Why This Matters for Your Career
I’ve sat on both sides of the technical interview table. As a candidate, I’ve fumbled through command-line questions that should have been easy. As a hiring manager, I’ve watched promising candidates crash and burn because they couldn’t navigate a filesystem under pressure.
Here’s what nobody tells you: interviewers aren’t testing whether you’ve memorised obscure flags. They’re testing whether you can think like someone who actually runs production systems. The difference between “I know Linux” and “I work in Linux daily” is immediately obvious—and it’s worth about £15-20k in salary.
This isn’t just another commands cheat sheet. This is the list of commands that actually come up in interviews for junior sysadmin, helpdesk-to-infrastructure, and DevOps roles paying £35-50k. I’ve grouped them by what interviewers are really testing.
The Commands Interviewers Actually Ask About
Navigation and File Operations
Every interview starts here. If you hesitate on these, the interview is already over in the interviewer’s mind.
| Command | What It Does | Interview Context |
|---|---|---|
pwd |
Print working directory | “Where are you right now?” |
cd /var/log |
Change directory | Navigate to logs when troubleshooting |
ls -la |
List all files with details | Check permissions, ownership, hidden files |
cp -r source/ dest/ |
Copy recursively | Backup before making changes |
mv old new |
Move or rename | Rename config files during changes |
rm -rf directory/ |
Remove recursively (dangerous) | They’ll ask if you know the risks |
mkdir -p path/to/new |
Create nested directories | Setting up directory structures |
If someone asks you to delete something with rm -rf, always confirm the path first. Saying “I’d double-check that path before running rm -rf” shows production awareness.
Reading and Searching Files
Troubleshooting is 80% reading logs and config files. These commands separate the googlers from the practitioners.
| Command | What It Does | When You’ll Use It |
|---|---|---|
cat filename |
Display entire file | Short config files |
less filename |
Page through file | Large logs (q to quit) |
head -n 20 file |
First 20 lines | Check file headers, recent entries |
tail -f /var/log/syslog |
Follow log in real-time | Watch logs while reproducing issues |
grep "error" logfile |
Search for pattern | Find specific issues in logs |
grep -r "text" /path/ |
Recursive search | Find config containing specific values |
find /path -name "*.log" |
Find files by name | Locate files across filesystem |
Combine them: tail -f /var/log/nginx/error.log | grep -i "502" shows you 502 errors in real-time. That’s the kind of one-liner that impresses interviewers.
User and Permission Management
Security questions are inevitable. If you can’t explain permissions, you won’t get a sysadmin role.
| Command | What It Does | Interview Context |
|---|---|---|
whoami |
Current username | Verify which account you’re using |
sudo command |
Run as root | Privilege escalation (explain the risks) |
chmod 755 file |
Change permissions | You must know octal notation |
chown user:group file |
Change ownership | Fix permission issues |
passwd username |
Change password | User management tasks |
useradd -m newuser |
Create user with home | Onboarding new team members |
- “What does 755 mean?” — Owner can read, write, execute (7). Group can read and execute (5). Others can read and execute (5). Have this memorised cold.
Process and Service Management
Production systems run services. You need to know how to check what’s running and restart things that aren’t.
| Command | What It Does | Real Scenario |
|---|---|---|
ps aux |
List all processes | Find what’s consuming resources |
top / htop |
Live process view | Identify runaway processes |
kill PID |
Terminate process | Stop misbehaving applications |
kill -9 PID |
Force kill | When normal kill doesn’t work |
systemctl status nginx |
Check service status | Is the web server running? |
systemctl restart nginx |
Restart service | Apply configuration changes |
journalctl -u nginx |
Service logs | Why did the service fail? |
- “The website is down. Walk me through your troubleshooting.” — Start with
systemctl status nginx, check the logs withjournalctl -u nginx -n 50, then investigate from there. Having a logical process matters more than knowing every command.
