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import commit from './images/commit.png'; import status from './images/status.png'; import conflict from './images/conflict.png'; import git_log from './images/git_log.png'; import status_with_change from './images/status_with_change.png' import DefaultLayout from './components/default-layout' import { FullScreenCode } from 'mdx-deck' import { FFMergeBefore, FFMergeAfter, MergeBefore, MergeAfter } from './components/graphs' import { Split } from 'mdx-deck' import { TwoColumn, Column } from './components/two-column' import ImageText from './components/image-text'

Introduction To Git

Sebastian Csar

What and Why Version Control?

  • Keep track of changes to the code
  • Revert bad changes
  • Allow multiple people to work in parallel more easily
  • Git is but one of many distributed version control systems
    • Mercurial
    • svn

Key Definitions

  • Repository - a git "project"
    • has all the current files
    • but also all the history
  • Branch - a split from the "main line" so you can make changes without messing with the main line
  • main/master - (by convention) the name for the "main line" branch
  • commit (n./v.) - a unit of change/the act of making that change
  • merge - bringing the changes from one branch into another
  • rebase - relocating where a branch splits from its parent

Branches

  • git checkout <id> moves HEAD to <id> (which can be a branch name, a commit hash, a tag, ...)
  • creating a branch: git checkout -b <branch name>
    • short for git branch <branch name> + git checkout <branch name>

Making a commit.

<Image src={status} style={{maxWidth: '75%', maxHeight: '75%', objectFit: 'contain', backgroundSize: 'contain'}} alt='foo'/>

 ~/dotfiles   setup_scripts  git status
 On branch setup_scripts
 Your branch is up to date with 'origin/setup_scripts'.

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

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

Making a commit

  • git status: see what files have changed
  • git add: stage each file that you want to commit
    • tip: git add -p my_file.py will show you all changes individually and ask if you want to stage them
  • git commit: make the commit
    • shortcut: git commit -m "The commit message"

commit

<Image src={commit} style={{maxWith: '75%', maxHeight: '75%', backgroundSize: 'contain'}}/>

Add .vscode to .gitignore

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# On branch setup_scripts
# Your branch is up to date with 'origin/setup_scripts'.
#
# Changes to be committed:
#       new file:   .gitignore
#
# Untracked files:
#       git_aliases/
#

A quick vim digression

  • git tends to default to using vim as its editor for commit messages
  • To commit:
    • i (enter insert mode)
    • write your commit message
    • <esc> (back to command mode)
    • :w (save)
    • :q (quit)
  • To set a different editor:
git config --global core.editor nano

What about merging?

Merge the branch featureA into main:

git checkout main # make sure you're in main to start
git merge featureA

There are two scenarios:

  • a fast-forward merge--your commits stack on top of the main branch
  • a three-way merge

Fast-Forward Merges

before

We have two branches: `main` and `some_branch`. There are three commits on `main` before `some branch` splits off and makes two commits with messages "start doing something" and "finish it up"

after

We merge `some_branch` into `main`. `some_branch` no longer exists and `main` is the only branch in the image. It contains the same three commits it did before, _plus_ the two from `some_branch`.

Three-Way Merges

before

In this example, we again have `some_branch` and `main` which diverge after three commits. We have the same two commits on `some_branch`: "start doing something" and "finish it up". However, `main` also has an additional commit with message "main advances" _after_ `some_branch` split off.

after

We again merge `some_branch` into `main`. Both `some_graph` and `main` remain in the image. The tip of `main` is now at the "merge commit"--a commit created by git with message "Merge branch some_branch".

Merge Conflicts

$ git diff main setup_scripts --name-only
.gitignore
setup.sh
$ git diff main conflict_demo --name-only
.gitignore
setup.sh

What happens when I try to merge setup_scripts into conflict_demo?

<Image src={conflict} style={{maxWidth:'75%', backgroundSize: 'contain'}}/>

 ~/dotfiles   conflict_demo  git merge setup_scripts
 CONFLICT (add/add): Merge conflict in setup.sh
 Auto-merging setup.sh
 Automatic merge failed; fix conflicts and then commit the result.

<<<<<<< HEAD
# first, let's start by installing stow

sudo apt-get install stow

# get nvim
mkdir -p $HOME/bin
cd $HOME/bin
curl -LO https://github.com/neovim/neovim/releases/latest/download/nvim.appimage
chmod u+x nvim.appimage
=======
#!/usr/bin/env bash

if ! command stow &> /dev/null
then
echo "Stow not found -- go download it from http://ftpmirror.gnu.org/stow/"
exit
fi

# install vimplug for nvim
sh -c 'curl -fLo "${XDG_DATA_HOME:-$HOME/.local/share}"/nvim/site/autoload/plug.vim --create-dirs \
https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim'
>>>>>>> setup_scripts

Resolving Conflicts

For each conflicted file:

  • open the file and edit it, taking the lines from the version you want
    • caution: they may mix together in a way that results in broken code
  • stage the file (git add) Then, when you're done, git commit.

More on confilcts...

  • Editors often have a merge tool built to help
  • Github also allows you to resolve conflicts in the pull-request UI

.gitignore

  • There's a special file called .gitignore which contains a list of files/directories that you want git to ignore
  • These are usually things like files generated by your text editor/IDE or generated code
  • Github will offer to make one for you or you can google for a "standard" list for the language you're using

Links and References