git

Setup

• Login with your username found on your name badge and set the initial password for your account: https://git.aspp.school/user/login
• You'll have to type that password many many times this week: choose wisely!
• We will use the exercise in the repo for the rest of the lecture

A cautionary quote

My first instinct is to sell all my computers, fake my own death, move to another planet, and reinvent computing from scratch, rather than try to understand Git.

I rarely actually do that, mind you. But the urge is there.

— Lars Wirzenius (Linux kernel developer)

Warm-Up

• how to start a repo from scratch?
  • git init local method
  • on an online forge (GitHub, GitLab, …): git clone
• how to revert mistakes?
  • before commit:
    • git restore <file> [discard changes in the working directory] changes files
    • git restore --staged <file> [unstage changes ➔ opposite of git add <file>, does not modify the working directory]
  • after commit:
    • git revert <commit> [creates a new commit, modifies the working directory]
    • git reset <commit> [only reset the HEAD pointer, does not modify the working directory] rewrites history ➔ can not be used if you have already pushed to some remote
    • git reset --hard <commit> [reset HEAD and modify working directory] rewrites history and changes files ➔ can not be used if you have already pushed to some remote
• how to move the whole working directory to a specific point in history?
  • git checkout <commit> ➔ DETACHED HEAD problem, changes files
  • interaction with branches: git branch <branch_name> + git switch <branch_name>
• how to copy a file from a different branch:
  • git checkout <branch> <file> ➔ the file is staged automatically
  • git restore --source=<branch> <file ➔ the file is not staged
• git gui: building commits along the way interactively (for the mess around type of workflows)
• check out these sketches for a graphical visualization of git commands!

The Open Source model

• remotes: git pull <from_where> <what>, git push <where> <what>, git fetch <from_where> <what>, git merge <another_branch>
• GitHub: forks, branches and PRs: important ➔ explain fork vs. clone!!!
• strategies for keeping your fork up-to-date: your main, origin's and upstream's main, short-lived and long-lived topic branches
• a more thorough and detailed explanation can be found on the SciPy Contributor's Guide. This guide can be adapted to your own needs, see gitwash.
• make it clear that GitHub and GitLab are just options (git≠GitHub)

Scenarios

1. lone scientist working alone in the cellar without Internet (local git)
2. lone scientist uploading their software to the Internet in the hope it can be useful for other people (local git + one personal GitHub repo)
3. lone scientist sharing one software project with some other befriended lone scientist working in a different place (local git + one personal GitHub repo + permissions)
4. research group sharing software among members (local git + several GitHub repos + permissions + branches + [optional] PRs)
5. fully distributed software development using the most typical open source software workflows as used by numpy, scipy, sklearn, etc. (like above + we don't trust our contributors, i.e. work strictly with forks)