Introduction to Version
Control with SVN
& Git
CSC/ECE 517, Fall 2011
Titus Barik (tbarik@ncsu.edu)
& Ed Gehringer, with help from
Gaurav Tungatkar (gstungat@ncsu.edu)
Local version control: RCS
check out
File
• Keep many
copies of files
• Error prone
• RCS stores deltas
Version 3
Version 2
Version 1
Centralized Version Control
• If you need to work with other
programmers …
Computer A
File
check out
Computer B
File
Version 3
Version 2
check out
Version 1
File Server vs.
Version-Control Server
At first glance, the clientserver architecture of a
version-control system
looks much like a typical
file server.
So why do we need
version control?
File Sharing Issues, 1
• Assume your project has 15 developers:
– Each developer wants to make 1 change and
must communicate this change to all other
developers.
– What are the total number of interactions
that must occur?
• C(15, 2) = N(N – 1)/2 = 15(14)/2 = 105.
That’s a lot of information to track!
File Sharing Issues, 2
The problem is that
users are stepping on
each other’s feet!
Image: Version Control with Subversion
Approach 1: Lock, Modify,
Unlock
1. Locking may cause
administrative
problems.
2. Locking may cause
unnecessary
serialization.
3. Locking may create a
false sense of security.
Image: Version Control with Subversion
Approach 2: Copy-Modify-Merge
Sounds chaotic, but in
practice, runs extremely
smoothly.
Question: When is locking
necessary?
Image: Version Control with Subversion
Subversion (SVN)
• Open source, Apache License.
• Available on all major operating systems.
• Improvements such as “cheap” copying
and branching, remote options (https),
over legacy CVS.
• However, CVS has more mature tools,
and SVN has it own flaws (DVCS).
Exercise 1
• Answer these questions
• Give one advantage of using a version-control server
for source-code management over using a fileserver.
• Suppose a project has 15 developers and no shared
repository. Can you come up with a better way of
communicating changes than N(N-1)/2 interactions?
• Explain how locking can cause administrative
problems.
• Explain how locking can create a false sense of security.
• With copy-modify-merge, when is locking necessary?
Branches and Tags
Trunk: Location where main development occurs.
Branches: Location used to isolate changes to another
development line (e.g., experimental features).
Tags: Snapshot of the content (e.g., RTM, service packs,
EOL).
Image: http://en.wikipedia.org/wiki/Subversion_(software)
Populated Repository
A Subversion repository
layout. The folder names
are just a convention, and
have no special meaning
to the repository.
Image: Version Control with Subversion
Creating a Branch
In Subversion, the
underlying mechanism of
a branch is implemented
by performing a simple
directory copy.
Image: Version Control with Subversion
Exercise 2
• Answer these questions about branches.
o Suppose, in fixing a bug, you modify three lines
of code in two source files. Should you create a
new branch? Why or why not?
o Which would probably be more common,
branches or tags?
o What are some of the risks of copying files in a
repository? How do version-control systems
minimize this risk?
Distributed Version Control
• Clients don’t check
out individual files;
• they mirror the Computer A
repository.
File
• What’s the
advantage?
Version 3
Version 2
Version 1
Computer B
File
Version 3
Version 3
Version 2
Version 2
Version 1
Version 1
Git
• Came out of the Linux project, in 2005.
• Simple design
• Strong support for non-linear development
(thousands of parallel branches)
• Fully distributed
• Able to handle large projects like the Linux
kernel efficiently (speed and data size)
Integrity & Checksums
• Everything checksummed with an SHA-1 hash
– 40-character string
– composed of hex characters
– calculated based on the contents of a file or
directory structure in Git
• Example
– 24b9da6552252987aa493b52f8696cd6d3b00373
• Git knows everything by hash, not filename
Snapshots, not Diffs
• See http://progit.org/book/ch1-3.html
• Every time you commit, Git takes a
snapshot of your files.
• Files that have not changed are not
copied.
  Almost all ops are local
• browse history
• commit
3 States of a File in Git
• Modified
• Staged
working
directory
• Committed
git directory
(repository)
staging area
check out the project
stage files
commit
File Status Lifecycle
untracked
unmodified
modified
staged
edit the file
add the file
remove the file
stage the file
Checking Status
• To check the status of your files:
$ git status # On branch master
nothing to commit (working directory clean)
• Creating new files
$ vim README
$ git status # On branch master
# Untracked files:
# (use "git add <file>..." to include in what will be
committed)
#
# README nothing added to commit but untracked files present
(use "git add" to track)
Checking status, cont.
• Begin to track the file:
$ git add README
• The file is now tracked:
$
#
#
#
#
#
git status
On branch master
Changes to be committed:
(use "git reset HEAD <file>..." to unstage)
new file: README #
• For more info:
http://progit.org/book/ch2-2.html
Remotes
• On a project, you may be working with several
remote directories.
• “Origin” is the server you cloned your
repository from
$ git clone git://github.com/schacon/ticgit.git
Initialized empty Git repository in
/private/tmp/ticgit/.git/
remote: Counting objects: 595, done.
remote: Compressing objects: 100% (269/269), done.
remote: Total 595 (delta 255), reused 589 (delta 253)
Receiving objects: 100% (595/595), 73.31 KiB, done.
Resolving deltas: 100% (255/255), done.
$ cd ticgit $ git remote origin
• http://progit.org/book/ch2-5.html
Pulling, pushing to remote
• $ git fetch [remote-name]
• E.g., git fetch origin
• git push origin master
Common Workflow using Git
• Centralized workflow …
• http://progit.org/book/ch5-1.html
• Integration-manager workflow …
• Common use case:
http://progit.org/book/ch5-2.html
Exercise 3
• Answer these question about DVCSs.
o What is an advantage of referring to files
and branches by hash, rather than by
filename?
o Can you think of one disadvantage of a
DVCS compared to a centralized VCS?