Git for Version Control

These slides are heavily based on slides created

by Ruth Anderson for CSE 390a. Thanks, Ruth!
images taken from http://git-scm.com/book/en/

http://www.cs.washington.edu/403/
 About Git
•  Created by Linus Torvalds,
creator of Linux, in 2005
–  Came out of Linux development community
–  Designed to do version control on Linux kernel

•  Goals of Git:
–  Speed
–  Support for non-linear development
(thousands of parallel branches)
–  Fully distributed
–  Able to handle large projects efficiently

–  (A "git" is a cranky old man. Linus meant himself.)

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 Installing/learning Git
•  Git website: http://git-scm.com/
–  Free on-line book: http://git-scm.com/book
–  Reference page for Git: http://gitref.org/index.html
–  Git tutorial: http://schacon.github.com/git/gittutorial.html
–  Git for Computer Scientists:
• http://eagain.net/articles/git-for-computer-scientists/

•  At command line: (where verb = config, add, commit, etc.)

–  git help verb

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 Centralized VCS
•  In Subversion, CVS, Perforce, etc.
A central server repository (repo)
holds the "official copy" of the code
–  the server maintains the sole
version history of the repo

•  You make "checkouts" of it

to your local copy
–  you make local modifications
–  your changes are not versioned

•  When you're done, you

"check in" back to the server
–  your checkin increments the repo's version
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 Distributed VCS (Git)
•  In git, mercurial, etc., you don't "checkout"
from a central repo
–  you "clone" it and "pull" changes from it

•  Your local repo is a complete copy

of everything on the remote server
–  yours is "just as good" as theirs

•  Many operations are local:

–  check in/out from local repo
–  commit changes to local repo
–  local repo keeps version history

•  When you're ready, you can "push" changes back to server

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 Git snapshots
•  Centralized VCS like Subversion Subversion
track version data on each
individual file.

•  Git keeps "snapshots" of the

entire state of the project.
–  Each checkin version of the
overall code has a copy of Git
each file in it.
–  Some files change on a given
checkin, some do not.
–  More redundancy, but faster.

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 Local git areas
•  In your local copy on git,
files can be:
–  In your local repo
• (committed)

–  Checked out and modified,

but not yet committed
• (working copy)

–  Or, in-between, in
a "staging" area Unmodified/modified Staged Committed
• Staged files are ready Files Files Files
to be committed.
• A commit saves a snapshot of all staged state.
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 Basic Git workflow
•  Modify files in your working directory.
•  Stage files, adding snapshots of them to your staging area.
•  Commit, which takes the files in the staging area and stores
that snapshot permanently to your Git directory.

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 Git commit checksums
•  In Subversion each modification to the central repo increments
the version # of the overall repo.
–  In Git, each user has their own copy of the repo, and commits
changes to their local copy of the repo before pushing to the
central server.
–  So Git generates a unique SHA-1 hash (40 character string
of hex digits) for every commit.
–  Refers to commits by this ID rather than a version number.

–  Often we only see the first 7 characters:

• 1677b2d Edited first line of readme
• 258efa7 Added line to readme
• 0e52da7 Initial commit

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 Initial Git configuration
•  Set the name and email for Git to use when you commit:
–  git config --global user.name "Bugs Bunny"
–  git config --global user.email bugs@gmail.com
–  You can call git config –list to verify these are set.

•  Set the editor that is used for writing commit messages:

–  git config --global core.editor nano
• (it is vim by default)

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 Creating a Git repo
Two common scenarios: (only do one of these)

•  To create a new local Git repo in your current directory:

–  git init
• This will create a .git directory in your current directory.
• Then you can commit files in that directory into the repo.
–  git add filename
–  git commit –m "commit message"

•  To clone a remote repo to your current directory:

