Version control with GIT

For zombies…… By Zeeshan Khan

 A method for centrally storing files
 Keeping a record of changes
 Who did what, when in the system
 Covering yourself when things inevitably go wrong
 Another “trendy” word combination
 Something that every software developer should deal with

 You can avoid using version control
 But it can’t last long
 You will need to collaborate eventually
 It might be tricky sometimes
 But you can avoid most problems
 Recommendations:
 Stick to basic working cycle
 Learn basic working cycle commands
 Practice on sandbox project

 Allows a team to share code
 Maintains separate “production” versions of code that are always
deployable
 Allows simultaneous development of different features on the same
codebase
 Keeps track of all old versions of files
 Prevents work being overwritten

 There are version control tools even for designers:
 There is version control functionality embedded in:
6
Adobe version cue PixelNovel Timeline
Microsoft Word OpenOffice.org Writer

 Branch - a copy of a set of files under version control which may be developed at
different speeds or in different ways
 Checkout - to copy the latest version of (a file in) the repository to your working copy
 Commit - to copy (a file in) your working copy back into the repository as a new
version
 Merge - to combine multiple changes made to different working copies of the same
files in the repository
 Repository - a (shared) database with the complete revision history of all files under
version control
 Trunk - the unique line of development that is not a branch
 Update - to retrieve and integrate changes in the repository since the update.
 Working copy - your local copies of the files under version control you want to edit

8
• CVS
• Subversion
• VSS,TFS,Vault
• ClearCase
• AccuRev
Centralized (client-server model)
• Git
• Mercurial
• Bazzar
• Perforce
• BitKeeper
Distributed

Users commits changes to the central repository and a
new version is born to be checked out by other users

CENTRALIZED WORKFLOW
 Branch by Release
Release 1
Release 2
Branch
V1.0 V1.1 V1.2
V2.0 V2.1 V2.2
Merge
• Branch by Feature / Task
Feature 1
Main Trunk
Branch Merge
BranchFeature 2 Merge

CENTRALIZED WORKFLOW
 Access the central server and ‘pull’ down the changes
others have made
 Make your changes, and test them
 Commit (*) your changes to the central server, so other
programmers can see them.
 (*) Work out the merge conflicts (windiff, built in tools
etc.)

Canonical Repository
Local Repository
Jeff
3. Local repository is update
from canonical repository
2. Pushes changes to the
canonical repository
4. Working copy is updated
from local repository
1. Commits changes to
the local repository
Each user has a full local copy of the repository.Users commit changes
and when they want to share it,they push it to the shared repository

DISTRIBUTED WORKFLOW
• Simple
Add Commit
• Branch by Member/ Features
Init/Clone Push
Development Trunk
V1.0 V1.1
Main Trunk
Developer 1
Developer 2

DISTRIBUTED WORKFLOW
 Each developer ‘clones’ a copy of a repository to their own machine.
The full history of the project is on their own hard drive.
 Two phase commits:You commit first to your local staging area, and then
push to the repository.
 Central Repository is not mandatory, but you usually have one
 Examples of distributed source control systems
 Git, Mercurial, Bazaar

 Single repository
 Commit requires
connection (no staging
area).
 Impossible to commit
changes to another user
 All history in one place
 Reintegrating the branch
might be a pain
 Considered to be not so
fast as DVCS
 Multiple repositories
 Commit does not require
connection (due to staging
area)
 Possible to commit changes
to another user
 Impossible to get all history
 Easier branches
management (especially
reintegration)
 Considered to be faster than
CVCS
Centralized Distributed

• Speed
• Simple design
• Strong support for thousands of parallel branches
• Fully distributed
• Able to handle larges projects like Linux kernel effectively
• Ensure integrity

Snapshots of the filesystem are saved in every commit instead of saving the differences

• Fetch or clone (create a copy of the
remote repository) (compare to cvs
check out)
• Modify the files in the local branch
• Stage the files (no cvs comparison)
• Commit the files locally (no cvs
comparison)
• Push changes to remote repository
(compare to cvs commit)

• Git directory: stores the metadata and
object database for your project.
• Working directory: a single checkout of
one version of the project
• Staging area (Index): file contained in
your Git directory that stores information
about what will go into he next commit

Untracked: files in your working directory
that were not in the last snapshot and are not
in staging area.
Unmodified: tracked but not modified
(initial clone)
Modified: tracked and modified
Staged: identified for next commit

 There are four elementary object types in Git:
 blob - a file.
 tree - a directory.
 commit - a particular state of the working directory.
 tag - an annotated tag (we will ignore this one for now).

 A blob is simply the content of a particular file plus some
 meta-data.
 A tree is a plain text file, which contains a list of blobs and/or trees with
their corresponding file modes and names.
 A commit is also a plain text file containing information about the author
of the commit, a timestamp and references to the parent commit(s) and
the corresponding tree.
 All objects are compressed with the DEFLATE algorithm and stored in
the git object database under .git/objects.

 Everything is check-summed before it is stored
 Everything is referred to by that checksum.
 SHA-1 hash is used for making checksum hash.
 Every commit is referred to by that SHA-1 hash.
 Cannot change the contents of any file or directory without Git
knowing about it

 The Secure Hash Algorithm is a 160 bit cryptographic hash
 function used in TLS, SSH, PGP, . . .
 Every object is identified and referenced by its SHA-1 hash.
 Every time Git accesses an object, it validates the hash.
 Linus Torvalds: ”Git uses SHA-1 in a way which has nothing at all to do
with security. [...] It’s about the ability to trust your data.”
 If you change only a single character in a single file, all hashes up to the
commit change!

