Continuing from the previous set, here are more essential Linux questions and topics that commonly arise in interviews, with practical use cases, troubleshooting tips, and best practices.

Q25: How do you declare a variable in a shell script?

Answer:

# Basic variable declaration
name="John"
count=42

# Array declaration
declare -a array=("one" "two" "three")
declare -A map=([key1]="value1" [key2]="value2")

Common Use Cases:

# Configuration variables
DB_HOST="localhost"
DB_PORT=3306

# Dynamic variables
timestamp=$(date +%Y%m%d)
user_count=$(who | wc -l)

Q26: What do $?, $#, and $ represent in shell scripting?

Answer:
Special parameters in shell:

$?  # Exit status of last command (0=success)
$#  # Number of arguments passed to script
$   # Current process ID
$0  # Script name
$1  # First argument
$@  # All arguments as separate strings

Practical Examples:

# Error handling
if [ $? -ne 0 ]; then
    echo "Previous command failed"
    exit 1
fi

# Argument validation
if [ $# -lt 2 ]; then
    echo "Usage: $0 <source> <destination>"
    exit 1
fi

Q27: How do you read command line input in a shell script?

Answer:

# Basic input
read name
echo "Hello, $name"

# With prompt
read -p "Enter username: " username

# Secure password input
read -s -p "Password: " password

Interactive Script Example:

#!/bin/bash
read -p "Database name: " db_name
read -s -p "Password: " db_pass
echo

mysql -u admin -p"$db_pass" "$db_name"

Q28: What is umask in Linux?

Answer:
Sets default permissions for new files/directories:

# View current umask
umask

# Set new umask
umask 022  # Files=644, Dirs=755
umask 027  # Files=640, Dirs=750

Common Applications:

# For secure environments
umask 077  # Private files only

# In .bashrc for permanent setting
if [ "$UID" -gt 199 ]; then
    umask 002
else
    umask 022
fi

Gotchas: Misconfigured umask can cause security risks by making files too accessible.

Q29: How do you change file permissions in Linux?

Answer:

# Symbolic mode
chmod u+x file        # Add execute for user
chmod g-w file        # Remove write for group
chmod o= file         # No permissions for others

# Octal mode
chmod 755 file        # rwxr-xr-x
chmod 600 file        # rw-------

Security Best Practices:

# Secure configuration files
chmod 600 ~/.ssh/config
chmod 644 public_html/*
find . -type d -exec chmod 755 {} \;  # Directories

Q30: How can you connect to remote servers without a password?

Answer:
Using SSH key-based authentication:

# Generate SSH key pair
ssh-keygen -t rsa -b 4096

# Copy public key to server
ssh-copy-id user@remote-server

# Alternative manual method
cat ~/.ssh/id_rsa.pub | ssh user@remote-server "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"

Automation Use Case:

# In scripts for automated deployments
#!/bin/bash
for server in $(cat servers.txt); do
    ssh-copy-id -i ~/.ssh/deploy_key user@$server
done

Q31: How do you open a file in read-only mode in the vi editor?

Answer:

# Method 1
view filename

# Method 2
vi -R filename

# Method 3
vim -r filename

Practical Applications:

# Safely viewing logs
sudo view /var/log/syslog

# Checking configuration
view /etc/nginx/nginx.conf

Q32: What is the purpose of the export command in Linux?

Answer:
Makes variables available to child processes:

# Export variable
export PATH=$PATH:/new/path
export DEBUG=true

# List exported variables
export -p

Common Use Cases:

# In .bashrc/.profile
export JAVA_HOME=/usr/lib/jvm/java-11
export NODE_ENV=production

# Temporary environment setup
export PGHOST=localhost
export PGUSER=admin

Gotchas: Be mindful of modifying critical variables like PATH.

Q33: How do you send error logs and stdout logs to different files?

Answer:

# Redirect stdout and stderr separately
command > output.log 2> error.log

# Both to same file
command &> combined.log

# Append mode
command >> output.log 2>> error.log

Real-world Example:

#!/bin/bash
# Script with comprehensive logging
./backup.sh > >(tee -a backup.log) 2> >(tee -a error.log >&2)

Q34: What is the nohup command in Linux?

Answer:
Runs processes immune to hangups:

nohup command &
nohup ./script.sh > output.log 2>&1 &

Common Applications:

# Long-running processes
nohup ./backup.sh &

# With custom output
nohup ./process.sh > /dev/null 2>&1 &

Use Case: Ideal for running scripts that should survive logout.
Gotchas: nohup creates a nohup.out file for output by default.

Q35: What does the netstat command do in Linux?

Answer:
Network statistics and connections:

netstat -tuln  # TCP/UDP listening ports
netstat -anp   # All connections with PID
netstat -s     # Protocol statistics

Monitoring Examples:

# Monitor TCP connections
watch -n 1 'netstat -t'

# Check service availability
netstat -nlp | grep :80

Gotchas: netstat is deprecated in favor of ss on many distributions.

Q36: How do you run a script at boot level in Linux?

Answer:
Multiple methods available:

# Using systemd
sudo systemctl enable myservice

# Using rc.local
echo "/path/to/script.sh" >> /etc/rc.local

# Using crontab
@reboot /path/to/script.sh

Service Configuration Example:

# /etc/systemd/system/myscript.service
[Unit]
Description=My Boot Script
After=network.target

[Service]
ExecStart=/path/to/script.sh
Type=simple

[Install]
WantedBy=multi-user.target

Advanced Troubleshooting Matrix

Issue	Command	Solution
High CPU	top, htop	Identify and kill resource-heavy processes
Memory Leak	free -h, smem	Monitor memory usage patterns
Disk Space	df -h, du -sh	Clean up large files/old logs
Network	netstat, ss	Check connections and port status

Best Practices Summary

1. Always use version control for scripts
2. Implement proper error handling
3. Document your configurations
4. Regular security audits
5. Maintain backup strategies

Common Pitfalls to Avoid

1. Not testing scripts in dev environment
2. Forgetting to handle script errors
3. Neglecting file permissions
4. Poor password management
5. Insufficient logging

Remember: Linux administration is about understanding both the individual commands and how they work together in a system context.

Hope this will help anyone that comes across this blog. I will do something like that for DevOps maybe in the future.