Version Control
Notes adapted from Michael Jantz Pro Git, 2nd Edition by Chacon and Straub. Available here.
Table of Contents
What is Version Control?
- A system that records changes to a file or set of files over time so that
you can recall specific versions later
- Often used to write software
- Useful for any collaborative document / project
- Version control systems (VCS) can:
- Revert specific files to a previous state
- Revert the entire project to a previous state
- Compare changes over time
- See who introduced an issue and when
- Reproduce *EVERY* state
Types of Version Control
- Local Version Control
- Backup Files
- VMS from VAX minicomputers
- tar, diff, patch
- Examples: rcs, sccs
- co - checkout
- ci - checkin
- rcslog - see history
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- Centeralized VCS
- Distributed VCSEnables collaboration with developers on other systems
- Single server contains all versioned files, clients check files in and out from the central repository
- Risks from keeping all files in one central location
- Examples: CVS, Subversion, Perforce
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- Distributed VCS
- Clients fully mirror the repository
- Every clone is a full back-up of the data
- Examples: Git, Mercurial, Bazaar, Darcs
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Git Basics
- Git is actually a content tracking not version control: each commit represents a full filesystem
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- Nearly every operation is local
- Fast and easy to look up and compare files from the past
- Can work offline
- Git has integrity
- Everything is check-summed
- Check-summing is done by computing sha-1 hash based on the contents of a file or directory structure
- Git generally only adds data
- No danger of really screwing things up
Three States for Files in Git
- Committed
- Data is safely stored in your local database
- Modified
- File has been changed, but not committed to your database
- Stage
- A modified file is marked to go into your next commit snapshot
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Tracked vs. Untracked Files
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Creating pull request
- Fork and clone students repository
- Add a
yourutkid.md
file, commit, and push:
cd students
- Edit
yourutkid.md
git add yourutkid.md
git commit -m "added my interests"
git push -u origin master
- Create and submit pull request
Working with Files
-
add stages a file or directory (directories are added recursively)
-
status tells you the status of files in the repo
git status
-
git status -s
(short version)
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diff compares files
-
git diff
(compares working directory with staging area)
-
git diff --staged
(compares staged changes to last commit)
-
git diff --cached
(same as git diff –staged)
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commit creates a new revision with your staged changes
-
git commit
(will open a text editor for you to document your commit)
-
git commit -m "document string"
(to avoid opening an editor)
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git commit -v
(displays differences of what you’re committing)
-
git commit -a
(automatically stages every file that is tracked and then commit)
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rm stages a removal of a file
git rm file.txt
-
git rm --cached file.txt
(removes file from staging area)
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mv renames a file
git mv file.txt new_file.txt
The .gitignore File
- Tells git that some classes of files should not be automatically added or even shown as untracked.
- Lists filenames patterns that should be ignored
- Placed in the directory in which you want the rules to apply (rules are applied recursively to all subdirectories)
- List of useful
.gitignore
files here:
Viewing the Commit History
-
log shows commit history
git log
-
git log -p
(shows differences in each commit)
-
git log -p -2
(shows differences of only the last two versions)
-
git log --pretty=oneline
(easy-to-read one line format)
-
git log --pretty=format:"..."
(allows you to specify your own format string)
-
git log --since=2.weeks
(show only commits in the last 2 weeks)
-
git log --author="Audris Mockus"
(show only commits by that autor)
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git log -Sstring
(show only commits that added or removed the string ‘string’)
-
git log -- file.txt
(show only commits that modified file.txt)
Undoing Things
- To add to a previous commit, use
--amend
:
git commit -m "initial commit"
git add forgotten_file.txt
git commit --amend
- To unstage a stage file, use reset:
- To unmodify a modified file, use checkout:
Working with REmote Repositories
-
remote shows your remote repositories
-
fetch gets data from your remote repository
-
git fetch [remote-name]
(leave remote-name blank to fetch from origin)
-
push pushes your data to the remote repository
git push [remote-name] [branch-name]
-
git push origin master
(most common)
Branching in Git
- Branching means to diverge from the main line of development
- Allows you to continue work without messing with the main line
- Git branching is lightweight
- Does not copy entire source tree
- Encourages workflows that branch and merge often
Commit Objects
- A commit object that contains a pointer to the snapshot of the content you stored. The commit object includes:
- Author name and email
- Message attached to the commit
- Pointers to the commit(s) that came directly berfore it (its parents)
- Zero parents fro the initial commit, 1 parent for a normal commit, multiple parents for a merge of two or more branches
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> git add README test.rb LICENSE
> git commit -m "The initial commit of my project"
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Commit Objects Point Back to Their Parents
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- Next commit stores a pointer to the commit(s) that came before it
A Branch is a Pointer to a Commit Object
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- A branch in Git is a lightweight movable pointer to one of these commits
- The default branch in Git is master.
