Linux Interview Questions: Essential Commands for Developers

Linux skills are expected for backend, DevOps, and full-stack roles. Even if you develop on macOS or Windows, your code likely runs on Linux servers. Interviewers want to see you're comfortable navigating, troubleshooting, and automating in a Linux environment.

This guide covers the essential Linux commands and concepts that come up in developer interviews.

File System Navigation

Q: What are the essential commands for navigating the Linux file system?

These are the basics, but interviewers expect fluency:

# Navigation
pwd                     # Print working directory
cd /path/to/dir         # Change directory
cd ..                   # Go up one level
cd ~                    # Go to home directory
cd -                    # Go to previous directory

# Listing files
ls                      # List files
ls -la                  # Long format, include hidden files
ls -lh                  # Human-readable sizes
ls -lt                  # Sort by modification time

# File info
file document.pdf       # Determine file type
stat file.txt          # Detailed file information
du -sh directory/       # Directory size
df -h                   # Disk space usage

Directory structure to know:

Path	Purpose
/	Root directory
/home	User home directories
/etc	Configuration files
/var	Variable data (logs, databases)
/tmp	Temporary files
/usr	User programs and data
/opt	Optional/third-party software

---

File Operations

Q: How do you create, copy, move, and delete files?

# Create
touch file.txt              # Create empty file
mkdir directory             # Create directory
mkdir -p path/to/nested     # Create nested directories

# Copy
cp source dest              # Copy file
cp -r source/ dest/         # Copy directory recursively
cp -p source dest           # Preserve permissions/timestamps

# Move/rename
mv oldname newname          # Rename file
mv file /path/to/dest       # Move file

# Delete
rm file.txt                 # Delete file
rm -r directory/            # Delete directory recursively
rm -rf directory/           # Force delete (careful!)
rmdir empty_directory/      # Delete empty directory only

Interview tip: Always mention being careful with rm -rf, especially with variables:

# DANGEROUS - if $DIR is empty, deletes everything!
rm -rf $DIR/*

# SAFER - quotes and checks
if [ -n "$DIR" ]; then
    rm -rf "$DIR"/*
fi

---

File Permissions

Q: How do Linux file permissions work?

This is a common interview topic. Understand both symbolic and numeric notation.

# View permissions
ls -la
# -rw-r--r-- 1 user group 1234 Jan 1 12:00 file.txt
# drwxr-xr-x 2 user group 4096 Jan 1 12:00 directory/

Breaking down -rw-r--r--:

Position	Meaning
1	File type (- file, d directory, l link)
2-4	Owner permissions (rw-)
5-7	Group permissions (r--)
8-10	Others permissions (r--)

Permission values:

• r (read) = 4
• w (write) = 2
• x (execute) = 1

# Numeric notation
chmod 755 script.sh     # rwxr-xr-x (owner: full, others: read+execute)
chmod 644 file.txt      # rw-r--r-- (owner: read+write, others: read)
chmod 600 secrets.txt   # rw------- (owner only)

# Symbolic notation
chmod u+x script.sh     # Add execute for owner
chmod g-w file.txt      # Remove write for group
chmod o=r file.txt      # Set others to read only
chmod a+r file.txt      # Add read for all

# Change ownership
chown user:group file.txt
chown -R user:group directory/

Special permissions to know:

• SUID (4): Execute as file owner (e.g., passwd command)
• SGID (2): Execute as group / new files inherit directory group
• Sticky bit (1): Only owner can delete files (e.g., /tmp)

chmod 4755 file         # SUID
chmod 2755 directory    # SGID
chmod 1777 directory    # Sticky bit

---

Hard Links vs Soft Links

Q: What's the difference between hard links and soft links?

# Create soft link (symlink)
ln -s /path/to/original link_name

# Create hard link
ln /path/to/original link_name

Feature	Hard Link	Soft Link (Symlink)
Points to	Inode directly	File path
Original deleted	Still works	Broken link
Cross filesystem	No	Yes
Link to directory	No	Yes
File size	Same as original	Small (path length)

Use cases:

• Soft links: Shortcuts, pointing to executables, config file alternatives
• Hard links: Backups, ensuring file persists while any link exists

# Check if files are hard linked (same inode)
ls -li file1 file2
# 123456 -rw-r--r-- 2 user group 100 Jan 1 12:00 file1
# 123456 -rw-r--r-- 2 user group 100 Jan 1 12:00 file2

---

Text Processing: grep, sed, awk

Q: How do you search and manipulate text in Linux?

grep - Search for patterns:

grep "error" logfile.log           # Find lines containing "error"
grep -i "error" logfile.log        # Case insensitive
grep -r "TODO" ./src               # Recursive search in directory
grep -n "error" logfile.log        # Show line numbers
grep -v "debug" logfile.log        # Invert match (exclude debug)
grep -c "error" logfile.log        # Count matches
grep -E "error|warning" log.log    # Extended regex (OR)
grep -A 3 "error" log.log          # Show 3 lines after match
grep -B 2 "error" log.log          # Show 2 lines before match

sed - Stream editor:

