Simplest POSIX ACL example

This is an exhibit of POSIX Access Control Lists.

user|tmp/sand-PzO % # create an inaccessible file called "test" with "123" inside
user|tmp/sand-PzO % # NOTE: I could've written 0 instead of 000.
user|tmp/sand-PzO % install -m 000 <(echo 123) test

user|tmp/sand-PzO % # which the "User nobody (can) Read"
user|tmp/sand-PzO % setfacl -m u:nobody:r test

user|tmp/sand-PzO % ls -l test
----r-----+ 1 user user 4 May 24 11:03 test

user|tmp/sand-PzO % cat test
cat: test: Permission denied
user|tmp/sand-PzO % sudo -u nobody cat test
[sudo] password for user:
123