theme: Next, 1

GIT GUD

[.build-lists: true]

Agenda

Background
Internals
Tips & Tricks
Best Practices

^

=> Lecture
=> Lecture & Interactive
=> Interactive
=> Lecture

Background

what is `Git`?

^ Ask this question?

VCS

Version Control System

^ What does a VCS do?

[...] a system that records changes to a file or set of files over time -- Scott Chacon & Ben Straub ¹

Version Control Systems

CVS
Perforce
SVN
Mercurial
Git

^ Who has tried what? Later: How they differ

Open Source

GNU GPL2

^ published under GNU General Public License version 2 ^ Next Slide: What makes Git different from, for example, SVN

Distributed

^ What does that mean? For this we have understand:

Centralised

VS

Distributed

Centralised

^

Central repository = Single source of truth
Server/Client communication

^ Examples: SVN, CVS, Perforce

Distributed

^

Each repository a full backup (with exceptions)
Communication between repositories
Possibel to do centralised approach (usual approach)

^ Examples: Git, Mercurial

Internals

let's get our hands dirty

^ I could bore with commands, but you probably know those already. Let's talk about how Git works internally => Helps a lot for advanced stuff

Terminology
Object model

Expected Knowledge

add
reset
commit
merge

^ How to add changes? (Probably familiar with this) Follow up: How does Git track changes?

How does `Git` track changes?

`Git` is different

^ Not only "Distributed" => The way Git thinks about it's files

The major difference [...] is the way Git thinks about its data. -- Scott Chacon & Ben Straub ¹

Snapshots

not

Deltas

^ Deltas: What changed? Snapshots: Current state Q: Any advantages you can think of?

^ How does Git build this to a commit?

What is a

Commit

^ Commit -> Tree -> Tree | BLOB Commit: Tree + Parents + Metadata

^ Git is closer to a filesystem then to a classic VCS Next slide: How does Git identify objects?

SHA1

^ Hash built over all properties of the object (Commit => Author, Committer etc.) You might have heard to never change commits after pushing: Hash changes when properties change

Try it out!

Clone workshop repository

`git clone`

`git@github.com:sascha-wolf/git-workshop.git`

$ cd git-workshop

$ echo 'Some random text' > my_file

$ git hash-object -w my_file
1a76b8a41993e2c667f5b191fb57abdab2102a8b

$ git cat-file -t 1a76b8a41993e2c667f5b191fb57abdab2102a8b
blob

$ git cat-file -p 1a76b8a41993e2c667f5b191fb57abdab2102a8b
Some random text

^ You can actually find the blob in .git/objects/1a/76b8a41993e2c667f5b191fb57abdab2102a8b Git only uses the content; create an identical file and the hash will be the same

How history is made

`git` history at least

^ Each commit has a reference on it's parent => Single linked list Not really single though ...

Honour thy parents

Navigate through history

^ Q: You want to remove the last commit, how?

`^`

and

#~<N>

^ Better have an example

$ git checkout ancestry
...

$ git log --oneline -1 HEAD
822eca1 Add the number 5 to numbers

$ git log --oneline -1 HEAD^
d4bd446 Add the number 4 to numbers

$ git log --oneline -1 HEAD^^
1b94d52 Add the number 3 to numbers

$ git log --oneline -1 HEAD~2
1b94d52 Add the number 3 to numbers

What is a

Branch

^ Q: Ask this!

Let's take a look

$ ls -l .git/refs/heads
total 32
-rw-r--r--  1 swolf  staff  41 Jan 16 09:06 ancestry
-rw-r--r--  1 swolf  staff  41 Jan 16 07:00 dev
-rw-r--r--  1 swolf  staff  41 Jan 16 07:00 master
-rw-r--r--  1 swolf  staff  41 Jan 16 07:02 waldo-came-and-left

^ All references in .git/refs:

branches in heads
remote branches in remotes/<remote-name>
tags in tags

Huh, `master` is a file ...

^ Q: What do you think is in master?

I wonder what's in there ...

`$ cat .git/refs/heads/master`

`f20c96538c6dca6fb37e631388814fe941afc2ae`

It's just a hash!