Branch Example
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- Creates a new pointer to the same commit you’re currently on
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- The HEAD pointer tells you which branch you’re currently on
- Currently still on master
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- Switches HEAD pointer to point to an existing branch
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> git commit -a -m "made some change"
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- Next commit moves the testing branch forward
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- Moves the HEAD pointer back to the master and reverts your files in the working directory to the master branch
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> git commit -a -m "more changes to master"
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- Changes now isolated in separate branches
Basic Branching and Merging
- An example workflow:
1. Do work on a website
2. Create a branch for the new story you’re working on
3. Do some work in the new branch
– A Critical issue needs a hotfix –
1. Switch to the production
2. Create a branch to add the hotfix
3. After testing, merge the hotfix branch, and push to production
4. Switch back to your original story and continue working
Branch and Merge Example
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> git checkout -b iss53
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- Switched to a new branch “iss53”
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> vim index.html
> git commit -a -m 'added a new footer [issue 53]'
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> git checkout master
Switched to branch 'master'
> git checkout -b hotfix
Switched to a new branch 'hotfix'
> vim index.html
> git commit -a -m 'fixed the broken email address'
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> git checkout master
> git merge hotfix
Updating f42c576..3a0874c
Fast-forward
index.html | 2 ++
1 file changed, 2 insertions(+)
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> git branch -d hotfix
Deleted branch hotfix (3a0874c).
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> git checkout iss53
Switched to branch 'iss53'
> vim index.html
> git commit -a -m 'finished the new footer [issue 53]'
[iss53 ad82d7a] finished the new footer [issue 53]
1 file changed, 1 insertion(+)
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> git checkout master
Switched to branch 'master'
> git merge iss53
Merge made by the 'recursive' strategy.
index.html | 1 +
1 file changed, 1 insertion(+)
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- Result of three way merge stored in new commit (C6)
- Delted iss53 branch after merge is complete
Merge Conflicts
- If you try to merge two branches with different changes to the same parts of the same file, git will report a merge conflict
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> git merge iss53
Auto-merging index.html
CONFLICT (content): Merge conflict in index.html
Automatic merge failed; fix conflicts and then commit the result.
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- When a conflict occurs, git pauses the commit process
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> git status
On branch master
You have unmerged paths.
(fix conflicts and run "git commit")
Unmerged paths:
(use "git add <file>..." to mark resolution)
both modified: index.html
no changes added to commit (use "git add" and/or "git commit -a")
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- Git adds standard conflict resolution markers:
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<<<<<<< HEAD:index.html
<div id="footer">contact : email.support@github.com</div>
=======
<div id="footer">
please contact us at support@github.com
</div>
>>>>>>> iss53:index.html
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- Conflicts should be resolved manually
- There are tools to assist with merging (‘git mergetool’)
- In our example, the resolved code might be:
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<div id="footer">
please contact us at email.support@github.com
</div>
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- To resolve conflicts, add the conflicted file(s) to the staging area
- To complete the merge, commit the resolved files
git commit -m "merged commit"
Git Workflow
- A standard set of best practices for developing a project with git
- Includes development model for branching / merging / deploying code
- Encourages good development practices (feature-driven development, code review, continuous delivery)
- Can be applied to large development teams or across different projects
GitHub Flow
- Anything in the
master
branch is deployable
- To work on something new, create a descriptively named branch off of
master
- Commit to that branch locally and regularly push your work to the same named branch on the server
- When you need feedback or help, or you think the branch is ready for merging, open a merge / pull request
- After someone else has reviewed and signed off on the feature, you can merge it into
master
- Once it is merged and pushed to
master
, you can and should deploy immediately
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