`userdo' opens specified files with the priviledge then execute the given command without the priviledge but with the file descriptors of the opened files.

usage: userdo [-h][-o path,mode,fd] user cmd [cmdarg]...

$ sudo ./userdo -o /hoge,w,3 nobody sh -c "whoami >&3"

check the result

$ ls -la /hoge
-rw-r--r--  1 root  wheel  7 Nov  7 13:35 /hoge

the file is created by `root',

$ cat /hoge
nobody

the command is executed by `nobody'.