Intro to git (one hour version)

Git: a brief introduction

Randal L. Schwartz, [email protected] 5.0.2 on 28 Jul 2012

This document is copyright 2011, 2012 by Randal L. Schwartz, Stonehenge Consulting Services, Inc.This work is licensed under Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License

http://creativecommons.org/licenses/by-nc-sa/3.0/

Saturday, August 4, 12

Overview

• Key concepts of git• Using git by yourself• Using git with others


Key concepts• Manages trees, not files• Stores entire history for a commit• Serverless (mostly)• Source-code manager• No meta-data• Handles binaries badly

• Optimized for branch/merge• If not branching or merging, bad fit


The moving parts• Stored in “.git”:• Object repository• files (“blob”), directories, commits

• Operational files (config, etc.)• Working directory• Low-level CLI (“plumbing”)• High-level CLI (“porcelain”)• Various GUIs• Various web UIs


Commits

• Unit of work• Usually has a parent• Two or more on a merge• Zero on a “root” commit

• Metadata (committer, message, timestamps)• Tree of files at time of commit


Trees

• SHA1 of each file in a directory• SHA1 of each directory• ... recursively

• These are kept efficiently in the object store• Identical files map to same SHA1, stored once• And so do identical trees!

• No penalty to “move” a subdirectory or file


Making commits• Changes made to work dir• Added to index with “git add”• Committed with “git commit”• Commit updates HEAD• Prior value of HEAD is the parent

• HEAD almost always points at a branch name• Thus, branch name is also moved forward

• Series of commits loosely also called “branch”• Commit also gets a SHA1


The SHA1 is king

• SHA1 of commit defines:• Metadata of commit• Tree of files• Parentage

• Parentage defines previous commits• Thus, SHA1 uniquely defines complete history• Useful when working with others


Naming commits• SHA1 (can often be abbreviated)• HEAD, branch-name, tag• Optionally followed by @{historical}• “historical” can be:• yesterday, 2011-11-22, etc (date ref)• 1, 2, 3, etc (prior version of this ref)• “upstream” (upstream version of local)

• Optionally followed by ~n for “n’th ancestor”


Branches

• Initial branch is “master” (name not special)• Fork the current branch:

git checkout -b topic1• topic1 is set to same SHA1 as previous branch• HEAD now points at topic1• New commits now apply to topic1

• Switch back with “git checkout master”


Multiple branches• Start a second topic based on original master:

git checkout -b topic2 master• Work, work, commit, commit• Switch back and forth at will:

git checkout topic1• Topics record independent deltas to master• A B C (master) D E F (topic1)• A B C (master) G H I (topic2)


Combining the work

• Merge the work back in to master:git checkout mastergit merge topic1git branch -d topic1

• This was a fast-forward merge:A B C D E F (new “master”)A B C G H I (topic2)


Really merging• Now to bring topic2 back into the mix:

git checkout master # already theregit merge topic2

• Three-way merge between master, topic2, relative to common ancestor (“C”)

• Might cause conflicts• New commit will have both F and I as parents• History is no longer linear• DEF and GHI will appear as parallel lines


Linear history• We can “rebase” commits:

A B C D E F (master)A B C G H I (topic2)git checkout topic2; git rebase master

• Git replays new commits on top of ancestor:A B C D E F (master) G’ H’ I’ (topic2)

• Might cause conflicts at each replay• Need to clean up when done:

git checkout master; git merge topic2git branch -d topic2


Rebase vs merge

• Both can have conflicts• Merge makes a bushy tree• Hard to peel out “linear” history

• Rebase makes a linear tree• But gets rid of commits

• Rebase is nicer, but you must take care• Shared commits should not be rebased


Using git yourself

• Create the repo at your top-level dir:git initgit add -A . # or git add *git commit # invokes a text editor

• You now have a single “.git” dir• The current working dir snapshotted as root


Work a bit• Edit some files• Check the status: git status• Add all the changes and commit them:

git add -A .git commit

• Shortcut if you haven’t added new files:git commit -a

• You’re making commits on the master branch


What’s changed?• git diff• Diff between index and working tree• These are things you should “git add”• “git commit -a” will also make this list empty

• git diff HEAD• Difference between HEAD and working tree• “git commit -a” will make this empty

• git diff --cached• between HEAD and index• “git commit” (without -a) makes this empty


Useful commands

• git archive: export a tree as a tar/zip• git bisect: find the offensive commit• git cherry-pick: selective merging• git mv: rename a file/dir• git rm: ditto for delete• git revert: add commit to undo previous commit• git blame: who wrote this?


Read the history• git log• print the changes

• git log -p• print the changes, including a diff between

revisions• git log --stat• Summarize the changes with a diffstat

• git log -- file1 file2 dir3• Show changes only for listed files or subdirs


Using git with others

• Commits can be transmitted by many protocols• On disk• HTTP[S]• git protocol over TCP• git protocol over SSH

• git over SSH is preferred


Getting commits

• Most likely: git clone over SSH:git clone user@host:/some/path/to/repo.git

• Makes local repo.git dir• Checks out remote repo’s “HEAD” as master• Adds remote repo as “origin”• Nothing special about that name


Working with upstream• Work on local master:

edit edit; git commit -a• Refresh from upstream• By merge: git pull origin master• By rebase: git pull --rebase origin master

• Push to upstream:git push origin master

• Check what’s different before pull:git fetch origin; git diff master origin/master


For further info• See “Git (software)” in Wikipedia• And the git homepage http://git-scm.com/• Git wiki at https://git.wiki.kernel.org/• Wonderful Pro Git book: http://progit.org/book/• Get on the mailing list• Helpful people there• You can submit bugs, patches, ideas

• And the #git IRC channel (on Freenode)• Now “git” to it!


Intro to git (one hour version)

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