Linux Interview Questions

What is the difference between a process and a thread in Linux?

What is the difference between `soft` and `hard` links in Linux?

Explain Linux file permissions. What does `chmod 755` mean?

What is the purpose of `/etc/passwd` and `/etc/shadow`?

What is a zombie process? How do you find and kill it?

What is the difference between `kill`, `kill -9`, and `kill -15`?

What does `top` show and what are key metrics to watch?

What is `sudo` and how is it different from `su`?

How do you check disk usage in Linux?

What is `cron` and how do you schedule a cron job?

What is the `/proc` filesystem?

What is the difference between `grep`, `awk`, and `sed`?

How do you check which port a process is listening on?

What is `systemd` and how do you manage services with it?

What is the difference between `>` and `>>` in shell?

How does Linux boot? Explain the boot process.

What is `inode` in Linux? What happens when you run out of inodes?

Explain Linux memory management: what is swap, page cache, and OOM killer?

What is `strace` and when would you use it?

How does `iptables` work? Explain tables and chains.

What is the difference between `ps aux` and `ps -ef`?

How do you find and delete files older than N days in Linux?

What is `ulimit` and why is it important for production services?

Explain Linux namespaces and cgroups — how do they enable containers?

How does SSH key-based authentication work?

What is `journald` and how is it different from traditional syslog?

How do you troubleshoot high CPU usage in Linux?

Explain the Linux load average. When is it a problem?

What are Linux runlevels / systemd targets?

How do you manage environment variables in Linux?

What is `tcpdump` and how would you use it to debug network issues?

Explain Linux signals. What are SIGHUP, SIGINT, SIGTERM, SIGKILL?

What is `lsof` and what can you learn from it?

How do you create and manage Linux users and groups for a service?

What is NFS and when would you use it in a Kubernetes environment?

What is SELinux and AppArmor? How do they differ?

Explain Linux kernel tuning parameters relevant to high-traffic servers.

What is eBPF and how is it changing Linux observability?

How does Linux handle virtual memory and the page table?

What are Linux capabilities and how do they relate to container security?

How does `perf` work and when would you use it over `strace`?

What is the difference between soft IRQ, hard IRQ, and kernel threads in Linux?

How do you implement log rotation for application logs in Linux?

What is NUMA (Non-Uniform Memory Access) and why does it matter for performance?

How would you harden a Linux server for production?

What are Linux namespaces used for in container isolation? How would you create one manually?

What is `systemd-resolved` and how does DNS resolution work in Linux?

How does `rsync` work and when would you use it over `scp`?

What is io_uring and how does it improve I/O performance?

How would you debug a server that is unresponsive but still SSH-accessible?