–  git clone url localDirectoryName
• This will create the given local directory, containing a working copy of
the files from the repo, and a .git directory (used to hold the
staging area and your actual local repo)
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 Git commands
command description
 git  clone  url  [dir]   copy a Git repository so you can add to it
 git  add  file   adds file contents to the staging area
 git  commit   records a snapshot of the staging area
 git  status   view the status of your files in the working
directory and staging area
 git  diff   shows diff of what is staged and what is
modified but unstaged
 git  help  [command]   get help info about a particular command

 git  pull   fetch from a remote repo and try to merge

into the current branch
 git  push   push your new branches and data to a remote
repository
others:  init,  reset,  branch,  checkout,  merge,  log,  tag  

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 Add and commit a file
•  The first time we ask a file to be tracked, and every time
before we commit a file, we must add it to the staging area:
–  git add Hello.java Goodbye.java
• Takes a snapshot of these files, adds them to the staging area.
• In older VCS, "add" means "start tracking this file." In Git, "add"
means "add to staging area" so it will be part of the next commit.

•  To move staged changes into the repo, we commit:

–  git commit –m "Fixing bug #22"

•  To undo changes on a file before you have committed it:

–  git reset HEAD -- filename (unstages the file)
–  git checkout -- filename (undoes your changes)
–  All these commands are acting on your local version of repo.
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 Viewing/undoing changes
•  To view status of files in working directory and staging area:
–  git status or git status –s (short version)

•  To see what is modified but unstaged:

–  git diff

•  To see a list of staged changes:

–  git diff --cached

•  To see a log of all changes in your local repo:

–  git log or git log --oneline (shorter version)
1677b2d Edited first line of readme
258efa7 Added line to readme
0e52da7 Initial commit
• git log -5 (to show only the 5 most recent updates), etc.
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 An example workflow
[rea@attu1 superstar]$ emacs rea.txt
[rea@attu1 superstar]$ git status
no changes added to commit
(use "git add" and/or "git commit -a")
[rea@attu1 superstar]$ git status -s
M rea.txt
[rea@attu1 superstar]$ git diff
diff --git a/rea.txt b/rea.txt
[rea@attu1 superstar]$ git add rea.txt
[rea@attu1 superstar]$ git status
# modified: rea.txt
[rea@attu1 superstar]$ git diff --cached
diff --git a/rea.txt b/rea.txt
[rea@attu1 superstar]$ git commit -m "Created new text file"

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 Branching and merging
Git uses branching heavily to switch between multiple tasks.

•  To create a new local branch:

–  git branch name

•  To list all local branches: (* = current branch)

–  git branch

•  To switch to a given local branch:

–  git checkout branchname

•  To merge changes from a branch into the local master:

–  git checkout master
–  git merge branchname
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 Merge conflicts
•  The conflicting file will contain <<< and >>> sections to
indicate where Git was unable to resolve a conflict:

<<<<<<< HEAD:index.html
<div id="footer">todo: message here</div> branch 1's version
=======
<div id="footer">
thanks for visiting our site branch 2's version
</div>
>>>>>>> SpecialBranch:index.html

•  Find all such sections, and edit them to the proper state
(whichever of the two versions is newer / better / more
correct).

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 Interaction w/ remote repo
•  Push your local changes to the remote repo.
•  Pull from remote repo to get most recent changes.
–  (fix conflicts if necessary, add/commit them to your local repo)

•  To fetch the most recent updates from the remote repo into
your local repo, and put them into your working directory:
–  git pull origin master

•  To put your changes from your local repo in the remote repo:
–  git push origin master

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 GitHub
•  GitHub.com is a site for online storage of Git repositories.
–  You can create a remote repo there and push code to it.
–  Many open source projects use it, such as the Linux kernel.
–  You can get free space for open source projects,
or you can pay for private projects.
• Free private repos for educational use: github.com/edu

•  Question: Do I always have to use GitHub to use Git?

–  Answer: No! You can use Git locally for your own purposes.
–  Or you or someone else could set up a server to share files.
–  Or you could share a repo with users on the same file system, as
long everyone has the needed file permissions).

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