vagrant@precise32:/$ ls -la /var/log/
drwxr-xr-x+ 17 root          root              4096 Jul 23 01:57 .
drwxr-xr-x  15 root          root              4096 Jul 23 01:06 ..
drwxr-xr-x+  2 logstash      logstash          4096 Jul 23 01:57 logstash
drwxr-xr-x+  2 nginx         root             12288 Jul 22 04:30 nginx

vagrant@precise32:/$ sudo -u nginx ls -la /var/log/nginx
total 12812
drwxr-xr-x+  2 nginx    root   12288 Jul 22 04:30 .
drwxr-xr-x+ 17 root     root    4096 Jul 23 01:57 ..
-rw-r--r--+  1 nginx    root       0 Nov 24  2012 access.log

vagrant@precise32:/$ sudo -u nginx ls -la /var/log/logstash
total 8
drwxr-xr-x+  2 logstash logstash 4096 Jul 23 01:57 .
drwxr-xr-x+ 17 root     root     4096 Jul 23 01:57 ..
-rw-r--r--+  1 root     root        0 Jul 23 01:57 logstash.log

vagrant@precise32:/$ sudo -u logstash ls -la /var/log/nginx
ls: cannot access /var/log/nginx: Permission denied

vagrant@precise32:/$ sudo -u logstash ls -la /var/log/logstash
ls: cannot access /var/log/logstash: Permission denied