Master Linux Command‑Line Essentials for Interview Success

File and Directory Operations

Basic file and directory commands are essential in Linux interviews, covering listing, creating, deleting, and moving files.

1. ls – List directory contents

Displays files and subdirectories in the current or specified directory.

Common options: -l: long format with permissions, owner, size, etc. -a: include hidden files (starting with .). -h: human‑readable sizes.

ls -lah /var/log

2. cd – Change directory

Switches to the specified directory.

Common usage:

cd /path/to/dir

cd ..

– parent directory cd ~ – home directory cd - – previous directory

cd /etc/nginx
cd ..

3. pwd – Print working directory

Shows the absolute path of the current directory.

pwd

4. mkdir – Create directory

Creates a new directory.

Common option: -p creates parent directories as needed.

mkdir -p /tmp/test/dir1/dir2

5. rm – Remove files or directories

Deletes specified files or directories.

Common options: -r: recursive delete -f: force, no prompt

rm -rf /tmp/test

6. cp – Copy files or directories

Copies files or directories to a target location.

Common options: -r: recursive copy -p: preserve attributes

cp -r /etc/nginx /backup/nginx

7. mv – Move or rename files/directories

Moves or renames a file or directory.

Common option: -i prompts before overwriting.

mv file1.txt /tmp/file2.txt

8. find – Search for files

Searches for files or directories matching criteria.

Common options: -name: pattern matching -type f|d: file or directory -exec: execute a command on results

find /var/log -name "*.log" -type f -mtime +7 -exec ls -lh {} \;

9. touch – Create empty file or update timestamp

touch test.txt

File Content Operations

10. cat – View file contents

Outputs file content to standard output.

Common option: -n shows line numbers.

cat -n /etc/passwd

11. less / more – Paginated view

Displays file content page by page, useful for large files.

Key commands: / search, q quit.

less /var/log/syslog

12. head / tail – View beginning or end of a file

head

shows the start; tail shows the end.

Common option: -n number of lines.

tail -f /var/log/nginx/access.log

13. grep – Search file contents

Searches for matching strings.

Common options: -i: ignore case -r: recursive -n: show line numbers -v: invert match

grep -r "error" /var/log

14. awk – Text processing

Processes columns, e.g., extracting fields.

awk '{print $1,$3}' /etc/passwd

15. sed – Stream editor

Performs text substitution, deletion, insertion.

sed 's/old/new/g' file.txt

Process Management

16. ps – View processes

Shows current processes.

Common options: aux (all users), -ef (full format).

ps aux | grep nginx

17. top / htop – Real‑time process monitor

Displays live system processes; htop offers a friendlier UI.

Key keys: q quit, k kill process.

htop

18. kill / killall – Terminate processes

Kill by PID ( kill) or by name ( killall).

Common signals: -9 (SIGKILL) – force kill -15 (SIGTERM) – graceful termination

kill -9 12345
killall nginx

19. jobs / fg / bg – Manage background jobs

List, bring to foreground, or send to background.

sleep 100 &
jobs
fg %1

System Monitoring

20. df – Disk usage

Shows filesystem space.

Option: -h human‑readable.

df -h

21. du – Directory/file size

Summarizes disk usage for files/directories.

Option: -sh total size, human‑readable.

du -sh /var/log/*

22. free – Memory usage

Displays RAM and swap usage.

Option: -h human‑readable.

free -h

23. uptime – System uptime and load

uptime

Network Related

24. ping – Test connectivity

ping -c 4 baidu.com

25. netstat / ss – Network status

Shows connections, listening ports, etc.

Common option (ss): -tuln display TCP/UDP listening ports.

ss -tuln

26. curl / wget – Download/request resources

curl -O https://example.com/file.txt

27. ifconfig / ip – Configure network interfaces

ip addr

Permission Management

28. chmod – Change file permissions

Modify access rights.

Numeric mode: e.g., 755 (rwxr-xr-x).

Symbolic mode: e.g., u+x (add execute for owner).

chmod 755 script.sh

29. chown – Change file owner

Change owner and group.

Option: -R recursive.

chown -R user:group /var/www

30. sudo – Execute with elevated privileges

sudo apt update

Other Useful Commands

31. tar – Archive and compress

Common options: -c create -x extract -z gzip -f specify file

tar -czvf backup.tar.gz /var/www

32. crontab – Schedule tasks

Common options: -e edit, -l list. crontab -e Add a daily 2 AM backup:

0 2 * * * /backup.sh

33. man – Command manual

man ls

Scenario‑Based Interview Questions

Scenario 1 – Find and clean old log files

Find .log files in /var/log older than 7 days, list sizes, and safely delete.

find /var/log -type f -name "*.log" -mtime +7 -exec ls -lh {} \;

Save list for review:

find /var/log -type f -name "*.log" -mtime +7 > /tmp/old_logs.txt
cat /tmp/old_logs.txt

Move to backup before deletion:

mkdir -p /tmp/log_backup
find /var/log -type f -name "*.log" -mtime +7 -exec mv {} /tmp/log_backup/ \;
rm -rf /tmp/log_backup/*

Scenario 2 – Identify and kill high‑CPU process

Use top or htop to locate the highest CPU consumer, note PID, then:

ps -p <PID> -o pid,ppid,cmd,%cpu
kill -15 <PID>   # graceful
kill -9 <PID>    # force if needed

Scenario 3 – Batch modify configuration files

Replace listen 80 with listen 8080 in all .conf files under /etc/nginx/conf.d:

cp -r /etc/nginx/conf.d /etc/nginx/conf.d.bak
find /etc/nginx/conf.d -type f -name "*.conf" -exec sed -i 's/listen 80/listen 8080/g' {} \;

Scenario 4 – Real‑time log monitoring

tail -f /var/log/nginx/error.log | grep --line-buffered "error"

Scenario 5 – Check and free disk space

df -h
du -sh /var/* | sort -hr | head -n 5
cp /var/log/app.log /backup/app.log.bak
> /var/log/app.log

Scenario 6 – Configure daily backup via cron

# /scripts/backup.sh
#!/bin/bash
DATE=$(date +%Y%m%d)
 tar -czf /backup/www_${DATE}.tar.gz /var/www
 find /backup -name "www_*.tar.gz" -mtime +30 -delete
chmod +x /scripts/backup.sh
crontab -e   # add: 0 2 * * * /scripts/backup.sh

Scenario 7 – Diagnose network connectivity

ping -c 4 baidu.com
nslookup baidu.com
ip addr
ip route
cat /etc/resolv.conf
# optionally add DNS
 echo "nameserver 8.8.8.8" | sudo tee -a /etc/resolv.conf

Scenario 8 – Set directory permissions

chown -R www-data:www-data /var/www/html
find /var/www/html -type d -exec chmod 755 {} \;
find /var/www/html -type f -exec chmod 644 {} \;

Scenario 9 – Release occupied port

ss -tuln | grep :80   # or lsof -i :80
kill -15 12345   # graceful
kill -9 12345    # force

Scenario 10 – Extract recent IPs and URLs from Nginx access log

tail -n 100 /var/log/nginx/access.log | awk '{print $1, $7}' > /tmp/access.txt

These examples provide ready‑to‑use commands and best‑practice explanations to help you excel in Linux‑related interview questions.