What is git?

• Git: Version Control System (VCS)
• Distributed: Lives on more than one computer
• Keeps track of snapshots of a directory: [VCS] repository = files + history
• Free software
• Really complex (300,000 lines of code, written in C, Bash, and Perl)
• Really useful (helps keep track of what you do, so you make fewer mistakes)
• Started by the Linus Torvals, who also started the Linux kernel
• Git GitHub, though Git’s code is available there.

Using Git

• Read the manual
• git help <command>
  • Warning: the documentation is pretty gross
• The following are the most useful bits of git knowledge I’ve picked up over the years, starting from the basics.
• If you’re experienced with the basics of git (add, commit, push, pull) and you don’t want a refresher, you could try digging a little deeper with Git From The Bottom Up for a few minutes.

Basic Usage (local repository)

• git init – Create a repository

    repo   files
     ___
    [   ]   *
    [___]

• git add – I want you to record the state of the following files

     ___
    [ * ]
    [___]

• git commit – Actually do it. (And attach a message describing changes)

     __________________
    [ * (HEAD) message ]
    [__________________]

• git commit (with no arguments) will open vim. Press i, write the commit message, press Esc then save and quit, by typing :wq
• HEAD: the current commit

Basic Usage (local repository)

• rinse, repeat

     ___________________
    [ * (HEAD) message3 ]
    [ * message2        ]
    [ * message         ]
    [___________________]

Other Useful Basic Commands

• git rm <filename>
  • Remove a file from the repo
• git mv <from-filename> <to-filename>
  • Rename a file in the repo

GitHub Basics

• Git repositories can be synchronized between multiple local and multiple remote computers (e.g. your laptop, halligan, and GitHub).
• GitHub offers public remote repositories
• Remote repositories allow distributed development
• Get set up here
  1. Create an account
  2. Create a GitHub repo
  3. Follow the directions (git remote add origin blah, and git push -u origin master)

Basic Usage (local + remote repository)

• A repository’s .git/config file has details
• Default remote name is origin
• git remote add origin https://github.com/Hnasar/test.git
• Local and remote repositories has benefits:
  • Work on stuff without an Internet connection
  • Work on a project from different computers
• Added complexity:
  • Manually keep changes synchronized.
  • Combining some changes requires intervention (a conflict)

Remote: Initial State

• Empty remote, new repository

     local           remote
    (laptop)        (GitHub)
     ____            ____
    [ *  ]          [    ]
    [ *  ]          [    ]
    [ *  ]          [    ]
    [____]          [____]

Remote: Updating the remote

• Update the remote with local changes with git push

     local           remote
    (laptop)        (GitHub)
     ____    push    ____
    [ *  ]  ----->  [ *  ]
    [ *  ]          [ *  ]
    [ *  ]          [ *  ]
    [____]          [____]

• (DANGEROUS if the remote repo gets completely messed up, try git push -f It’s usually better to resolve conflicts rather than do this. This can lead to lost data)

Remote: Creating a new local

• Download an entire remote repository to a new local copy with git clone

     local           remote         local
     (laptop)       (GitHub)        (halligan)
     ____            ____    clone    ____
    [ *  ]          [ *  ]  ------>  [ *  ]
    [ *  ]          [ *  ]           [ *  ]
    [ *  ]          [ *  ]           [ *  ]
    [____]          [____]           [____]

Remote: Updating the local

• Update local repositories with remote changes with git pull

     local           remote         local
     (laptop)       (GitHub)        (halligan)
     ____            ____             ____
    [ *  ]  (push)  [ *  ]   pull    [ *  ]
    [ *  ]  ----->  [ *  ]  ------>  [ *  ]
    [ *  ]          [ *  ]           [ *  ]
    [ *  ]          [ *  ]           [ *  ]
    [____]          [____]           [____]

• git pull is usually bad form. Use git pull --rebase
• Read this article for more info.

Viewing a Repository 1

• gitk --all
• 
• available on halligan, Ubuntu/Debian, homebrew

Viewing a Repository 2

• git log --graph --oneline --all --decorate
• mnemonic: (git log g.o.a.d.), goad, meaning it’s annoying to type all that
• (Shortcut: Ctrl + r, then start typing git log --graph …)
• 

Adding and Committing

• Commits are the basic unit of a repository
• Mark a new state of files at a point in time
• Commits are referenced by their hash (e.g. 0dc4e6e – see Viewing a Repository 2, above)
• Commit message message indicates to viewers what the changes in the commit did.
• (Use git diff to see what was changed from the last commit)
• 2-part command
  1. git add <path[s]> – record these changes in the next commit
  2. git commit – make the commit, and add a message
• (try git add -p to select exactly which changes within files are added)
• Before a commit is made, git reset (without any arguments!) will undo git add

Commit Style

• A good commit will contain only the changes necessary to some new feature of a repository.

• E.g. If the feature is: “ensure all img tags have an alt attribute”, a good commit will add alt tags for every img in one go, and NOT create a new commit for every changed img tag, or every file that I change things in.

• Good commit message form:
  • Feature in present tense
  • One blank line
  • Explanation/reasoning of changes

Add alt attribute to every img

As per Section 508 Amendment to the Rehabilitation Act of 1973
and the HTML 5 specification, every img should have an alt
attribute which "provides equivalent content for those who
cannot process images or who have image loading disabled".

