Fifteen common sysadmin interview questions
- How do you check disk, memory and CPU usage on Linux?
- Explain Linux file permissions and chmod 755.
- How do you manage services with systemd?
- What is Active Directory, and what are domains, OUs and Group Policy?
- How does DNS work, and how do you troubleshoot it?
- What does DHCP do?
- A user can't log in to their Windows machine. What do you check?
- How do you approach patch management?
- Describe your backup strategy for servers.
- How do you secure SSH access?
- What would you monitor on a server?
- How do you automate routine tasks?
- What is RAID, and which levels do you use?
- How do you handle a server that's out of disk space?
- Tell me about an outage you resolved.
Linux essentials
- Resources: df -h and du for disk, free -h for memory, top or htop for CPU and processes, iostat for disk I/O.
- Permissions: read, write and execute for owner, group and others; 755 means rwx for the owner and r-x for group and others.
- Services: systemctl start, stop, restart, enable and status; journalctl -u service for logs.
- SSH hardening: key-based authentication, disable root login and password authentication, restrict users, and use a bastion or VPN.
Windows and Active Directory
| Concept | Answer |
|---|---|
| Domain | A security boundary of users, computers and policies managed centrally |
| Organisational unit (OU) | A container for organising objects and applying policies |
| Group Policy | Centrally applied settings for users and computers |
| Domain controller | Server that authenticates users and holds the directory |
| Security vs distribution groups | For permissions vs for email lists |
For “a user can't log in”: check whether the account is locked or disabled, the password expired, the machine can reach a domain controller, time is in sync (Kerberos is sensitive to clock skew), and the network is working. Start with the cheap checks and the most common causes.
Networking and backups
DNS troubleshooting: confirm which resolver the machine uses, query it directly with nslookup or dig, compare against a public resolver, and check caching and records. DHCP assigns addresses and network settings automatically; a machine with a self-assigned address usually couldn't reach a DHCP server. For backups, follow the 3-2-1 rule — three copies, on two kinds of media, one offsite — and test restores. RAID provides redundancy, not backup: RAID 1 mirrors, RAID 5 uses parity across three or more disks, RAID 10 mirrors and stripes for performance and resilience.
Patching, monitoring and automation
- Patching: inventory, test patches on a pilot group, schedule maintenance windows, prioritise critical security updates, and keep rollback options.
- Monitoring: availability, CPU, memory, disk space and I/O, service health, certificate expiry, failed logins and backup success.
- Automation: shell or PowerShell scripts for routine tasks, configuration management tools for consistency across servers. See Ansible interview questions and shell scripting questions.
Scenario: a server is out of disk space
Check which filesystem is full with df -h, find what's growing with du sorted by size, and look for the usual suspects: logs without rotation, old packages or kernels, temporary files, core dumps and backups stored locally. If deleting a large log doesn't free space, a running process still holds the file open — lsof shows deleted-but-open files, and restarting or signalling that process releases them. Afterwards, add log rotation, monitoring with alerts at sensible thresholds and a note in the runbook.
Practising
Interviewers follow up on the most specific thing you say, so rehearse the second question as well as the first. CoPilot Interview, a free AI interview assistant, runs system administration mock interviews with follow-ups and produces a report of likely next-round questions. Prepare two troubleshooting stories with the steps you took in order — interviewers score your method. See also Linux interview questions.
Disclosure: CoPilot Interview is our product, so weigh what we say about it accordingly. Everything else on this page applies whichever tool you use, or none.
Frequently asked questions
What questions are asked in a system administrator interview?
Linux and Windows administration, permissions, services, Active Directory and Group Policy, DNS and DHCP, troubleshooting log-in and connectivity issues, backups, RAID, patching, monitoring, SSH security and automation.
What does chmod 755 mean?
The owner can read, write and execute; the group and others can read and execute but not write.
Is RAID a backup?
No. RAID protects against disk failure, but not against deletion, corruption, ransomware or site loss. You still need separate, tested backups.
How do you troubleshoot DNS problems?
Check which resolver is in use, query it directly with nslookup or dig, compare with a public resolver, check for stale caches and verify the records themselves.
Practise on real interview questions
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