ruby 環境で必要だったので、kazuho さんの https://gist.github.com/kazuho/6181648 を参考に書いた。(参考リンク - 2) http://d.hatena.ne.jp/hirose31/20130808/1375965331

setusergroups

#!/usr/bin/env ruby

# usage: setusergroups [-h|--help] username child
#
# setuidgid w. support for supplementary groups
# ruby porting of https://gist.github.com/kazuho/6181648
#
# see: http://d.hatena.ne.jp/hirose31/20130808/1375965331
# see: https://gist.github.com/kazuho/6181648

require 'etc'

will_show_help  = false
will_show_help |= ARGV.size < 2
will_show_help |= ["-h", "--help"].include? ARGV[0]

if will_show_help
  puts "usage: setusergroups [-h|--help] username child"
  exit 1
end

username = ARGV.shift

# get uid, gid
userent = Etc.getpwnam(username) \
  or fail "unknown user: #{username}"

# get supplementary groups
supp_groups = []
while e = Etc.getgrent
  supp_groups << e.name if e.mem.include?(username)
end

# setuid, setgid, setgroups
Process::Sys.setgid userent.gid
Process.groups = supp_groups
Process::Sys.setuid userent.uid

# run given program
program = ARGV.shift
exec(program, *ARGV)