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GitDeepDive.org
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GitDeepDive.org
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#+OPTIONS: ^:{} toc:nil author:nil num:nil
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* Git Deep Dive
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A Version 1 Technical Meetup talk covering the deep internals of git.
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** Introduction
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This is the slide-deck and set-up scripts used to give the /Git Deep Dive/ technical meet-up talk on 2018-06-25 in Version 1, by Éibhear Ó hAnluain.
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The pack comes with the following:
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- This document
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- The slide-deck, =GitDeepDive.pdf=
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- A script to set up a simple git repository for exploration purposes: =simple-setup.sh=
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- A script to set up the mimic of a development team and processes: =gitDemo.sh=
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- A script to set up another simple repository to run through the process for completely clearing a file out of a git database: =largeFile-setup.sh=.
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- A =.gitignore= file.
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** Setup
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1) Run the script =simple-setup.sh=. This will create a git repository in =simpleRepo= and populate it with some commits. The commits in this repository will correspond to the diagrams in the slide-deck that relate to merging.
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2) Run the script =largeFile-setup.sh=. This will create another git repository in =largeFileRepo= and populate it with the same commits as in =simpleRepo=, and then puts in some more to create and manipulate a large binary file, and then to remove it. This will be used for the demonstration to remove a file completely from the repository.
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3) Run the script =gitDemo.sh=. This will replicate a previous git repository as a bare repo in =devTeamDemo/javaBootcampNoEclipse.git=, and then create a number of clones to represent the actions of a team lead and two developers, and mimic collaborative development among them. =gitDemo.sh= takes one optional parameter, =-s=, which will cause the progression to stop for 15 seconds following each "actor's" push in order to afford the opportunity to look at the git log.
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4) Use the =gpg= utility to generate a key-pair so that you can work through the commit- and tag-signing slide and sample commands. If you have access to =gpg2= and not =gpg=, you'll need to set the following =git config= to make sure it's picked up: =git config --global gpg.program gpg2=.
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5) Open the file =GitDeepDive.pdf=, and as you go through the slides, refer to the /Sample Commands/ section below for commands you can execute to get further insights.
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** Sample commands
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*** Slide: Configuration
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#+BEGIN_SRC shell
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# Go to where the GitDeepDive.pdf file is and set your BASE_DIR
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# environment variable.
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cd <where-you-have-this-repo-cloned-to>
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export BASE_DIR=$(pwd)
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# List your basic git config
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git config --list
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# Go into the simple repo location and look at the various config
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# contexts
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cd ${BASE_DIR}/simpleRepo
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git config --local --list
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git config --global --list
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git config --system --list
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# Set some settings
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git config --global user.name "<YourName>"
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git config --global user.email "<YourEmailAddress>"
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# Use a text editor to edit your config
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git config -e
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#+END_SRC
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*** Slide: fetch and merge, not pull
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#+BEGIN_SRC shell
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# Go into the clone belonging to one of the developers in the
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# development team demo area
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cd ${BASE_DIR}/devTeamDemo/javaBootcampNoEclipse.dev1
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# Update the clone, but don't merge anything
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git fetch --prune
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# Review the local and remote branches.
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git branch -va
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#+END_SRC
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*** Slide: Merging approaches: fast-forward
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#+BEGIN_SRC shell
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cd ${BASE_DIR}/simpleRepo
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# Check out the master branch and review it's log
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git checkout master
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git log --decorate --graph --oneline --all
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# Merge in the Rel1 branch and review the new log.
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git merge Rel1
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git log --decorate --graph --oneline --all
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#+END_SRC
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*** Slide: Merging approaches: merging strategies
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#+BEGIN_SRC shell
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cd ${BASE_DIR}/simpleRepo
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# Check out the master branch, merge the Rel2 branch and review the
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# new log.
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git checkout master
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# You'll be prompted for a commit message here.
