# Overview
git_update.sh is a bash script performing a punctual synchronization of a git repository. It can either be a new clone or an update.
CAREFUL: git_update.sh is not working-directory safe and can delete your work.

# Download
Download the git_update.sh file.
Bash and Git have to be installed on your computer. 

# Use
## Help extract
NAME
    git_update.sh
SYNOPSIS
    git_update.sh [-h] [-r ref] [-d dest] [-H] [-a] repository
OPTIONS
	-h prints the help. 
	-r specifies the reference to the commit to be synchronized. It can be a tag or a branch. By default, it is the last commit of branch main.
	It can be different from the actual branch. CAREFUL, the command git branch will always show the original branch name even though a branch switch has happened.
	-d specifies the destination of the clone or update. Directory must be empty if a new clone is to be made.
	If the repository to be cloned is local, and its path is passed as a relative path, the path should start from the destination. 
	To avoid mistakes, absolute paths are advised. 
	-H allows the $HOME directory to be used by git_update.sh. By default, git_update.sh cannot access $HOME to prevent default behavior. 
	If you need the global .gitconfig located in your $HOME to be used, you should supply the -H option. 	
	-a specifies that the aggressive option of the git garbage collection must be used. Only advised when changes happen in many different objects. Will slow down the execution.   
DESCRIPTION
	This script will replace the destination with the wanted commit of a git repository. The history is not preserved but tags are. Untracked files remain.
	The git commands have been chosen so as to minimize the memory and bandwidth usages.

## Cloning
git_update.sh will only clone in an empty repository, by default the current working directory. 
If called in a repository with a .git directory, it will update (see below). If called in a repository with untracked files, it will fail. 
Only the commit and the necessary objects will be cloned. The commit can be indicated through a reference, either a tag or branch, otherwise the last commit of main is the default. 
CAREFUL: the command git branch will always show the original branch name and will not follow the eventual branch changes. 
git_update.sh clones using 
"	git clone --recurse-submodules --shallow-submodules --depth=1"

## Updating
git_update.sh will update if the repository already contains a .git. Untracked files and directories will be kept. Any local modification to a tracked file or created tracked file will be deleted in favor of the new commit. The history will only contain the very last commit. 
git_update.sh updates using 
"	git fetch --tags --depth=1 --prune --prune-tags --force origin $ref 
	git reset --hard --recurse-submodules FETCH_HEAD 
	git submodule update --init --recursive --force --depth=1 --remote 
	git reflog expire --expire=now --all 
	git gc --prune=now [--aggressive]"

## Examples
A) Cloning the last commit of a branch into a non-existing directory 
./path/to/git_update.sh -r myBranch -d myDirectory https://git.mydomain/myname/myrepository.git 
Result: a directory myDirectory has been created in the current working directory. It is filled with the files of the last commit of branch myBranch as well as the .git. The history only shows the last commit.

B) Updating the current working directory repository, moving it from its current state to a certain tag, calling for an aggressive garbage collection.
./path/to/git_update.sh -r myTag -a https://git.mydomain/myname/myrepository.git 
Result: untracked files are still here. all the tracked files have been changed to respect the tag commit. The history only shows the last commit.
git gc has been called with the --aggressive option, hence the objects in the .git have been repacked so as to optimize memory.
CAREFUL: git branch still displays the original branch name and not the one of the tag. 


# Development process
git_update.sh has been written by the French association Jean-Cloud (https://www.jean-cloud.net), in the process of developing Shlagernetes, an orchestration tool. 
Shlagernetes allows storing services on fallible second-hand servers and tries to consume the less possible resources. 

Several tests have been performed in order to find the most energy-saving Git commands. 
You can find them under /test/performance_tests.
Please refer to /doc/development_explanations for more information about the tests and choices made in the elaboration of git_update.sh.

# Testing
The script test_git_update.sh can be found at /test/functional_tests. It will create a repository to test on before testing.
Please refer to the readme of test and the readme of test/functional_tests. 

# How to contribute
If you have any suggestion or question about git_update.sh, please contact us at contact@jean-cloud.org. 
We would be delighted to hear your feedback. Feel free to send diff files with improvement suggestions.