Git - simple repository management

Agenda

VCS intro
tracking modifications
using a local git repository
remote repositories

Beware: commands contain small typos. You must fix them to properly complete the course!

VCS

The importance of tracking changes.

VCS basics:

initialize a local repo
change the software
acknowledge (commit) changes
eventually revert changes

Tracking modifications

Track modifications of our config files without messing with the real /etc.



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! mkdir -p /repo-path
!cp -v /etc/host* /etc/s* /repo-path

All operations are local to /repo-path



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cd /repo-path

Always timestamp backup copies, don't .ori.



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! SIMPLE_BACKUP_SUFFIX="$(date -I)" cp -v -bf hosts hosts
!ls -l hosts*

Exercise: Use date +%s to timestamp a backup copy of hosts.



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# Use this cell for the exercise.

Git

A better way of tracking changes.

Used to maintain the Linux Kernel.

Distributed approach.

Checkout and Push

Tracing requires identification

Declare who's modifying files. This will be inserted in the commit.



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!git config --global user.email "jon@example.com"
!git config --global user.name "Jon Doe"

Note: authentication can not be enforced on a local repository.

Create a repository

Track modifications with git



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!git init /repo-path
!ls -l /repo-path/.git  # this is the index directory

Exercise

get the previous git config ... user.email
remove the --global flag from the previous command
run it



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# Write here the command
# and show the git config file.
!cat .git/config

Enter in the repo directory and check the status: there are a lot of files we are not interested in...



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!git status

.gitignore lists the files we're not interested in



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# Ignore all files not starting with h
!echo "[^h]*" >> .gitignore
!git status

Now we have all host* files to be tracked.

Populate the repo

Add files to the index



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! git add hosts

The file is now staged for commit. It's not archived though.



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!git status

Save files to the local index



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! git commit -m "Initial snapshot of hosts"

Basic workflow

Adding a line to the file we discover that



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!echo "127.0.0.2  localhost2.localdomain" >> hosts 
!git diff hosts

If we like the changes, we can stage them



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!git add hosts
!git status

and finally save them in the repo.



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!git commit  "Added localhost2 to hosts"

History changes

Now we have an history with two changes, containing:

commit messages
a commit hash

HEAD is the last commit.



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!git log

Reverting changes

We can revert a change using the hash or an history log



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! git checkout HEAD~1 -- hosts  # revert hosts to the previous commit

Cheatsheet

Now some git commands, but first create a dir.



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! mkdir -p /repo-path
!date >> /repo-path/file.txt
!date >> /repo-path/hi.txt



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!git init /repo-path    # Initialize repo eventually creating a directory
!git add /repo-path/hi.txt # Add file to index
!git commit -m "My changes"  # Save changes

Exercise

add file.txt to the index and commit



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# Use this cell for the exercise



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!date >> /repo-path/file.txt
!git diff
!git commit -a -m "Save all previously added files"



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!git log /repo-path/file.txt  # show changes



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!git checkout HEAD~1 -- file.txt # revert file



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!git diff HEAD  # diff with reverted



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!git checkout HEAD -- . # get *all files* from the latest commit

Tags & Branches

Writing codes and configuration we may want to follow different strategies and save our different attempts.

tag makes an unmodifiable snapshot of the repo instead.



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!git tag myconfig-v1 # create a tag
!git tag -l    # list tags

branch create a modifiable copy of the code, allowing to save and work on different features

Branches

master is the default branch



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!git branch -a

Create a branch



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!git checkout -b work-on-my-changes

And list the branches, check the active one!



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!git branch -a

Modify a file in a branch



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!date > new-file.txt
!git add new-file.txt

With commit we consolidate the new file in the branch



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!git commit -m "Added a new file"

Compare branches



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!git diff mister

Diff supports some parameters



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!git diff --ignore-all-space master

We can now switch between branches



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!git checkout master
!cat new-file.txt

And switch back



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!git checkout work-on-my-changes
!cat new-file.txt

Exercise

Create a new branch named antani
modify new-file.txt as you please
open a terminal, and use git add -p to stage the changes. What does it do?
commit the changes



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# Use this cell for the exercise

Checkout troubleshooting

If you change a file, git won't make you checkout to avoid missing changes.



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!date >> new-file.txt
!git checkout master

You have to remove the changes or commit them (in another branch too)



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# Use this cell for the exercise.

Merge

Once we have consolidated some changes (Eg. test, ...) we can merge the changes into the master branch



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!git checkout master
!git diff work-on-my-changes
!git merge work-on-my-changes

After a merge, if the branch is no more useful, we can remove it.



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!git branch -d work-on-changes

If there are unmerged changes, git doesn't allow deleting a branch.

Exercise:

use git branch -d to remove the antani branch
what happens?
replace -d with -D. Does it work now?



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# use this cell for the exrcise

Selective adding

You can stage partial changes with:



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!git add -p

Remote repositories

Remote repos may be either https, ssh or files.



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! mkdir -p /repo-tmp && cd /repo-tmp # use another directory

https repo

Git clone downloads a remote repo, with all its changes and history. Try with a remote https repo.



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! git clone https://github.com/ioggstream/python-course/ python-course
cd /repo-tmp/python-course

Show repository configuration. Remote origin.



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! git config -l

The remote repo is retrieved with all its changes and history



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! du -ms .git

And log can show branches and merges.



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!git log --graph

file repo

A local repo can be cloned too, and has the same features of a remote one. It's actually a remote file:// uri.



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! git clone /repo-tmp/python-course /repo-tmp/my-course

Show repository configuration. Remote origin.



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! git config -l

Pull & push

You can add new files to a repo with the above workflow:

create a branch with git checkout -b test-1
add a new file
stage changes with git add
commit with git commit

Now that your changes are on your local repo, you can synchronize / upload them to the remote copy with:



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! git push origin test-1

Remember:

origin is the URI specified by git config -l
test-1 is the branch name where you want to upload

To upload changes to the remote master (default) branch, you need to

merge the changes to your local master



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!git checkout master
!git merge test-1

push changes to master



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!git push origin master

To make it work, you need to be authenticated/authorized with the remote repo ;)