git_update/git_update.sh

#!/bin/bash
summary="$0 [options] <repo>"
ref=main
dst='.'
nonempty_target=false
use_home=false
be_aggressive=false

Help(){
	echo "
NAME
    git_update.sh
SYNOPSIS
    git_update.sh [-h] [-r ref] [-d dest] [-H] [-N] [-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.
	-d specifies the destination of the clone or update. If non-empty, -N must be specified. 
	-H 
	-N indicates that the destination is non-empty. The files that do not conflict and if it is an update, are not committed, will be kept. 
	-a specifies that the aggressive option of git clean must be used. git clean is called wen -N is not specified. 
	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.

DESCRIPTION
	This script will replace the destination with the wanted commit of a git repository. The history is not preserved but tags are.
	The git commands have been chosen so as to minimize the memory and bandwidth usages."
}

while getopts ":hr:d:NHa" option; do
   case $option in
      h) # display Help
         Help
         exit;;
      r) # branch or tag wanted
            ref="$OPTARG";;
      d) # destination of clone
            dst="$OPTARG";; 
	  N) # clone to a Non-empty target. Existing files will be overwritten
	  		nonempty_target="true";;
	  H) # use real home dir
	  		use_home="false";;
	  a) # use git clean with the aggressive option
	  		be_aggressive="true";;
     \?) # invalid option
         echo "Error: Invalid option here"
         exit;;
   esac
done
shift $((OPTIND-1))


repo="$1"
if [ -z "$repo" ] ; then
	exit "$0: Empty repo given\n$summary"
fi

if [ ! $use_home ] ; then
	set -a
	export HOME=/dev/null
	set +a
fi

mkdir -p "$dst"
cd "$dst"


if [ -d .git ] ; then

	git submodule update --init --recursive --force --depth=1 --remote 
	git fetch --tags --depth=1 --prune --prune-tags --force origin $ref 
	git reset --hard FETCH_HEAD 
	git reflog expire --expire=now --all 
	if "$be_aggressive" ; then
		git gc --prune=now --aggressive
	else
		git gc --prune=now 
	fi
	# Preserve existing files in some cases
	if ! "$nonempty_target" ; then #we keep uncommitted files when in -N mode
		git clean -qfdx
	fi

else
	clone_dst='.'

	# To override an existing dir, we need to clone elsewhere first
	if "$nonempty_target" ; then
		clone_dst="$(mktemp -d)"
	fi
    git clone -b "$ref" --recurse-submodules --shallow-submodules --depth 1 "$repo" "$clone_dst" 

	# To override an existing dir, we then move everything to that dir
	if "$nonempty_target" ; then
		mv "$clone_dst/"{*,.*} .
		rmdir "$clone_dst"
	fi
fi