^ Let us inspect the object! We can use git cat-file --batch

$ cat .git/refs/heads/master | git cat-file --batch
f20c96538c6dca6fb37e631388814fe941afc2ae commit 540
tree 9ab6d12a4587fb4a02de1b5e745277c6c832cf5a
parent c9866a994539e01938ceec6c75ada013ce456e9b
parent 12e86b3370f931c423022a8c8d7d7fa7413e0bc4
author Sascha Wolf <sascha.wolf@grandcentrix.net> 1516082444 +0100
committer Sascha Wolf <sascha.wolf@grandcentrix.net> 1516082444 +0100
gpgsig -----BEGIN PGP SIGNATURE-----

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 =vi7g
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Merge branch 'dev'

What about `HEAD`?

^ Q: What do you think is HEAD? Q: Have you ever seen "detached HEAD"?

$ cat .git/HEAD
ref: refs/heads/master

$ git checkout --detach master
...

$ cat .git/HEAD
f20c96538c6dca6fb37e631388814fe941afc2ae

^

Reference onto a branch
Reference onto a commit (detached HEAD)

What is a

Merge

^ Q: How does a merge differ from a usual commit?

What does the log tell us?

$ git log --decorate --graph --oneline
*   f20c965 (origin/master, master) Merge branch 'dev'
|\
| * 12e86b3 (origin/dev, dev) Add bar
| * 4043e69 Add some content to foo
|/
* c9866a9 Add foo

Remember the commit object?

$ cat .git/refs/heads/master | git cat-file --batch
f20c96538c6dca6fb37e631388814fe941afc2ae commit 540
tree 9ab6d12a4587fb4a02de1b5e745277c6c832cf5a
parent c9866a994539e01938ceec6c75ada013ce456e9b
parent 12e86b3370f931c423022a8c8d7d7fa7413e0bc4
author Sascha Wolf <sascha.wolf@grandcentrix.net> 1516082444 +0100
committer Sascha Wolf <sascha.wolf@grandcentrix.net> 1516082444 +0100
gpgsig -----BEGIN PGP SIGNATURE-----

 iHUEABEIAB0WIQTQyMji07ff76Vkk26j80vFo8w6AAUCWl2VDAAKCRCj80vFo8w6
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 =vi7g
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Merge branch 'dev'

^ Q: Do you notice something? => Two parents

Tips & Tricks

We're going to look at

bisect
--patch mode
stash
reflog
rebase
filter-branch

One thing before we start ...

^ Use aliases for intersting command+option combinations

Aliase

`git config --global alias. ``

In Action

$ git config --global alias.l 'log --decorate --graph --oneline'

$ git l -3
*   f20c965 (HEAD -> master, origin/master) Merge branch 'dev'
|\
| * 12e86b3 (origin/dev, dev) Add bar
| * 4043e69 Add some content to foo
|/

^ --global to put it into your ~/.gitconfig

`git bisect`

a smart bug hunters weapon

`git bisect`

Search for the first commit which introduces an issue

$ git bisect start
$ git bisect bad <known bad commit>
$ git bisect good <known good commit>

Shortcut

$ git bisect <known bad commit> <known good commit>

Automate it!

`git bisect run <command>`

^ Exit codes are used 1: bad commit 0: good commit

Let's try it out!

`git bisect start waldo-came-and-left d175`

^ Show the asciinema as demonstration at the end

Stop `bisect`ing

`git bisect reset`

`--patch`

`add` | `checkout` | `reset`

Let's try it out!

$ git checkout patch-practice
...

$ git apply patch-practice.diff
...

$ git add --patch # Try to add only the above line!

^ Tip 1: ? for help Tip 2: s for split

Level up your `--patch`

`e`dit mode

^ It's possible to edit things before adding it to the index! => Show it on the example patch

`git stash`

quicksave for git

^ Q: Who knows this already?

`git stash`

push create a stash from current changes
pop apply and delete a stash
drop delete a stash

Use stash@{<n>} to reference older stashes

`git reflog`

a history for your `HEAD`

Try it out!

`git reflog`

Branches have `reflog`s too!

