10 CentOS Interview Questions and Answers

CentOS, a free and open-source Linux distribution, is widely recognized for its stability and reliability in enterprise environments. It is derived from the sources of Red Hat Enterprise Linux (RHEL) and offers a robust platform for managing servers, deploying applications, and ensuring security. CentOS is favored by many organizations for its long-term support and compatibility with a wide range of software.

This article provides a curated selection of interview questions designed to test your knowledge and proficiency with CentOS. By reviewing these questions and their detailed answers, you will be better prepared to demonstrate your expertise and problem-solving abilities in a technical interview setting.

CentOS Interview Questions and Answers

1. Explain the boot process.

The boot process in CentOS involves several stages:

1. BIOS/UEFI Initialization: When the system is powered on, the BIOS (Basic Input/Output System) or UEFI (Unified Extensible Firmware Interface) initializes the hardware components and performs a Power-On Self Test (POST) to ensure all hardware is functioning correctly.
2. Boot Loader: After the BIOS/UEFI initialization, the system control is handed over to the boot loader. In CentOS, the default boot loader is GRUB (GRand Unified Bootloader). GRUB is responsible for loading the kernel into memory and passing control to it. It also provides a menu for selecting different kernel versions or operating systems.
3. Kernel Initialization: The kernel is loaded into memory and initializes the system’s core components, such as memory management, process scheduling, and device drivers. It also mounts the root filesystem specified in the boot loader configuration.
4. initramfs: The initial RAM filesystem (initramfs) is a temporary root filesystem loaded into memory. It contains necessary drivers and tools to mount the actual root filesystem. Once the root filesystem is mounted, the initramfs is discarded.
5. Systemd Initialization: After the kernel has initialized the hardware and mounted the root filesystem, it starts the systemd process (PID 1). Systemd is the system and service manager for CentOS. It initializes the user space and manages system services, including starting and stopping services, managing dependencies, and handling system states.
6. Target Units: Systemd uses target units to group services and other units that need to be started to reach a specific system state. The default target in CentOS is usually multi-user.target, which brings the system to a state where multiple users can log in. Other targets include graphical.target for systems with a graphical user interface and rescue.target for single-user mode.
7. Login Prompt: Finally, once all necessary services are started, the system displays a login prompt, allowing users to log in and start using the system.

2. Write a shell script to check disk usage and send an email alert if it exceeds 80%.

To write a shell script that checks disk usage and sends an email alert if it exceeds 80%, you can use the df command to get the disk usage and mail command to send the email. Here is a simple example:

#!/bin/bash

# Set the threshold
THRESHOLD=80

# Get the disk usage percentage
USAGE=$(df / | grep / | awk '{ print $5 }' | sed 's/%//g')

# Check if the usage is greater than the threshold
if [ $USAGE -gt $THRESHOLD ]; then
  # Send an email alert
  echo "Disk usage is at ${USAGE}% on $(hostname)" | mail -s "Disk Usage Alert" [email protected]
fi

3. Write a command to find all files larger than 100MB in the /var directory.

To find all files larger than 100MB in the /var directory, you can use the find command in Linux. The find command is a powerful utility that allows you to search for files and directories based on various criteria.

find /var -type f -size +100M

In this command:

• /var specifies the directory to search in.
• -type f ensures that only files are considered (not directories).
• -size +100M filters files that are larger than 100MB.

4. How would you install and configure Apache?

To install and configure Apache on CentOS, follow these steps:

1. Install Apache: Use the package manager yum to install Apache.
2. Start and Enable Apache: Ensure that Apache starts on boot and is currently running.
3. Configure Firewall: Allow HTTP and HTTPS traffic through the firewall.
4. Verify Installation: Check that Apache is running by accessing the default web page.
5. Configure Apache: Edit the configuration files to customize the server settings.

Example commands:

# Install Apache
sudo yum install httpd -y

# Start Apache
sudo systemctl start httpd

# Enable Apache to start on boot
sudo systemctl enable httpd

# Allow HTTP and HTTPS through the firewall
sudo firewall-cmd --permanent --add-service=http
sudo firewall-cmd --permanent --add-service=https
sudo firewall-cmd --reload

# Verify Apache is running
curl http://localhost

To configure Apache, you can edit the main configuration file located at /etc/httpd/conf/httpd.conf. For example, to change the document root, you would modify the DocumentRoot directive:

# Open the configuration file in a text editor
sudo vi /etc/httpd/conf/httpd.conf

# Change the DocumentRoot directive
DocumentRoot "/var/www/html"

After making changes to the configuration file, restart Apache to apply the changes:

sudo systemctl restart httpd

5. Write a script to backup a directory to another location, including error handling.

#!/bin/bash

# Variables
SOURCE_DIR="/path/to/source"
DEST_DIR="/path/to/destination"
LOG_FILE="/path/to/logfile.log"

# Function to log messages
log_message() {
    echo "$(date '+%Y-%m-%d %H:%M:%S') - $1" >> $LOG_FILE
}

# Check if source directory exists
if [ ! -d "$SOURCE_DIR" ]; then
    log_message "Source directory does not exist: $SOURCE_DIR"
    exit 1
fi

# Check if destination directory exists, create if not
if [ ! -d "$DEST_DIR" ]; then
    mkdir -p "$DEST_DIR"
    if [ $? -ne 0 ]; then
        log_message "Failed to create destination directory: $DEST_DIR"
        exit 1
    fi
fi

# Perform the backup using rsync
rsync -av --delete "$SOURCE_DIR/" "$DEST_DIR/"
if [ $? -eq 0 ]; then
    log_message "Backup successful from $SOURCE_DIR to $DEST_DIR"
else
    log_message "Backup failed from $SOURCE_DIR to $DEST_DIR"
    exit 1
fi

6. Explain how to configure a software RAID array.

RAID (Redundant Array of Independent Disks) is a technology that combines multiple physical disk drives into a single logical unit for data redundancy, performance improvement, or both. There are different levels of RAID, such as RAID 0, RAID 1, RAID 5, and RAID 10, each offering different benefits and trade-offs.

