intro to git (one hour version)

Git: a brief introduction

Randal L. Schwartz, merlyn@stonehenge.comVersion 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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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”

Saturday, August 4, 12

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”

Saturday, August 4, 12

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)

Saturday, August 4, 12

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)

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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?

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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

Saturday, August 4, 12

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!

Saturday, August 4, 12