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Understanding Git and GitHub

Ultra terse basics

Git helps you to keep track of changes in the code of a project and to develop together with people. GitHub is a site where you can host Git repositories (projects).

You clone a repository from GitHub to your machine, you make a local branch, you fix a bug, you commit that change, you push your change online, and finally you open a pull request so the owner can inspect it and decide to merge it or not with its original repository.

Some definitions

  • VCS: version control system, which Git is.
  • repository: a project, basically; where the project is stored: could be GitHub and/or your PC.
  • remote: the generic word for a repository hosted online, generally at GitHub. Also the reference to it.
  • fork: your personal online copy of a repository of someone else.
  • commit / snapshot: the "revision", as called in other VCSs; a given state of the project; also as a verb: to commit: to create a snapshot.
  • branch: a simple pointer (or reference) to the head of a line of work.
  • tag: just another pointer, used e.g. to say that from this tagged commit starts the v2.1 of the project.
  • Naming conventions for repositories and branches:
    • master (main on GitHub...): the primary branch of the repository.
    • origin: your fork (or your own original repository).
    • upstream: the repository you forked from.
  • merging: putting a forked history back together again; the result is a merge commit.
  • merge conflict: when trying to merge, two commits changed the same file; needs to be solved before successfully concluding a merge.
  • reference / ref: basically a "pointer" to a commit; in practice it's a SHA-1 value of the commit.
  • HEAD: a symbolic reference to the branch you're currently on.
  • branch_name: a local branch.
  • origin/branch_name: a remote branch.
  • tracking / setting the upstream to: a tracking branch has a direct correlation to a remote branch, so if you do e.g. git push, Git automatically knows which server and branch to push to. After cloning a repository, it generally automatically creates a master branch that tracks origin/master; however you can switch to and create other branches.
  • fast-forwarding: what happens when you update your local repository with git pull and you get no issues; technically it's merging two or more commits in one go, when there is no divergent work to merge together (and therefore no merge conflicts). In yet other words, to fast-forward means to advance your local HEAD until it matches the last commit (or a given specified commit) of origin.

Understand visually

(adapted from https://git-scm.com/docs/git-checkout)

           HEAD (refers to branch 'master')
            |
            v
a---b---c (master)
    ^
    |
  tag 'v2.0'

Commits: a, b, and c

master: a branch which refers to commit c

HEAD: a symbolic reference to the branch you're currently on

tag 'v2.0': a tag which refers to commit b

Now we change some code; then, we commit it:

git add...; git commit...:

               HEAD
                |
                v
a---b---c---d (master)
    ^
    |
  tag 'v2.0'

We've created commmit d.

master has been updated, and so has HEAD, which always points to master.

Now, if we want, we can git push such commit online.

INDEX