Linux Interview Questions and Practice Labs

# CHAPTER 19

Linux Interview Questions and Practice Labs #

1. Introduction #

2. Learning Objectives #

• Confidently articulate solutions to complex Linux administration interview questions.

• Execute rapid-fire command combinations using pipes and filters.

• Demonstrate a logical, bottom-up troubleshooting methodology for server outages.

• Decipher and fix broken Bash scripts.

• Synthesize multiple Linux tools (e.g., ps, grep, kill) into single workflows.

3. Core Technical Interview Questions #

Q1: "A user complains that a website hosted on our Linux server is throwing a 502 Bad Gateway error. Walk me through your exact command-line troubleshooting steps from the moment you SSH into the machine." *How to answer (The Gold Standard):*

1. 1. Check Resources: "First, I run top or free -m to ensure the server isn't simply out of RAM or suffering a CPU lockup."

1. 2. Check the Service: "Next, I run sudo systemctl status nginx (or apache2) to see if the web daemon has crashed."

1. 3. Check the Ports: "I run sudo ss -tulpn | grep 80 to verify the server is actually actively listening on the HTTP port."

1. 4. Check the Logs: "If the service is running but throwing errors, I will inspect the application logs using tail -f /var/log/nginx/error.log or query the system journal using journalctl -u nginx to find the exact syntax or database connection error causing the 502."

Q2: "You type ls -l and see a file with the permissions drwxr-x---. Explain exactly what this means, who has access, and how you would change it to 755." *How to answer:* "The d means it is a directory. The owner has read, write, and execute permissions (rwx, which is a 7). The group has read and execute permissions (r-x, which is a 5). The rest of the world has zero access (---, which is a 0). To change it to 755 so the world can enter the directory, I would run the command chmod 755 [directoryname]."

Drill 2: The Rogue Process *Task:* A script named cryptominer.sh is running. Find its PID and kill it instantly using a single command string. *Answer:* ps aux | grep "cryptominer.sh" (Find the PID, e.g., 5043). Then kill -9 5043. *(Bonus point answer: killall -9 cryptominer.sh)*

Drill 3: The Archival Backup *Task:* Compress the /etc configuration directory into a gzip tarball named backup.tar.gz and place it in the /tmp folder. *Answer:* sudo tar -czvf /tmp/backup.tar.gz /etc

5. Troubleshooting Scenario Labs #

Scenario 1: The "Command Not Found" Panic *The Setup:* A developer writes a custom Python script, saves it as build.py in their /home/dev/scripts folder, makes it executable with chmod +x build.py, and types build.py. The terminal says Command Not Found. *The Solution:* The terminal only searches directories listed in the $PATH environment variable. The developer's personal script folder is not in the PATH. They must execute it using a relative path (./build.py) or an absolute path (/home/dev/scripts/build.py), or they must add the folder to their ~/.bashrc file via an export statement.

Scenario 2: The Locked Out Admin *The Setup:* You configure a new Ubuntu server. You install UFW (the firewall). You change the SSH port in /etc/ssh/sshd_config from 22 to 2222. You type sudo ufw enable. You log out. You can never log back in. *The Solution:* You enabled the firewall with a default deny policy, but you forgot to tell the firewall about the new SSH port! You should have executed sudo ufw allow 2222 BEFORE enabling UFW. Because you locked the only door, you must use the cloud provider's emergency console GUI to log in and fix the firewall rules.

6. Deciphering Broken Bash Scripts #

*The Broken Script:*

#!/bin/bash
TARGET="/var/log"
if [ $TARGET = "/var/log" ]
then
echo "Directory matches"
fi

*The Error:* The script lacks the mandatory spaces inside the if statement brackets. It must be written as if [ $TARGET = "/var/log" ]. Without the spaces, Bash interprets [ as a missing command and crashes.

7. Preparing for the Technical Assessment #

1. 1. Acknowledge the OSI Model: Always state you will check Layer 1/2 (cables/interfaces via ip a), then Layer 3 (routing via ping), then Layer 4 (ports via ss), then Layer 7 (application logs via journalctl).

1. 2. Read the Logs: The ultimate trump card in a Linux interview is saying, "Before making any changes to the system, I would inspect the application logs in /var/log or systemd to identify the exact error code." Guessing is dangerous; logs are facts.

8. Summary #

9. Next Chapter Recommendation #