Intro to git (one hour version)

Git: a brief introduction Randal L. Schwartz, merlyn@stonehenge.com Version 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 othersSaturday, 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 ﬁtSaturday, 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 UIsSaturday, 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 commitSaturday, 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 fileSaturday, 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 SHA1Saturday, 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 othersSaturday, 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 master git merge topic1 git 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 there git 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 linesSaturday, 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 topic2 git branch -d topic2Saturday, 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 rebasedSaturday, August 4, 12

Using git yourself • Create the repo at your top-level dir: git init git add -A . # or git add * git commit # invokes a text editor • You now have a single “.git” dir • The current working dir snapshotted as rootSaturday, 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 branchSaturday, 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 emptySaturday, 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 subdirsSaturday, 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 preferredSaturday, 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 nameSaturday, 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/masterSaturday, 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

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Intro to git (one hour version)

by Randal Schwartz on Aug 04, 2012

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