Git reset

• git reset --hard <commit>
  • DANGEROUS – you will lose any uncommitted changes
  • used to undo commits
  • Moves branch label, and HEAD to commit specified

Git reset example

    * 31a3f57 (HEAD, master) Third commit
    * 20ea82d Second commit
    * 9ef5cfb First commit

• git reset --hard 20ea82d

Git reset example

    * 20ea82d (HEAD, master) Second commit
    * 9ef5cfb First commit

Un-undoing Commits

• Commits are only truly deleted after a given time passes (several days)

• git reflog
  • displays most recent commits which have been HEAD

    20ea82d HEAD@{0}: reset: moving to HEAD~1
    31a3f57 HEAD@{1}: checkout: moving from 20ea82d to master
    20ea82d HEAD@{2}: checkout: moving from master to HEAD~1
    31a3f57 HEAD@{3}: commit: Third commit
    20ea82d HEAD@{4}: commit: Second commit
    9ef5cfb HEAD@{5}: commit (initial): First commit

• git reset --hard 31a3f57

Un-undoing Commits

• Back to the start!

    * 31a3f57 (HEAD, master) Third commit
    * 20ea82d Second commit
    * 9ef5cfb First commit

Branches

• Branches allow multiple lines of commits, which may be dealing with differing features, to not overlap (which might cause confusion).

• A branch is a label attached to a commit.
• Default branch name is master
• View branches (including the current one) with git branch -a

      * 7a0fc15 Patch.hs: Fix incorrect editsToChangeHunks offsets
      * e564f63 Make the type of Edit more general.
    * | 0bbe999 Implements applyPatch
    * | b6d7003 Implements sequencePatches
    |/
    * 6f2a864 Paralell patch changes

Using Branches

• Create a branch with git branch <branch-name>
• Delete a branch with git branch -d <branch-name>
• Switch branches with git checkout <branch-name>
• When you commit, the new commit’s parent is the tip of the current branch, and the branch will now point to the new commit.
• A successful Git branching model

Combining Branches

1. git merge <branch to merge in>
   • Produces a commit with multiple parents

    *   ca5ac46 Merge branch 'master' of github.com:jmont/nor
    |\
    | * 7a0fc15 Patch.hs: Fix incorrect editsToChangeHunks offsets
    | * e564f63 Make the type of Edit more general.
    * | 0bbe999 Implements applyPatch
    * | b6d7003 Implements sequencePatches
    |/
    * 6f2a864 Paralell patch changes

2. git rebase <branch to rebase onto>
   • Removes the branch by making the branch’s commits stem from the end of the other.

    * 7a0fc15 Patch.hs: Fix incorrect editsToChangeHunks offsets
    * e564f63 Make the type of Edit more general.
    * 0bbe999 Implements applyPatch
    * b6d7003 Implements sequencePatches
    * 6f2a864 Paralell patch changes

Conflicts

• Git is smart about what lines changed in which files in a commit

• Some commits indicate contradicting changes.
• If git can’t figure it out, it writes both version the file, complains of a conflict and tells you to fix it.
• Make the file look how you want, then do git add . and git commit

    these lines
    are not
    in conflict
    <<<<<<
    THESE ARE IN CONFLICT!
    ======
    These are in conflict.
    >>>>>> version 2
    these lines
    are good too

GitHub: Forking

• GitHub “Forking” is something GitHub invented (not a part of git)

• GitHub “Forking” is a way to copy a remote git repo from one GitHub user to another.
• “fork” typically means taking an existing project, and developing it in a new direction. This is what happened when Ubuntu forked from Debian. We say Debian is the upstream.

     remote          remote
     tuftsdev/       hnasar/
     running-dogs    running-dogs
     ____            ____
    [ *  ]   fork   [ *  ]
    [ *  ]  ----->  [ *  ]
    [ *  ]          [ *  ]
    [____]          [____]

GitHub: Pull Request

• Typically, free software software developers share patches (modifications to code, try git format-patch <commit>) via email or posting on websites.

• GitHub created a notion of a “Pull Request” to easily allow GitHub “Forked” projects to collaborate in a similar fashion as sharing patches.
• Good explanation
• Pull Requests must be accepted by the recipient.

     remote          remote
     tuftsdev/       hnasar/
     running-dogs    running-dogs
     ____           ____
    [_*__] <------ [ *  ]
    [ *  ]   pull  [ *  ]
    [ *  ] request [ *  ]
    [ *  ]         [ *  ]
    [____]         [____]

GitHub: No passwords

• Possible to use GitHub without typing in username & password each time
• Set up SSH keys
• Make sure that your remote URIs are set to git@github.com/…
• 
• Check a repository’s .git/config file

Checkout & Blame

• checkout moves HEAD (the current commit, and the corresponding state of the files)
• (remember git log --graph --oneline --all --decorate)
• git checkout <commit-hash> (e.g. git checkout 0dc4e6e)
  • ‘detached HEAD’ state, which means HEAD isn’t on a branch
  • git checkout a branch to “reattach” the HEAD
• git blame <file> to see when and who last made changes to a part of a file.
• git show <commit> displays the contents of a given commit.

Commit-ishs

• commit-ish: some way to reference commits
  • branch-name
  • commit-hash
  • relationships
• ‘~’ - ancestor (defaults to ~1)
• Ex: HEAD~2 (parent of parent of current commit)

Rewriting History

• git rebase -i <commit-ish>
• reorder, delete, squash commits
• Used to make it look like you knew what you were doing
• Never rewrite commits which you have already pushed (messes up everyone else)

Questions/Comments

• More reference available here
• Still unclear?
• Did I miss something?

Quiz

1. Create a repository
2. Commit thrice
3. Create a branch from the first commit and add another commit or two.
4. Merge the new branch into master
5. Undo the merge in master
6. Rebase the new branch onto master instead
7. Squash all the commits on top of the initial commit

This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/.