# Replace first occurrence per line
sed 's/old/new/' file.txt

# Replace all occurrences
sed 's/old/new/g' file.txt

# Edit file in-place
sed -i 's/old/new/g' file.txt

# Delete lines matching pattern
sed '/pattern/d' file.txt

# Print specific lines
sed -n '5,10p' file.txt            # Lines 5-10

awk - Pattern scanning:

# Print specific columns
awk '{print $1, $3}' file.txt

# With delimiter
awk -F',' '{print $2}' data.csv

# Conditional printing
awk '$3 > 100 {print $1}' data.txt

# Sum a column
awk '{sum += $1} END {print sum}' numbers.txt

# Common log parsing
awk '{print $1}' access.log | sort | uniq -c | sort -rn | head
# ^ Count requests by IP, sorted by frequency

---

Process Management

Q: How do you find and manage processes?

# View processes
ps aux                    # All processes, detailed
ps aux | grep node        # Find specific process
pgrep -a node            # Find by name (PID and command)
top                      # Interactive process viewer
htop                     # Better interactive viewer

# Process details
ps aux --sort=-%mem | head    # Top memory consumers
ps aux --sort=-%cpu | head    # Top CPU consumers

# Background/foreground
command &                 # Run in background
jobs                      # List background jobs
fg %1                     # Bring job 1 to foreground
bg %1                     # Resume job 1 in background
nohup command &           # Run immune to hangups

Killing processes:

kill PID                  # SIGTERM (graceful shutdown)
kill -9 PID               # SIGKILL (force kill)
kill -15 PID              # SIGTERM (same as kill PID)

pkill process_name        # Kill by name
pkill -f "node server"    # Kill by full command match
killall node              # Kill all processes with name

# Find what's using a port
lsof -i :3000
# Kill it
kill $(lsof -t -i :3000)

Common signals:

Signal	Number	Purpose
SIGHUP	1	Hangup (reload config)
SIGINT	2	Interrupt (Ctrl+C)
SIGKILL	9	Force kill (can't be caught)
SIGTERM	15	Graceful termination
SIGSTOP	19	Pause process
SIGCONT	18	Resume process

---

Networking Commands

Q: What networking commands should every developer know?

# Test connectivity
ping google.com
ping -c 4 google.com      # Only 4 packets

# DNS lookup
nslookup example.com
dig example.com

# HTTP requests
curl https://api.example.com
curl -X POST -d '{"key":"value"}' -H "Content-Type: application/json" url
curl -I https://example.com    # Headers only
curl -o file.zip https://url   # Download to file
wget https://url/file.zip      # Download file

# View network connections
netstat -tulpn            # Listening ports with PIDs
ss -tulpn                 # Modern replacement for netstat
lsof -i :3000             # What's using port 3000

# Network interfaces
ip addr                   # Show IP addresses
ifconfig                  # Legacy command
ip route                  # Show routing table

# Trace route
traceroute google.com

Port checking:

# Check if port is open locally
nc -zv localhost 3000

# Check remote port
nc -zv example.com 443

# Common ports to know
# 22 - SSH
# 80 - HTTP
# 443 - HTTPS
# 3000 - Node.js default
# 5432 - PostgreSQL
# 3306 - MySQL
# 6379 - Redis
# 27017 - MongoDB

---

Log Analysis

Q: How do you analyze logs in Linux?

# View logs
cat /var/log/syslog             # Full file
tail /var/log/syslog            # Last 10 lines
tail -f /var/log/syslog         # Follow (live updates)
tail -n 100 app.log             # Last 100 lines
head -n 50 app.log              # First 50 lines
less /var/log/syslog            # Paginated viewing

# Search logs
grep "ERROR" app.log
grep -i "error\|warning" app.log
zgrep "ERROR" app.log.gz        # Search compressed logs

# Count occurrences
grep -c "ERROR" app.log

# Time-based filtering (if logs have timestamps)
awk '$0 >= "2024-01-15 10:00" && $0 <= "2024-01-15 11:00"' app.log

# Analyze log patterns
cut -d' ' -f1 access.log | sort | uniq -c | sort -rn | head
# ^ Count occurrences of first field (e.g., IPs in access log)

journalctl (systemd logs):

journalctl                       # All logs
journalctl -u nginx              # Logs for specific service
journalctl -f                    # Follow mode
journalctl --since "1 hour ago"
journalctl --since "2024-01-15" --until "2024-01-16"
journalctl -p err                # Only errors

---

find and locate

Q: How do you find files in Linux?

