Applied online and received an e-mail the next day to setup a screening call. Screening was 20-30 basic Linux/Networking questions, such as common port numbers, how to check disk usage, disk space. A couple were MySQL-specific, and a couple were RedHat/CentOS specific. Was contacted by recruiter a couple of days after screening to schedule a "break/fix" session, which was for the next day.
The break/fix was done over the phone with a systems administrator, where you're asked to SSH into a remote host to conduct the assessment. The person wasn't personable, outgoing or empathetic at all, which didn't exactly help with the nervousness of the interview process. I felt I did terribly in the session, despite having years of professional Linux Sysadmin experience. This was done on a CentOS system, so be sure to brush up on RHEL/CentOS. The break/fix had some easy scenarios and some that consisted of strange issues or caveats.
Tips for candidates: Be familiar with RHEL/CentOS (e.g. yum, where things are located, what they're called, etc). Be familiar with MySQL. Check the simple, easy things first. Be familiar with all components of the LAMP stack. Be familiar with using netstat, tcpdump, and iptables. I was never asked about my resume, past experience, anything personal, etc. It was strictly skills assessments, and I assume that more personal questions would have come later.