Creating a new repository:
 $ git init
Cloning from an existing repository:
 $ git clone https://github.com/dbrgn/fahrplan

SPECIFIC CHANGES:
 $ git add *.py
 $ git add README.rst
 $ git commit -m 'First commit'
ALL CHANGES:
 $ git commit -am 'First commit'

FROM STAGING AREA
 $ git rm --cached file.py
FROM INDEX AND FILE SYSTEM
 $ git rm file.py

Git tracks content, not files. Although there is a move command...
 $ git mv file1 file2
...this is the same as...
 $ mv file1 file2
 $ git rm file1
 $ git add file2

SHOWING STATUS:
$ git status
SHOWING LOG (ENTIRE PAGED)
 $ git log
SHOWING LOG (DATE FILTERING)
 $ git log --since=2.weeks
 $ git log --since="2 years 1 day 3 minutes ago"

LAST COMMIT
 $ git show
SPECIFIC COMMIT
 $ git show 1776f5
 $ git show HEAD^

UNSTAGED CHANGES
 $ git diff
STAGED CHANGES
 $ git diff --cached
RELATIVE TO SPECIFIC REVISION
 $ git diff 1776f5
 $ git diff HEAD^

CHANGE LAST COMMIT
 $ git commit --amend
UNSTAGE STAGED FILE
 $ git reset HEAD file.py
UNMODIFY MODIFIED FILE
 $ git checkout -- file.py
REVERT A COMMIT
 $ git revert 1776f5

 This is a file describing the files that are to be ignored from git tracking
 Blank lines or lines starting with # are ignored
 Standard glob patterns work
 End pattern with slash (/) to specify a directory
 Negate pattern with exclamation point (!)
 $ cat .gitignore
*.pyc
/doc/[abc]*.txt
.pypirc

 Other clones of the same repository
 Can be local (another checkout) or remote (coworker, central
server)
 There are default remotes for push and pull
 $ git remote -v
origin git://github.com/schacon/ticgit.git (fetch)
origin git://github.com/schacon/ticgit.git (push)

WITHOUT DEFAULT
 $ git push <remote> <branch>
SETTING A DEFAULT
 $ git push -u <remote> <branch>
THEN...
 $ git push

FETCH & MERGE
 $ git pull [<remote> <branch>]
FETCH & REBASE
 $ git pull --rebase [<remote> <branch>]
-> Rebasing should be done cautiously!

 Like most VCSs, Git has the ability to tag specific points in history as
being important. Generally, people use this functionality to mark
release points (v1.0, and so on)
 Git uses two main types of tags: lightweight and annotated. A
lightweight tag is very much like a branch that doesn’t change — it’s
just a pointer to a specific commit. Annotated tags, however, are
checksummed; contains the tagger name, e-mail, and date; have a
tagging message; and can be signed and verified with GNU Privacy
Guard (GPG).

LIGHTWEIGHT TAGS
 $ git tag v0.1.0
ANNOTATED TAGS
 $ git tag -a v0.1.0 -m 'Version 0.1.0'

Branches are "Pointers" to commits.

Any reference is actually a text file which contains nothing more than the
hash of the latest commit made on the branch:
 $ cat .git/refs/heads/master
57be35615e5782705321e5025577828a0ebed13d
HEAD is also a text file and contains only a pointer to the last object that
was checked out:
 $ cat .git/HEAD
ref: refs/heads/master

Scenario 1 – Interrupted workflow
You’re finished with part 1 of a new feature but you can’t continue with
part 2 before part 1 is released and tested

Scenario 2 – Quick fixes
While you’re busy implementing some feature suddenly you’re being
told to drop everything and fix a newly discovered bug

Different auto-merge strategies are there like fast-forward, 3 way , etc...
If it fails, fix by hand…..
 $ git merge <branch>
Auto-merging index.html
CONFLICT (content): Merge conflict in index.html
Automatic merge failed; fix conflicts and then commit the result.
Then mark as resolved and trigger merge commit
 $ git add index.html
 $ git commit

 Linear alternative to merging
 Rewrites tree! Never rebase published code!

 Often, when you’ve been working on part of your project, things are
in a messy state and you want to switch branches for a bit to work on
something else.The problem is, you don’t want to do a commit of
half-done work just so you can get back to this point later.The answer
to this issue is
$ git stash
 Stashing takes the dirty state of your working directory — that is,
your modified tracked files and staged changes — and saves it on a
stack of unfinished changes that you can reapply at any time.

CREATE NEW BRANCH
 $ git branch iss53
 $ git checkout -b iss53 master
SWITCH BRANCH
 $ git checkout iss53
DELETE BRANCH
 $ git branch -d iss53

SHOW ALL BRANCHES
 $ git branch
iss53
*master
testing
SHOW LAST BRANCH COMMITS
 $ git branch -v
iss53 93b412c fix javascript issue
*master 7a98805 Merge branch 'iss53'
testing 782fd34 add scott to the author list in the readmes

SHOW MERGED BRANCHES
 $ git branch --merged
iss53
*Master
SHOW UNMERGED BRANCHES
 $ git branch --no-merged
testing

 AKA feature branches
 For each feature, create a branch
 Merge early, merge often
 If desired, squash commits

Version control with GIT

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