`git reflog <some branch>`

`git rebase`

one command to rule them all

^ Q: What rebase do?

git rebase dev feature/login-button

Let's try it out

$ git checkout rebase-practice
...

$ git rebase dev
???

^ Huh, seems like we got a conflict ... foo was removed but we want to keep it! Q: What can we do?

`git rebase --interactive`

Let's fix this

We don't want to remove foo!

$ git rebase --abort
...

$ git rebase --interactive dev
???

Avoid rebasing pushed commits!

^ Q: Do you know why?

`rebase` rewrites history

with great power comes great responsibility

^ Q: Why is that bad?

Changing parent

=

Changing SHA1

=

Chain reaction

MOAR POWER!

over history

`git filter-branch`

^ Lot's of ways to rewrite history

`git filter-branch`

Allows to rewrite

--env-filter <command> --index-filter <command> --msg-filter <command> --parent-filter <command> --subdirectory-filter <directory> --tag-name-filter <command> --tree-filter <command>

^ Need to remove a sensitive file from all commits? tree-filter to the rescue

Honourable mentions

cherry-pick: "copy" one+ commits onto the current branch
clean: remove untracked files and folders
commit --amend: include added changes in last commit
grep: search pattern in tracked files
worktree: checkout a reference into another worktree

Best Practices²

Commit often

Perfect later

Publish once

^ You can use interactive rebase for "Perfect later"

Use shortlived branches

or at least merge upstream work regularly

Do not rewrite published history!

^ N: Otherwise your colleagues might do this

Commit Messages

<Topic|File>: <Short description> (50 characters sort limit)

<More detailed description> (bullet points are fine)
- problem (what is the issue with the current implementation)
- choosen approach with reasoning
- maybe rejected approaches

Use imperativ

Add foo instead of Added foo

^ N: Don't do this

Thank you

Footnotes

https://git-scm.com/book/en/v2 ↩ ↩²
http://sethrobertson.github.io/GitBestPractices ↩

Files

git-gud.md

Latest commit

History

git-gud.md

File metadata and controls

GIT GUD

Agenda

Background

what is Git?

VCS

Version Control System

Version Control Systems

Open Source

GNU GPL2

Distributed

Centralised

VS

Distributed

Centralised

Distributed

Internals

let's get our hands dirty

Expected Knowledge

How does Git track changes?

Git is different

Snapshots

not

Deltas

What is a

Commit

SHA1

Try it out!

Clone workshop repository

git clone

[email protected]:sascha-wolf/git-workshop.git

How history is made

git history at least

Honour thy parents

Navigate through history

^

and

What is a

Branch

Let's take a look

Huh, master is a file ...

I wonder what's in there ...

$ cat .git/refs/heads/master

f20c96538c6dca6fb37e631388814fe941afc2ae

It's just a hash!

What about HEAD?

What is a

Merge

What does the log tell us?

Remember the commit object?

Tips & Tricks

We're going to look at

One thing before we start ...

Aliase

`git config --global alias. ``

In Action

git bisect

a smart bug hunters weapon

git bisect

Shortcut

Automate it!

git bisect run <command>

Let's try it out!

git bisect start waldo-came-and-left d175

Stop bisecting

git bisect reset

--patch

add | checkout | reset

Let's try it out!

Level up your --patch

edit mode

git stash

quicksave for git

git stash

git reflog

what is `Git`?

How does `Git` track changes?

`Git` is different

`git clone`

`[email protected]:sascha-wolf/git-workshop.git`

`git` history at least

`^`

Huh, `master` is a file ...

`$ cat .git/refs/heads/master`

`f20c96538c6dca6fb37e631388814fe941afc2ae`

What about `HEAD`?

`git bisect`

`git bisect`

`git bisect run <command>`

`git bisect start waldo-came-and-left d175`

Stop `bisect`ing

`git bisect reset`

`--patch`

`add` | `checkout` | `reset`

Level up your `--patch`

`e`dit mode

`git stash`

`git stash`

`git reflog`

a history for your `HEAD`

`git reflog`

Branches have `reflog`s too!

`git reflog <some branch>`

`git rebase`

`git rebase --interactive`

`rebase` rewrites history

`git filter-branch`

`git filter-branch`

Best Practices²