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.

Q13: What is an inode in Linux?

Answer:

An inode is a data structure containing metadata about a file, like permissions and ownership:

ls -i           # Show inode numbers
stat filename   # Show detailed inode info

find / -inum [inode_number]  # Find files with same inode

Use Case: Understand file structure and manage disk space.
Gotchas: Running out of inodes can lead to storage issues despite free space.

Q14: How do you check CPU utilization in Linux?

Answer:

top/htop         # Real-time system monitoring
mpstat      # CPU-specific statistics
vmstat      # Virtual memory statistics
perf/sar         # System activity reporter

Best Practice: Use htop for interactive monitoring.
Advanced Use Case: For detailed analysis, consider perf or sar.
Troubleshooting: Look for processes with high CPU in top and investigate as needed.

Example Use Case:

# Check CPU usage every 2 seconds
top -b -n 1 | grep "Cpu(s)"
# Monitor specific process
top -p $(pgrep process_name)

Q15: What are the differences between the top and htop commands?

Answer:

top: Basic system monitor.

htop: Enhanced features:

• Color output
• Vertical/horizontal scrolling
• Tree view
• Mouse support

When to Use Which:

top    # Quick system check, available everywhere
htop   # Detailed analysis, process management

Q16: What is a mount in Linux, and how do you create one?

Answer:

Mount connects storage devices to filesystem:

# Basic mount
mount /dev/sdb1 /mnt/data

# With specific options
mount -o rw,user /dev/sdb1 /mnt/data

# Automatic mount (in /etc/fstab)
/dev/sdb1  /mnt/data  ext4  defaults  0  2

Common Use Cases – Mounting network shares:

mount -t nfs server:/share /local/mount
mount -t cifs //server/share /local/mount

Q17: How do you troubleshoot live logs in Linux?

Answer:

# Follow log in real-time
tail -f /var/log/syslog

# Multiple logs simultaneously
tail -f /var/log/syslog /var/log/auth.log

# With grep
tail -f /var/log/syslog | grep --line-buffered "error"

Advanced Use Case – Monitoring multiple services:

# Using multitail
multitail /var/log/nginx/access.log -I /var/log/mysql/error.log

Q18: What is the sed command in Linux?

Answer:
Stream editor for text manipulation:

# Replace text
sed 's/old/new/g' file

# Delete lines
sed '/pattern/d' file

# Insert text
sed '2i\new line' file

Gotchas: Use caution with -i (in-place editing), as it overwrites files.

Practical Applications:

# Configuration file updates
sed -i 's/debug=false/debug=true/' config.ini

# Batch file renaming
for f in *.txt; do
    sed -i 's/error/warning/' "$f"
done

Q19: What is the awk command in Linux?

Answer:
Pattern scanning and text processing:

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

# Sum numbers
awk '{sum += $1} END {print sum}' file

# Filter rows
awk '$3 > 100' file

Use Case: Ideal for data extraction and reporting from logs or CSV files.
Advanced Usage: Use awk for complex filtering or field calculations.
Gotchas: Understand file delimiters, especially with non-standard formats.

Real-world Use Cases:

# Log analysis
awk '$9 >= 400' access.log  # Find HTTP errors

# Resource monitoring
ps aux | awk 'NR>1{sum += $3} END {print "CPU%: " sum}'

Q20: What are the grep and egrep commands in Linux?

Answer:
Search for patterns in text files:

grep pattern file     # Basic search
egrep "pat1|pat2" file # Extended regex
grep -r pattern dir   # Recursive search

Use Case: Quickly find log entries, error messages, or configuration lines.
Gotchas: By default, grep is case-sensitive. Use -i for case-insensitive matches.

Common Applications:

# Security monitoring
grep "Failed password" /var/log/auth.log

# Code analysis
grep -r "TODO" ./src/

Q21: How can you list only directories in Linux?

Answer:

ls -d */              # Basic directory listing
ls -l | grep "^d"     # With detailed info
find . -maxdepth 1 -type d  # Using find

Use Cases:

# Backup directories only
for d in */; do
    tar czf "${d%/}.tar.gz" "$d"
done

Q22: How do you check the processes running in Linux?

Answer:

ps aux               # All processes
ps -ef              # Full format listing
pgrep process_name  # Get process ID

Monitoring Scenarios:

# Memory-hungry processes
ps aux --sort=-%mem | head

# Process tree
pstree -p

Q23: How do you get a Java thread dump in Linux?

Answer:

Command: jstack PID
Use Case: Useful for debugging Java application issues.

jstack PID > thread_dump.txt
kill -3 PID  # Send SIGQUIT

# For multiple dumps
for i in {1..3}; do
    jstack PID > dump_$i.txt
    sleep 10
done

Best Practice: Save dumps with timestamps for detailed analysis later.
Troubleshooting: If jstack isn’t available, install JDK tools or use kill -3 PID.

Q24: How can you check the running ports on a Linux machine?

Answer:

netstat -tulpn     # All listening ports
ss -tulpn         # Modern alternative
lsof -i          # File-based approach

Security Applications:

# Find unauthorized services
netstat -tulnp | grep LISTEN

# Monitor specific port
watch -n 1 'netstat -an | grep :80'

Gotchas: Only root or privileged users can access full port lists.

I will continue with other questions later on.

In the next parts, we’ll cover questions focused on:
Questions 25-36: Shell Scripting and Advanced Topics