This slide deck is intended for Git beginners and focuses on the underlying concepts, the usage of the command line, especially the commands git reset, git checkout, git revert, git commit, and eventually introduces models for using Git and why git rebase may play an important role in some those models.

1. Business Informatics Group
Institute of Software Technology and Interactive Systems
Vienna University of Technology
Favoritenstraße 9-11/188-3, 1040 Vienna, Austria
phone: +43 (1) 58801-18804 (secretary), fax: +43 (1) 58801-18896
office@big.tuwien.ac.at, www.big.tuwien.ac.at
A Brief Introduction to Working with Git
10.03.2014
VUT, Vienna, Austria
Philip Langer

2. History of Versioning Systems
Based on http://codicesoftware.blogspot.com/2010/11/version-control-
timeline.html
local to central …
central to distributed …
1972
sccs
discourage branching …
everything is a branch …

3. Git: A Distributed Versioning System
 There is no single repository server
 Every peer is a full repository instance
 Single-peer versioning possible
 Including the complete history
 Committing changes is independent of peers
 Allows disconnected operation
 Collaboration is done by pushing/pulling commits
 Comparable to peer-to-peer networks
 The role of peers is purely organizational
 No technical difference among peers
 Single common central server possible
 One peer happens to be the central repo
 But several other architectures are possible too
 Gate-keeper architecture
 Teams of teams
3
commit
push & pull
push & pull
push & pull
<Bob>
<Alice>
<Sally>
<Server>
<Harry>
push & pull

4. The Four Layers & Transferring Changes Among Them
 Every repository consists of a
 Workspace
 Index (“staging area”)
 Local repository
 Remote repositories
 …
 Transferring changes
 git add
 workspace to index
 git commit
 index to local branch
 git push
 local branch to remote branch
 …
 Changes are organized in commits
 Each commit has a parent
 The history is basically a DAG
4

5. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft
 Only history
 …
5
default:
HEAD
default:
--mixed
History
Index
Workspace
Head

6. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft
 Only history
 …
6
default:
HEAD
default:
--mixed
History
Index
Workspace
Head

7. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft
 Only history
 …
7
default:
HEAD
default:
--mixed
History
Index
Workspace
Head

8. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard HEAD~
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft
 Only history
 …
8
default:
HEAD
default:
--mixed
History
Index
Workspace
Head

9. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard HEAD~
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft
 Only history
 …
9
default:
HEAD
default:
--mixed
History
Index
Workspace
Head
!

10. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft
 Only history
 …
10
default:
HEAD
default:
--mixed
History
Index
Workspace
Head

11. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft
 Only history
 …
11
default:
HEAD
default:
--mixed
History
Index
Workspace
Head
Reset index
after wrong
add

12. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed] HEAD~
 History and index
 git reset --soft
 Only history
 …
12
default:
HEAD
default:
--mixed
History
Index
Workspace
Head

13. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed] HEAD~
 History and index
 git reset --soft
 Only history
 …
13
default:
HEAD
default:
--mixed
History
Index
Workspace
Head
Undo last
commit, however
it‘s still there

14. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft
 Only history
 …
14
default:
HEAD
default:
--mixed
History
Index
Workspace
Head
Does nothing
actualy
(HEAD is default)

15. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft HEAD~
 Only history
 …
15
default:
HEAD
default:
--mixed
History
Index
Workspace
Head

16. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is HEAD?
 A reference pointing to the last commit (of the current branch)
 Thus, it will be the parent of the next commit
 Relative commit expressions e.g. with HEAD~2
 git reset [--soft | --mixed | --hard] [commit | reference to commit]
 Resets HEAD in history, index, and/or workspace to [commit]
 git reset --hard
 History, index, and workspace
 git reset [--mixed]
 History and index
 git reset --soft HEAD~
 Only history
 …
16
default:
HEAD
default:
--mixed
History
Index
Workspace
Head
Undo last
commit, leaving
index untouched

17. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is checkout?
 Get a version from the history
 And overwrite workspace
 (also for creating and changing branches … later)
 git checkout -- <file/s>
 Get version of <file/s> from HEAD
 Overwrite version in workspace
17
History
Index
Workspace
Head
Undo last
commit, leaving
index untouched

18. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is checkout?
 Get a version from the history
 And overwrite workspace
 (also for creating and changing branches … later)
 git checkout -- <file/s>
 Get version of <file/s> from HEAD
 Overwrite version in workspace
18
History
Index
Workspace
Head
Undo local
changes to
workspace.
!

19. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is revert?
 Does not modify history (good if you pushed already)
 Takes a commit from history
 Creates reverse patch
 Commits reverse patch (new commit)
 git revert HEAD~1
19
History
Index
Workspace
Head
Must be clean!

20. Before talking about branches…
Demystifying “git reset”, “git checkout”, and “git revert”
 What is revert?
 Does not modify history (good if you pushed already)
 Takes a commit from history
 Creates reverse patch
 Commits reverse patch (new commit)
 git revert HEAD~1
20
History
Index
Workspace
Head
Undo commits after
you‘ve already
pushed them.
Must be clean!

21. Branching
 After all every change (local or remote) is a fork leading to a new branch
 So make branches a first-class concept
 No logical difference between local and remote branches
 Every merge is a branch merge
21
<Bob>
<Server>
<Alice>

22. Branching
 Each commit has a parent
 A branch is nothing more than a pointer to a commit
22
Current
Branch

23. Branching
 Each commit has a parent ( DAG)
 A branch is nothing more than a pointer to a commit
23

24. Branching
 Local branches may link to “tracking branches”
24
Current
Branch

25. Branching
 Branches can be linked in ./git/config
(links are also created automatically in certain cases;
or may be linked explicitly using git checkout --track)
[remote "origin"]
fetch = +refs/heads/*:refs/remotes/origin/*
url = https://code.google.com/a/eclipselabs.org/p/moliz/
[branch "master"]
remote = origin
merge = refs/heads/master
[branch "issue_18_alf"]
remote = origin
merge = refs/heads/issue_18_alf
25

26. Merging Branches
 git checkout master
git merge iss53
26

27. Merging Branches
 git checkout master
git merge iss53
27
Note that git pull is a git fetch & git merge

28. Branching Models of Git
 Several different “models”
 All entirely organizational
 The svn-like model:
 Only one branch
 Only one central repo
 git pull (origin master)
 Apply changes
 git commit (master)
 git push (origin master)
 Good for e.g. papers
 The isolated-development model
 Own branch for current dev
 Own branch for releases
 Own branch for each feature
28

29. Keeping the History of Topic Branches Clean: Rebase
 git checkout experiment
… // apply changes
git commit -a
git checkout master
git merge experiment
29

30. Keeping the History of Topic Branches Clean: Rebase
 git checkout experiment
… // apply changes
git commit -a
git checkout master
git merge experiment
30

31. Keeping the History of Topic Branches Clean: Rebase
 git checkout experiment
… // apply changes
git commit -a
git rebase master
31Rebase is rewriting history! Never rebase pushed history!

32. Keeping the History of Topic Branches Clean: Rebase
 Improving previous commits
… // apply changes
git commit -a
… // apply changes
git commit -a
git rebase -i HEAD~2
# Rebase 710f0f8..a5f4a0d onto 710f0f8
#
# Commands:
# p, pick = use commit
# e, edit = use commit, but stop for amending
# s, squash = use commit, but meld into previous commit
32