To configure a software RAID array on CentOS, you can use the mdadm utility, which is a powerful tool for creating, managing, and monitoring RAID arrays.

Example:

# Install mdadm if not already installed
sudo yum install mdadm

# Create a RAID 1 array with two devices
sudo mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1

# Create a filesystem on the RAID array
sudo mkfs.ext4 /dev/md0

# Mount the RAID array
sudo mkdir -p /mnt/raid1
sudo mount /dev/md0 /mnt/raid1

# Save the RAID configuration
sudo mdadm --detail --scan | sudo tee -a /etc/mdadm.conf

# Update the initramfs
sudo dracut -H -f /boot/initramfs-$(uname -r).img $(uname -r)

7. Write a script to monitor CPU usage and log it to a file every minute.

To monitor CPU usage and log it to a file every minute in CentOS, you can use a combination of the mpstat command and a cron job. The mpstat command provides CPU usage statistics, and cron can be used to schedule the logging at regular intervals.

First, create a script that logs the CPU usage:

#!/bin/bash
# cpu_monitor.sh
mpstat 1 1 | awk '/Average/ {print strftime("%Y-%m-%d %H:%M:%S"), $3, $4, $5, $6, $7, $8, $9, $10, $11}' >> /var/log/cpu_usage.log

Make the script executable:

chmod +x cpu_monitor.sh

Next, set up a cron job to run this script every minute:

crontab -e

Add the following line to the crontab file:

* * * * * /path/to/cpu_monitor.sh

This cron job will execute the cpu_monitor.sh script every minute, logging the CPU usage to /var/log/cpu_usage.log.

8. Describe the steps to set up a high-availability cluster using Pacemaker and Corosync.

To set up a high-availability cluster using Pacemaker and Corosync on CentOS, follow these high-level steps:

• Install Required Packages: Install Pacemaker, Corosync, and any other necessary packages on all nodes in the cluster.
• Configure Corosync: Set up the Corosync configuration file to define the cluster nodes and communication settings.
• Start and Enable Services: Start and enable the Corosync and Pacemaker services on all nodes.
• Configure Pacemaker: Use the pcs command-line tool to configure the cluster, including setting up resources, constraints, and failover policies.
• Test the Cluster: Verify that the cluster is functioning correctly by testing failover and resource management.

9. Explain how to use yum or dnf for package management.

Yum (Yellowdog Updater, Modified) and DNF (Dandified Yum) are package management tools used in CentOS to manage software packages. They allow users to install, update, and remove packages from the system repositories. DNF is the next-generation version of Yum and is used in CentOS 8 and later versions.

Yum and DNF handle dependencies automatically, ensuring that all required packages are installed or updated together. They also provide features like package groups, repository management, and transaction history.

Here are some common commands for using yum and dnf:

# Installing a package
sudo yum install package_name
sudo dnf install package_name

# Updating a package
sudo yum update package_name
sudo dnf update package_name

# Removing a package
sudo yum remove package_name
sudo dnf remove package_name

# Listing installed packages
yum list installed
dnf list installed

# Searching for a package
yum search package_name
dnf search package_name

10. Describe how to set up and manage virtual machines using KVM or another hypervisor.

To set up and manage virtual machines using KVM on CentOS, follow these steps:

1. Install KVM and related packages: First, ensure that your system supports hardware virtualization. Then, install KVM and related packages such as qemu-kvm, libvirt, and virt-manager.

2. Enable and start the libvirtd service: Enable and start the libvirtd service to manage the virtualization environment.

3. Create a virtual network: Use virsh or virt-manager to create a virtual network for your virtual machines.

4. Create and manage virtual machines: Use virt-install or virt-manager to create and manage virtual machines. You can specify various parameters such as CPU, memory, storage, and network configurations.

5. Manage virtual machines: Use virsh commands to start, stop, and manage your virtual machines. You can also use virt-manager for a graphical interface.

Example commands:

# Install KVM and related packages
sudo yum install -y qemu-kvm libvirt virt-install bridge-utils virt-manager

# Enable and start the libvirtd service
sudo systemctl enable libvirtd
sudo systemctl start libvirtd

# Create a virtual network (example using default network)
sudo virsh net-start default
sudo virsh net-autostart default

# Create a virtual machine (example)
sudo virt-install \
--name centos-vm \
--ram 2048 \
--disk path=/var/lib/libvirt/images/centos-vm.img,size=20 \
--vcpus 2 \
--os-type linux \
--os-variant centos7.0 \
--network network=default \
--graphics none \
--console pty,target_type=serial \
--location 'http://mirror.centos.org/centos/7/os/x86_64/' \
--extra-args 'console=ttyS0,115200n8 serial'