Linux Interview Questions for DevOps Engineers (2026)

How do you troubleshoot a server with high load?

Start with uptime/top to see load vs CPU count, then vmstat and iostat to tell CPU-bound from I/O-bound, pidstat to find the process. High load with low CPU usually means I/O wait or blocked processes.

Explain Linux file permissions and ownership.

Each file has owner/group/other with read(4)/write(2)/execute(1) bits (e.g. 755). chmod changes bits, chown changes owner. Special bits: setuid, setgid, sticky bit (common on /tmp).

What is systemd and how do you manage a service?

systemd is the init system and service manager. systemctl start/stop/enable/status a unit; journalctl -u reads its logs; unit files define dependencies and restart policy.

How do you find what's using a port?

ss -tulpn (or lsof -i :PORT) shows the listening process and PID. This is a daily troubleshooting command.

How do you diagnose a full disk?

df -h finds the full filesystem; du -sh * (drill down) finds the big directories; lsof | grep deleted finds space held by deleted-but-open files (a classic gotcha fixed by restarting the holder).

What's the difference between a process and a thread?

A process has its own address space; threads share the process's memory. ps and top show processes; threads share the PID's resources.

How do signals work (SIGTERM vs SIGKILL)?

SIGTERM (15) asks a process to shut down gracefully (it can clean up); SIGKILL (9) force-kills immediately and can't be caught. Always try SIGTERM first.

How do you schedule recurring jobs?

cron (crontab -e) for classic schedules, or systemd timers for better logging/dependencies. For one-off future runs, 'at'.

How do you check memory and detect OOM?

free -h for usage; dmesg or journalctl for 'Out of memory: Killed process' lines when the OOM killer fires. High swap usage warns before OOM.

What happens from typing a command to it running?

The shell parses it, resolves the binary via PATH, forks a child, execs the binary, the kernel loads it and sets up memory, then it runs and returns an exit code.