BASH script to port a Git commit from one Git repo to another unrelated Git repo (can run with BASH on Windows)

Example output

$ GitPortPatch.sh 22b7f80c22ac1861e945ddf30e30671c46b53d6e
Getting commit message
Getting patch
Getting affected filenames
Getting current status of those files...
Checking them...
Ensuring affected files are ready for patching...
        Preparing Assets/TerraVolPack/TerraVol/Scripts/Data/MeshData.cs
        Preparing Assets/TerraVolPack/TerraVol/Scripts/Map/Chunk.cs
Apply patch? y/n : y
Patching...
patching file Assets/TerraVolPack/TerraVol/Scripts/Data/MeshData.cs
patching file Assets/TerraVolPack/TerraVol/Scripts/Map/Chunk.cs
Adding affected files to staged group...
        Assets/TerraVolPack/TerraVol/Scripts/Data/MeshData.cs
        Assets/TerraVolPack/TerraVol/Scripts/Map/Chunk.cs
Commit patch? y/n : y
Committing...
[voxels-generations 2842944] MeshData: validation working (maybe doing extraneous calculations?) (from 22b7f80c22ac1861e945ddf30e30671c46b53d6e)
 2 files changed, 80 insertions(+), 25 deletions(-)
Done.

GitPortPatch.sh

#!/bin/bash
# Process a Git commit from one Git repository into another (unrelated) repository.
# (there are definitely better ways if they are related, e.g. cherry-pick)
# Currently leaves tmp files in /tmp (since it's sometimes handy to check).
# v1.0	2017/04/22	Rupert Key / @Arakade

showHelp() {
	echo -e "\n${CmdName} [-h -? -g -a -G -A -1 <DirFrom> -2 <DirTo> ] <SHA1>"
	echo -e "\t-h -?     : this help"
	echo -e "\t-g and -G : -g get patch (default), -G do not get patch"
	echo -e "\t-a and -A : -a apply patch (default), -A do not apply patch"
	echo -e "\t-1        :  set 'from' repository"
	echo -e "\t-2        :  set 'to' repository"
	echo -e "\tSHA1      :  commit to port\n"
	echo -e "\tYou will be asked before anything is changed."
	echo -e "\tIf you wish to tweak a patch before applying,"
	echo -e "\t- answer 'n' to not apply,"
	echo -e "\t- tweak the files in /tmp/<SHA1>* as needed,"
	echo -e "\t- resume without redoing the 'get' with $CmdName -G <SHA1>"
	echo -e "\n"
}

checkRepo() {
	if [ "XX" == "X${1}X" ] ; then
		showHelp
		echo -e "ERROR: No Git repository specified as $2\n"
		exit 1
	fi

	if [ ! -d "$1" ] ; then
		showHelp
		echo -e "ERROR: \"$1\" does not exist (or is not a directory)\n"
		exit 1
	fi

	if [ ! -d "$1/.git" ] ; then
		showHelp
		echo -e "ERROR: \"$1\" is not a Git repository\n"
		exit 1
	fi
}

GetPatch() {
	cd "$D1"
	# Get log for committing at end
	echo "Getting commit message"
	git log --pretty=%B "${commit}^..${commit}" | awk "NR==1 {printf(\"%s (from %s)\n\", \$0, \"${commit}\")} NR>1" > "$LogFile"
	if [ 0 -ne $? ] ; then
		echo "Failed to get log"
		exit 1
	fi

	# Get the actual patch
	echo "Getting patch"
	git diff "${commit}^" "${commit}" > "$PatchFile"
	if [ 0 -ne $? ] ; then
		echo "Failed to get diff"
		exit 1
	fi

	# Process filenames affected from the patch
	echo "Getting affected filenames"
	# --- a/Assets/TerraVolPack/TerraVol/Scripts/Data/ActionData.cs
	sed -n 's%^--- a/\(.*\)$%\1%p' "$PatchFile" > "$FilenamesFile"

	cd "$D2"

	# See whether any already modified
	echo "Getting current status of those files..."
	git status > "$StatusFileBefore"
	echo "Checking them..."
	AlreadyModified=""
	cat "$FilenamesFile" | while read f ; do
		if grep -q "$f" "$StatusFileBefore" ; then
			# Really changed (not just whitespace)
			linesDiff=$(git diff -w "$f" | wc -l)
			if [ $linesDiff -ne 0 ] ; then
				AlreadyModified="\t$f\n${AlreadyModified}"
			fi
		fi
	done
	if [ "XX" != "X${AlreadyModified}X" ] ; then
		echo -e "Some affected files already modified. They'll need commiting first:\n$AlreadyModified"
		exit 1
	fi

	# Start actually changing things
	echo "Ensuring affected files are ready for patching..."
	cat "$FilenamesFile" | while read f ; do
		if [ -f "$f" ] ; then
			echo -e "\tPreparing $f"
			dos2unix -q "$f"
		fi
	done
}

ApplyPatch() {
	read -n 1 -p "Apply patch? y/n : " response
	if [ "X${response}X" != "XyX" ] ; then
		echo ""
		exit 0
	fi

	cd "$D2"

	# Actually apply the patch
	echo -e "\nPatching..."
	patch -i "$PatchFile" -p 1 -F 10 -N

	echo "Adding affected files to staged group..."
	cat "$FilenamesFile" | while read f ; do
		if [ -f "$f" ] ; then
			echo -e "\t$f"
			git add "$f"
		fi
	done

	read -n 1 -p "Commit patch? y/n : " response
	if [ "X${response}X" != "XyX" ] ; then
		echo ".  OK, not committing."
		echo "Commit with git commit --file \"$LogFile\""
		exit 0
	fi

	echo -e "\nCommitting..."
	git commit --file "$LogFile"

	echo "Done."
}

##
## MAIN
##

CmdName="$(basename $0)"
D1=""
D2=""
doGetPatch=1
doApplyPatch=1
commit=""
OPTIND=1
while getopts "h?-gaGA1:2:" opt ; do
	case "$opt" in
		h|\?|-)
			showHelp
			exit 0
			;;
		g)
			doGetPatch=1
			;;
		G)
			doGetPatch=0
			;;
		a)
			doApplyPatch=1
			;;
		A)
			doApplyPatch=0
			;;
		1)
			D1="$(readlink -m ${OPTARG})"
			;;
		2)
			D2="$(readlink -m ${OPTARG})"
			;;
	esac
done

shift $((OPTIND-1))
[ "X${1}X" == "X--X" ] && shift

commit="$1"
if [ "X${commit}X" == "XX" ] ; then
	showHelp
	echo -e "ERROR: No commit supplied\n"
	exit 1
fi

checkRepo "$D1" "source"
checkRepo "$D2" "destination"

PatchFile="/tmp/${commit}.patch"
LogFile="/tmp/${commit}.log"
FilenamesFile="/tmp/${commit}.filenames" 
StatusFileBefore="/tmp/${commit}-before-status.txt"

[ $doGetPatch -eq 1 ] && GetPatch
[ $doApplyPatch -eq 1 ] && ApplyPatch