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git merge Rel2
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git log --decorate --graph --oneline --all
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#+END_SRC
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*** Slide: Merging approaches: Rebase
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#+BEGIN_SRC shell
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# Remove and refresh the simpleRepo
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${BASE_DIR}/simple-setup.sh
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cd ${BASE_DIR}/simpleRepo
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# Check out master, review its log, merge Rel1 and review the log
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git checkout master
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git log --decorate --graph --oneline --all
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git merge Rel1
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git log --decorate --graph --oneline --all
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# Check out Rel2
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git checkout Rel2
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# Rebase Rel2 onto the now-new master
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git rebase master
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# Review the log
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git log --decorate --graph --oneline --all
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#+END_SRC
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*** Slide: refs
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#+BEGIN_SRC shell
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# Remove and refresh the simpleRepo
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${BASE_DIR}/simple-setup.sh
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cd ${BASE_DIR}/simpleRepo
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# Look at the contents of .git/refs/heads and one of the heads itself
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ls -l .git/refs/heads/
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cat .git/refs/heads/Rel2
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# Tag a branch, look at the branch ref and the tag ref
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git tag Rel1.0 Rel1
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cat .git/refs/heads/Rel1
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cat .git/refs/tags/Rel1.0
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# Look at the HEAD ref
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cat .git/HEAD
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# Go to another clone and look at the refs for the origin remote
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cd ${BASE_DIR}/devTeamDemo/javaBootcampNoEclipse.dev1
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ls -l .git/refs/remotes/origin/
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# .git/packed-refs contains the refs that haven't been interacted with
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# yet.
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cat .git/packed-refs
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#+END_SRC
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*** Slide: Annotated tags
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#+BEGIN_SRC shell
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cd ${BASE_DIR}/simpleRepo
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# Look at a "lightweight" tag
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cat .git/refs/tags/Rel1.0
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# What type of object is it pointing to?
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git cat-file -t $(cat .git/refs/tags/Rel1.0)
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# What's in the object it's pointing to
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git cat-file -p $(cat .git/refs/tags/Rel1.0)
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# Create an annotated tag and look at *it*
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git tag -a -m "Formal release of 1.0" Rel1.0.prod Rel1
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cat .git/refs/tags/Rel1.0.prod
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# What type of object is it pointing to?
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git cat-file -t $(cat .git/refs/tags/Rel1.0.prod)
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# What's in the object it's pointing to
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git cat-file -p $(cat .git/refs/tags/Rel1.0.prod)
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#+END_SRC
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*** Slide: blame
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#+BEGIN_SRC shell
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# Look at the lines of the file on master
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git checkout master
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git blame information.md
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# .. and on Rel1
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git checkout Rel1
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git blame information.md
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# And on master after Rel2 has been merged in.
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git merge Rel2
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git blame information.md
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# Slide
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#+END_SRC
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*** Slide: Tag and commit signing
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#+BEGIN_SRC shell
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# Look at your secret keys
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gpg --list-secret-keys
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# Check out master and make a change to information.md
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git checkout master
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cat <<EOF >> information.md
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An additional line for demonstrating commit-signing.
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EOF
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# Add and commit the change, signing the commit.
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git add information.md
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git commit -S<secretKeyID> -m "Update to information.md"
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# Look at the commit object
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git cat-file -p master
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# Create an annotated tag from the Rel2 branch, signing it.
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git tag -s -u <secretKeyID> -m "Release 2." Rel2.0 Rel2
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# Look at the tag object
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git cat-file -p Rel2.0
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# Verify the signed tag and the signed commit.