Networking Essentials
Network troubleshooting comes up in almost every infrastructure interview.
| Command | What It Does | Troubleshooting Use |
|---|---|---|
ip addr / ifconfig |
Show IP addresses | Check network configuration |
ping google.com |
Test connectivity | Is the network working? |
curl -I https://site.com |
HTTP headers only | Test web server responses |
netstat -tulpn |
Listening ports | What services are exposed? |
ss -tulpn |
Modern netstat | Same as above, newer tool |
traceroute host |
Network path | Where is the connection failing? |
dig domain.com |
DNS lookup | DNS resolution issues |
Disk and Storage
| Command | What It Does | When It Matters |
|---|---|---|
df -h |
Disk free space | Is the disk full? (common issue) |
du -sh /path/* |
Directory sizes | What’s consuming space? |
lsblk |
List block devices | View disk structure |
mount /dev/sdb1 /mnt |
Mount filesystem | Attach storage |
- “A user reports the application won’t save files. What do you check first?” — Disk space with
df -h. Full disks cause more outages than people admit.
Common Interview Scenarios
Scenario 1: “The application is slow”
# Check system resources
top
# Or better
htop
# Check disk I/O
iostat -x 1
# Check memory
free -h
# Check what's using CPU
ps aux --sort=-%cpu | head -10
Scenario 2: “Users can’t reach the website”
# Is the service running?
systemctl status nginx
# What ports are listening?
ss -tulpn | grep :80
# Can we reach it locally?
curl -I http://localhost
# Check the firewall
sudo iptables -L -n
# Or on newer systems
sudo ufw status
Scenario 3: “Logs are filling up the disk”
# Check disk usage
df -h
# Find large files
du -sh /var/log/*
# Find files over 100MB
find /var/log -size +100M
# Truncate a log file safely
sudo truncate -s 0 /var/log/large.log
The Career Translation
Every command here maps to a real job responsibility:
| Command Category | Job Responsibility | Salary Impact |
|---|---|---|
| Navigation/Files | Basic system administration | Entry requirement for any Linux role |
| Permissions | Security and access control | +£3-5k for security awareness |
| Processes/Services | Application support | +£5-8k for production experience |
| Networking | Infrastructure troubleshooting | +£5-10k for network skills |
| Disk/Storage | Capacity planning | Essential for senior roles |
A helpdesk role at £25k expects you to know navigation and basic file operations. A junior sysadmin at £35-40k expects all of the above. A mid-level infrastructure engineer at £45-55k expects you to combine these fluently under pressure.
How to Practice
- Build a homelab – Even a Raspberry Pi running Ubuntu gives you real practice
- Break things intentionally – Fill a disk, kill processes, misconfigure permissions, then fix them
- Time yourself – Interview pressure is real. Practice until these commands are muscle memory
- Explain out loud – Talk through what you’re doing. Interviewers want to hear your thought process
Quick Reference Card
Print this. Stick it next to your monitor. Use it until you don’t need it.
# Where am I? What's here?
pwd && ls -la
# Find and read files
find /path -name "*.conf"
grep -r "search term" /etc/
# Check system health
df -h && free -h && uptime
# Service troubleshooting
systemctl status servicename
journalctl -u servicename -n 50
# Network basics
ip addr
ss -tulpn
curl -I http://localhost
# Process management
ps aux | grep processname
top
Next Steps
This cheat sheet gets you through the initial technical screen. To level up further:
- Master systemctl – Service management is where junior sysadmins spend most of their time
- Learn log analysis –
journalctl,grep, andawkfor parsing logs - Understand permissions deeply – Beyond chmod, learn about ACLs and SELinux basics
- Practice networking – Every outage starts with “is it the network?”
The goal isn’t to memorise 500 commands. It’s to have 30 commands so ingrained that you can focus on solving the problem, not remembering the syntax. That’s what separates a £25k helpdesk role from a £45k infrastructure position.
This is part of the Linux Fundamentals series for career-focused tech professionals. Coming up next: mastering systemctl for service management—the command you’ll use more than any other in production.