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LinuxLinux + IT series

Quiz: 3 Linux questions most admins get wrong

Three Linux questions that catch out experienced admins. Answer each one before you read the explanation.

Quick reference

**Q1. You delete a 10 GB log file, but df -h still shows the same usage. Why?**

  • a) The filesystem needs a reboot to update
  • b) A process still has the file open
  • c) Linux keeps deleted files in a trash folder

Answer: b) A process still has the file open. The space is freed only when the last process closes the file. Find it with lsof +L1, then restart that process.

Q2. A 4-core server shows a load average of 4.00. What does that mean?

  • a) It is overloaded at 400%
  • b) It is about fully busy, not yet overloaded
  • c) Four users are logged in

Answer: b) It is about fully busy, not yet overloaded. Load counts tasks running or waiting. On 4 cores, 4.00 means every core is busy. Above that, work starts to queue.

**Q3. Which file decides whether /etc/hosts is checked before DNS?**

  • a) /etc/resolv.conf
  • b) /etc/nsswitch.conf
  • c) /etc/hostname

Answer: b) /etc/nsswitch.conf. The hosts: line in nsswitch.conf sets the order, usually files then dns. resolv.conf only lists the DNS servers.

Explained

  1. You delete a 10 GB log file, but df -h still shows the same disk usage. Why?

Answer: a process still has the file open. Deleting a file removes its name, but the data stays on disk until the last process holding it closes the file. lsof +L1 lists these deleted-but-open files. Restart the process and the space comes back. Next time, empty the file with truncate -s 0 instead of deleting it.

  1. A 4-core server shows a load average of 4.00. What does that mean?

Answer: it is roughly fully busy, but not overloaded. The load average counts tasks that are running or waiting to run, including tasks waiting on disk I/O. On 4 cores, 4.00 means every core has work. A load of 8 on the same machine means tasks are queuing. The number only means something next to the core count, which nproc shows.

  1. Which file decides whether /etc/hosts is checked before DNS?

Answer: /etc/nsswitch.conf. The hosts: line sets the lookup order, usually files dns, meaning the hosts file first and DNS second. /etc/resolv.conf only says which DNS servers to ask. This is why an old entry in /etc/hosts can override a DNS change and make one server see a different site than everyone else.

None of these are rare situations. All three show up in real tickets every month.

In one line

Fundamentals never expire. Tools change every year. These answers do not.

Try the first one

lsof +L1

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