# find - search in real-time
find /path -name "*.log"                    # By name pattern
find /path -name "*.log" -mtime -7          # Modified in last 7 days
find /path -size +100M                      # Larger than 100MB
find /path -type f -name "*.tmp" -delete    # Find and delete
find /path -type d -name "node_modules"     # Find directories
find /path -user john                       # By owner
find /path -perm 755                        # By permissions

# Execute command on results
find . -name "*.js" -exec grep "TODO" {} \;
find . -name "*.log" -exec rm {} \;

# Using xargs (often faster)
find . -name "*.js" | xargs grep "TODO"
find . -name "*.log" -print0 | xargs -0 rm  # Handle spaces in names

# locate - uses database (faster but may be outdated)
locate filename
updatedb                # Update the locate database

---

Environment Variables

Q: How do environment variables work in Linux?

# View variables
env                         # All environment variables
echo $PATH                  # Specific variable
printenv HOME

# Set temporarily (current session)
export MY_VAR="value"
export PATH="$PATH:/new/path"

# Set for single command
MY_VAR="value" ./script.sh

# Persistent variables
# Add to ~/.bashrc or ~/.bash_profile:
export MY_VAR="value"
# Then reload:
source ~/.bashrc

Important variables:

Variable	Purpose
PATH	Directories to search for executables
HOME	User's home directory
USER	Current username
SHELL	Current shell
PWD	Current working directory
LANG	Language/locale settings

---

Shell Scripting Basics

Q: Can you write a basic shell script?

Interviewers often ask for simple scripts on the spot.

#!/bin/bash

# Variables
NAME="World"
echo "Hello, $NAME"

# Command line arguments
echo "Script name: $0"
echo "First argument: $1"
echo "All arguments: $@"
echo "Number of arguments: $#"

# Conditionals
if [ -f "$1" ]; then
    echo "File exists"
elif [ -d "$1" ]; then
    echo "It's a directory"
else
    echo "Not found"
fi

# Loops
for file in *.txt; do
    echo "Processing $file"
done

for i in {1..5}; do
    echo "Number $i"
done

while read -r line; do
    echo "$line"
done < file.txt

# Functions
greet() {
    local name=$1
    echo "Hello, $name"
}
greet "Developer"

# Exit codes
command
if [ $? -eq 0 ]; then
    echo "Success"
else
    echo "Failed"
fi

Common test operators:

Operator	Meaning
-f file	File exists and is regular file
-d dir	Directory exists
-e path	Path exists
-r file	File is readable
-w file	File is writable
-x file	File is executable
-z string	String is empty
-n string	String is not empty

---

I/O Redirection and Pipes

Q: Explain input/output redirection in Linux.

# Output redirection
command > file.txt          # Redirect stdout (overwrite)
command >> file.txt         # Redirect stdout (append)
command 2> error.log        # Redirect stderr
command > out.log 2>&1      # Redirect both stdout and stderr
command &> all.log          # Shorthand for above (bash)

# Input redirection
command < input.txt         # Read from file
command << EOF              # Here document
line 1
line 2
EOF

# Pipes
command1 | command2         # Pipe stdout to next command
command1 |& command2        # Pipe stdout and stderr

# Common pipe patterns
cat file | grep "pattern" | sort | uniq -c
ps aux | grep node | awk '{print $2}' | xargs kill

File descriptors:

• 0 = stdin
• 1 = stdout
• 2 = stderr

# Discard output
command > /dev/null          # Discard stdout
command 2> /dev/null         # Discard stderr
command > /dev/null 2>&1     # Discard all output

---

Quick Reference: Most Common Commands

Category	Commands
Navigation	cd, ls, pwd, find, locate
Files	cp, mv, rm, mkdir, touch, cat, less
Permissions	chmod, chown, ls -l
Text	grep, sed, awk, cut, sort, uniq
Processes	ps, top, kill, pkill, bg, fg
Network	curl, ping, netstat, ss, lsof
Disk	df, du, mount
Archives	tar, gzip, unzip

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Related Articles

If you found this helpful, check out these related guides:

• Complete DevOps Engineer Interview Guide - comprehensive preparation guide for DevOps interviews
• Docker Interview Guide - Containers run on Linux
• Kubernetes Interview Guide - Orchestrating Linux containers
• Node.js Advanced Interview Guide - Server-side JavaScript on Linux
• System Design Interview Guide - Architecture where Linux servers power everything
• CI/CD & GitHub Actions Interview Guide - Automation pipelines using shell commands

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What's Next?

These commands cover what most developers need day-to-day. As you go deeper, explore:

• systemd - Service management
• cron - Scheduled tasks
• iptables/nftables - Firewall rules
• strace/ltrace - System call debugging

The developers who stand out can navigate, troubleshoot, and automate on Linux without hesitation. Practice these commands until they're muscle memory.

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