util/groupmerge.sh

#!/bin/bash
#
# groupmerge.sh
#
# merge two /etc/groups together, avoiding id conflicts.
# to be used within dockerfiles

if [[ ! -e $1 || ! -e $2 ]]; then
	echo "usage: $0 <groupfile1> <groupfile2>"
	echo "Use groupfile1 as the basis and merges missing entries from groupfile2"
fi

cat $1 | grep -v ":1000:"  
# yes, group assignments of the host user will remain, but it won't hurt

echo "# merged entries:"
cat $2 | while read grp; do
 	name=$(echo $grp | cut -d ':' -f 1)
	pass=$(echo $grp | cut -d ':' -f 2)
	dgid=$(echo $grp | cut -d ':' -f 3)
	rest=$(echo $grp | cut -d ':' -f 4-)

	line=$(cat $1 | grep -E "^$name:")
	if [[ $line == "" ]]; then
		ngid=$(($dgid + 2000))
		echo $name:$pass:$ngid:$rest
		if [[ -e /.dockerenv ]]; then
			find / -group $dgid -mount -exec chgrp $ngid {} \;
		fi
	fi
done