Linux Admin

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Linux Admin

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Resources:

• Top 100+ Linux Interview Questions and Answers (2025) - Naukri Code 360

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What is the purpose of the /etc/passwd file?

The /etc/passwd file is a critical system file in Linux that stores essential information about user accounts. Each line in the file represents a user account and contains fields separated by colons, including:

• Username
• Encrypted password (typically stored in /etc/shadow for security)
• User ID (UID)
• Group ID (GID)
• User’s full name or description
• Home directory
• Login shell

This file is used by various system utilities to manage user accounts and permissions.

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How do you check system resource usage in Linux?

There are several commands to check system resource usage in Linux:

• top or htop: Provides a real-time, dynamic view of system processes and resource usage.
• free: Displays information about system memory usage.
• df: Shows disk space usage for mounted filesystems.
• iostat: Reports CPU statistics and I/O statistics for devices and partitions.
• vmstat: Reports information about processes, memory, paging, block I/O, and CPU activity.

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What is a symbolic link and how do you create one?

A symbolic link (also known as a symlink or soft link) is a special type of file that points to another file or directory. It’s similar to a shortcut in Windows.

To create a symbolic link, use the ln command with the -s option:

ln -s target_path link_name

For example:

ln -s /path/to/original/file /path/to/symlink

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How do you find large files on a Linux system?

You can use the find command combined with sort to locate large files:

find /path/to/search -type f -size +100M -exec ls -lh {} \; | sort -k5 -rh

This command will:

• Search for files larger than 100MB
• List them with human-readable sizes
• Sort them by size in descending order

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What is the purpose of the cron daemon?

The cron daemon is a time-based job scheduler in Unix-like operating systems. It enables users to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals. Common uses include:

• Automating system maintenance tasks
• Scheduling backups
• Running periodic reports
• Monitoring system health

Jobs are configured in crontab files, which specify the schedule and command to run.

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How do you check open ports on a Linux system?

To check open ports on a Linux system, you can use the following commands:

• netstat -tuln: Shows all active listening ports
• ss -tuln: A more modern alternative to netstat
• lsof -i: Lists all network connections and the processes using them

For example:

sudo netstat -tuln | grep LISTEN

This will show all TCP and UDP ports in the LISTEN state.

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What is the purpose of the /etc/fstab file?

The /etc/fstab (file system table) file contains information about how disk partitions and other block devices should be mounted into the file system. It includes:

• Device identifiers (UUID, device path)
• Mount points
• Filesystem types
• Mount options
• Dump and pass values for filesystem checks

This file is read by the system during boot to determine which filesystems to mount and how to mount them.

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How do you troubleshoot high CPU usage on a Linux server?

To troubleshoot high CPU usage:

• Use top or htop to identify which processes are consuming the most CPU.
• Check the load average with uptime to see if the system is overloaded.
• Use ps aux to get more detailed information about specific processes.
• Check system logs (/var/log/syslog or journalctl) for any errors or warnings.
• Use strace to trace system calls and signals for a specific process.
• If it’s a web server, check the access logs for unusual traffic patterns.
• Consider using more specialized tools like perf for performance analysis if needed.

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What is SELinux and what are its modes?

SELinux (Security-Enhanced Linux) is a security architecture integrated into the Linux kernel that provides mandatory access control (MAC) system. It enforces security policies to restrict programs’ access to system resources.

SELinux has three modes:

• Enforcing: SELinux policy is enforced. Security policy violations are prevented and logged.
• Permissive: SELinux policy is not enforced, but violations are logged. This is useful for troubleshooting.
• Disabled: SELinux is turned off completely.

You can check the current mode with getenforce and change it with setenforce (temporarily) or by editing /etc/selinux/config (permanently).

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How do you set up a basic firewall using iptables?

Here’s a basic example of setting up a firewall using iptables:

# Flush existing rules
iptables -F

# Set default chain policies
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT

# Allow loopback connections
iptables -A INPUT -i lo -j ACCEPT

# Allow established and related connections
iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

# Allow incoming SSH (adjust port if needed)
iptables -A INPUT -p tcp --dport 22 -j ACCEPT

# Allow incoming HTTP and HTTPS
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j ACCEPT

# Log dropped packets
iptables -A INPUT -j LOG --log-prefix "IPTables-Dropped: "

# Save rules (Debian/Ubuntu)
sudo iptables-save > /etc/iptables/rules.v4

This basic setup allows SSH, HTTP, and HTTPS traffic while blocking other incoming connections. Remember to adjust rules based on your specific needs and to use a persistent solution like iptables-persistent to ensure rules survive reboots.