on GitHub's computers, but under a different user/organization account.

info340/repo

student/repo

GitHub Classroom

Assignments use a tool called GitHub Classroom to automatically create private copies (forks) of repos for assignments.

DO NOT "FORK" ASSIGNMENT REPOS

Push to GitHub

Upload commits to the GitHub cloud repo.

 git push -u origin main

Upload to the origin remote, main branch
The -u sets branch tracking so that future push commands go to correct place.

 git push

Push to the remote

Git Commands II

Copy repo to local machine

 git push

Upload commits

 git pull

Downloads and merges commits 

 git clone url

Using GitHub (Assignments)

edit files

staging area

git add
git commit
git clone

your machine

git push

your copy

follow assignment
link

Collaboration

Multiple people's local repositories can be linked to the same remote repository, allowing them to push and pull to the same central location.

Git and Branching

Joel Ross

Commit History

"First"

 git commit -m "First"

"Second"

"Third"

"Fourth"

 git commit -m "Second"
 git commit -m "Third"
 git commit -m "Fourth"

Git history has been a linear sequence of commits.

HEAD

HEAD

HEAD

HEAD

Branches

Branches allow for non-linear commits.

main

main

main

main

experiment

bugfix

experiment

experiment

Branch Commands

 git branch

List available branches

 git branch [my_branch]

Create a new branch called "my_branch"

 git checkout [my_branch]

Switch to branch "my_branch"

 git checkout -b [my_branch]

Create and switch to branch "my_branch"

 git branch -d [my_branch]

Delete branch "my_branch"

Branches

main

main

experiment

experiment

HEAD

HEAD

HEAD

HEAD 

git branch experiment

git checkout experiment

git commit

git commit

git checkout main

git commit

git checkout experiment

HEAD

HEAD

experiment

HEAD

Merging

We can merge two branches back together, producing a commit that contains the combined changes from both branches

main

main

experiment

HEAD

HEAD

Merging

 git merge [other_branch]

Merges changes from other_branch into the current branch.

A new commit is created on the current branch containing the merged content.

What happens if branches have conflicting code changes?

Merge Conflicts

A merge conflict occures when two commits from different branches include different changes to the same code. Git does not know which version to keep, so makes you choose. 

Merge conflicts must be resolved manually

Merge conflicts are a common and normal part of merging. They are the "typos" and "bugs" of using git.

Resolving Merge Conflicts

In order to resolve a conflict, you need to edit the file (code) so that you pick which version to keep. git will add marker content where you need to make a decision:

<<<<<<< HEAD

// This is the code from the "local" version (the branch you merged INTO)
// a.k.a the version from the HEAD commit

const message = "I am an original";
const lyric = "I've got no strings to hold me down";

// There can be multiple lines that conflict, including lines being deleted

=======

// This is the code from the "remote" version (the branch you merged FROM)

const message = "I think I'm a clone now...";

// The lines need not be related in content, they've just changed in a way
// that git can't figure out which to keep!

>>>>>>> f292a3332aedc8df3e8e8cf22ca3debc214c6460

the two versions to pick from

a divider between the versions

end conflict area

git add .
git commit -m "Merge branch 'other'"
  1. Use git status to see which files have merge conflicts. Note that files may have more than one!
     
  2. Fix the code so that it reflects the "final" version you want.
     
  3. Remember to delete the <<<<<<< and ======= and >>>>>>> !!
     
  4. Once you're satisfied that the conflicts are all resolved, add and commit your changes (the code you "modified" to resolve the conflict):

Resolving Merge Conflicts

Merge conflicts are normal and expected

Pulling and Merging

 git pull

Shortcut for git fetch then git merge

downloads commits

The git pull command also will perform a merge; any commits from the remote "branch" will be merged into the local branch.

That means that you may need to resolve merge conflicts when pulling!

may have conflicts!

Pushing and Merging

Pushing commits to Github may require merging if the commits are interwoven. Git will not allow such pushes. Instead, you need to pull changes in order to merge them, and then push the new combined/merged commits.

# attempt to push, may be blocked
git push

# IF BLOCKED, pull down the remote commits
git pull

# IF ANY CONFLICTS, resolve them!
git add .
git commit -m "Resolve merge conflict"

# now push combined commit history
git push

Branches on GitHub

Joel Ross

GitHub and Branches

Because GitHub just hosts normal repositories, GitHub has branches as well! These can (but need not) correspond with the branches on your local machine.

Remote Branch Cmds

 git branch -a

List all branches (including remote ones) 

 git pull [remote] [branch]

Shortcut for git fetch then git merge 

 git fetch

Import remote branches into local repo
Are still listed as "remote" branches that need to be merged

Can cause conflicts!

 git push [remote] [branch]

Remote Branch Cmds

Upload commits to specific remote and branch.

 git push [remote] --all

Push commits from all branches to all branches

GitHub Pages

A GitHub service that provide (free) web hosting for repository content.

 

The content of the gh-pages branch will be available at https://username.github.io/reponame.

 

To "publish" your webpage, just push your content to that branch.

https://help.github.com/articles/user-organization-and-project-pages/

Development vs Production

# do all your coding on `main`!

# switch to gh-pages ("production" branch)
git checkout gh-pages

# merge the changes from main
git merge main

# push to GitHub to publish
git push origin gh-pages

# switch back to main branch for more coding
# DON'T FORGET THIS STEP!!
git checkout main

# remember to push your souce code
git push origin main

IMPORTANT: The gh-pages branch is your "production" branch. Never make changes on production. All changes should be made on the main branch, and then merged into production gh-pages branch.