created two users and added one user to the other user group without permission to operate
system: Ubuntu18.04
details:
drwxrwxr-x 2 www www 4096 July 4 10:15. /
drwxr-xr-x 6 root root 4096 July 4 10:14.. /
-rwxrwxr-x 1 www www 0 July 4 10:15 index.txt*
xun@xun-fyjh:/home/www$ rm-rf index.txt
rm: cannot delete "insufficient index.txt": permissions
xun@xun-fyjh:/home/www$ id xun
uid=1000 (xun) gid=1000 (xun) group = 1000 (xun), 1003 (www)
xun@xun-fyjh:/home/www$ groups xun
xun: xun www