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git tag -v Rel2.0
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git log --show-signature -1
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#+END_SRC
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*** Slide: Git Objects -- blobs
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#+BEGIN_SRC shell
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# Generate the SHA1 of the contents of the ~/.bash_history file as
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# though git would
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cat ~/.bash_history | git hash-object --stdin
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#+END_SRC
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*** Slide: Git Objects -- trees
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#+BEGIN_SRC shell
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cd ${BASE_DIR}/devTeamDemo/javaBootcampNoEclipse.dev1
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# Get the tree object for the latest version of the top-level of the
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# project
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git cat-file -p master | grep '^tree'
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# Look at the contents of that tree object:
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git cat-file -p $(git cat-file -p master | grep '^tree' | sed 's/^tree //')
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#+END_SRC
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*** Slide: Git Objects -- commits
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#+BEGIN_SRC shell
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cd ${BASE_DIR}/simpleRepo
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git log --graph --all --decorate --oneline
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git cat-file -p HEAD
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git cat-file -p <anyOtherCommitId>
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#+END_SRC
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*** Slide: Git Objects -- tags
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#+BEGIN_SRC shell
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# Find all the tag objects
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git cat-file --batch-check='%(objecttype) %(objectname) %(objectsize)' --batch-all-objects | grep tag
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git cat-file -p <anyOfTheTagObjects>
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#+END_SRC
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*** Slide: "Content Addressable Filesystem"
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#+BEGIN_SRC shell
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# Find all the objects, and select one that refers to a file
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git rev-list --all --objects
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# Look at the contents of the selected object
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git cat-file -p <selectedBlobObject>
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# Use git to get the SHA1 of the contents of that object
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git cat-file -p <selectedBlobObject> | git hash-object --stdin
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# The name of the file for that object is based on the SHA1.
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ls -l .git/objects/...
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#+END_SRC
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*** Slide: The reflog
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#+BEGIN_SRC shell
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# Look at the reflog, then clear it completely and look at it again.
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git reflog
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git reflog expire --expire=now --expire-unreachable=now --verbose --all
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git reflog
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#+END_SRC
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*** Slide: fsck and gc
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#+BEGIN_SRC shell
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git fsck
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git gc
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#+END_SRC
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*** Slide: Useful commands
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#+BEGIN_SRC shell
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# Go into the repo where the large file had been created
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cd ${BASE_DIR}/largeFileRepo
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# List all the objects in increasing order of object size.
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git cat-file --batch-check='%(objecttype) %(objectname) %(objectsize)' --batch-all-objects | sed -n 's/^blob //p' | sort -n --key=2
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# Look for the file name associated with an object
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git rev-list --objects --all | grep <blobID>
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# Look for the commits that made changes to a specific file
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git log --follow -- "largeInformation.md"
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#+END_SRC
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*** Slide: Permanently removing a file from your git db
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#+BEGIN_SRC shell
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# Preserve information on the tags, as you may need this later.
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for tag in $(git tag)
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do
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echo "${tag},$(git log --format="%H,\"%cn\",\"%ci\",\"%s\"" ${tag} | head -1)"
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done | tee /tmp/tag_list.csv
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# Create a local branch for all the remote branches. This does nothing
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# in this demo as there is no remote.
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for branch in $(git branch -r | grep -v HEAD | sed 's/\ \ origin\///')
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do
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git branch ${branch} origin/${branch}
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done
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# Check out each branch and determine the amount of space it uses
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for branch in $(git branch | sed 's/^..//')
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do
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git checkout -q ${branch}
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du -sk . | sed "s/\./${branch}/"
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done | tee /tmp/branch_sizes.out
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# Check out the Rel1 branch
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git checkout Rel1
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# Use git-filter-branch to remove the file from all the commits on the
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# branch
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git filter-branch --tree-filter 'rm -f largeInformation.md' --prune-empty HEAD
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# Update refs (will fail for some branches)
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git for-each-ref --format="%(refname)" refs/original/ | xargs -n 1 git update-ref -d
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# Do the same for all the other branches
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for branch in Rel2 Rel3 Rel4 master
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do
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git checkout ${branch}
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git filter-branch --tree-filter 'rm -f largeInformation.md' --prune-empty HEAD
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git for-each-ref --format="%(refname)" refs/original/ | xargs -n 1 git update-ref -d
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done | tee /tmp/cleanup.out
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# Clear out the reflog
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git reflog expire --expire-unreachable=now --all
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# Run the garbage collector on the repository
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git gc --prune=now
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# Run an FSCK on the repo
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git fsck --unreachable --no-reflogs
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# Check out each branch again and determine the amount of space it
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# uses
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for branch in $(git branch | sed 's/^..//')
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do
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git checkout -q ${branch}
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du -sk . | sed "s/\./${branch}/"
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done | tee /tmp/branch_sizes_post_process.out
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#+END_SRC
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