Question 1

How do you check which process is using the most CPU and memory on a Linux server?

Accepted Answer

I start with top or htop, which show live CPU and memory use per process, sorted so the heaviest is at the top. For a quick snapshot I use ps aux --sort=-%cpu | head or --sort=-%mem. Once I have the PID I look closer with ps -p, or check its logs to see why it is busy.

Question 2

Explain Linux file permissions. What does chmod 755 mean?

Accepted Answer

Every file has permissions for three groups: the owner, the group and everyone else. Each can have read (4), write (2) and execute (1). 755 means the owner gets 7 (read, write, execute) and the group and others get 5 (read and execute). It is typical for scripts and folders that others need to use but not change.

Question 3

The disk on a server is full. How do you find what is using the space?

Accepted Answer

First df -h shows which filesystem is full. Then I go into that mount and use du -sh * | sort -h, or du -h --max-depth=1, to find the biggest folders and drill down. The usual suspects are logs in /var/log, old Docker images and temp files. I clean up safely, for example rotating logs or running docker system prune, and then add log rotation or monitoring so it does not happen again.

Question 4

What is the difference between a process and a service?

Accepted Answer

A process is any running program with a PID. A service is a process that runs in the background and is managed by the system, usually systemd, so it starts on boot, restarts if it crashes and has its logs collected. I manage services with systemctl start, stop, status and enable, and read their logs with journalctl -u.

Question 5

How do you search for a word inside log files?

Accepted Answer

I use grep. grep -i error app.log finds lines with 'error' in any case, -r searches a whole folder, -n shows line numbers and -C 3 shows 3 lines of context around each match. For live logs I combine it with tail -f, like tail -f app.log | grep ERROR.

Linux interview questions