The Ultimate Cell Organelles Cheat Sheet: Structure, Function & Significance

Introduction to CentOS

CentOS (Community Enterprise Operating System) is a free, enterprise-class Linux distribution derived from Red Hat Enterprise Linux (RHEL). Known for its stability, security, and long-term support, CentOS is widely used for servers, data centers, and enterprise applications. Understanding CentOS administration is essential for maintaining reliable, secure, and efficient Linux-based infrastructure.

System Information and Basic Commands

System Information

# System version and information
cat /etc/centos-release     # Show CentOS version
uname -a                    # Kernel information
hostnamectl                 # System hostname information
lscpu                       # CPU information
free -h                     # Memory usage (human-readable)
df -h                       # Disk usage (human-readable)
uptime                      # System uptime and load average

User Management

# User administration
useradd username            # Create new user
usermod -aG wheel username  # Add user to wheel group (sudo access)
passwd username             # Set/change user password
userdel -r username         # Delete user and home directory

# User information
id username                 # Display user ID and groups
who                         # Show who is logged in
w                           # Show who is logged in and what they're doing
last                        # Show last logins

File and Directory Operations

# Basic file operations
ls -la                      # List all files with details
cp -rp source destination   # Copy files/directories recursively preserving attributes
mv source destination       # Move/rename files or directories
rm -rf directory            # Remove directory recursively (use with caution!)
mkdir -p dir1/dir2/dir3     # Create directory structure
touch filename              # Create empty file or update timestamp

# File permissions
chmod 755 file              # Change file permissions (rwxr-xr-x)
chown user:group file       # Change file ownership
chcon -t httpd_sys_content_t file  # Change SELinux context

Process Management

# View processes
ps aux                      # List all running processes
top                         # Interactive process viewer
htop                        # Enhanced interactive process viewer (may need installation)

# Process control
kill PID                    # Terminate process by PID
kill -9 PID                 # Force terminate process
killall process_name        # Kill all processes by name
nice -n 19 command          # Run command with lower priority
renice +10 -p PID           # Change priority of running process

Package Management

YUM (CentOS 7) / DNF (CentOS 8+)

# Package installation
yum install package         # Install package (CentOS 7)
dnf install package         # Install package (CentOS 8+)
yum groupinstall "group"    # Install package group

# Package updates
yum update                  # Update all packages
yum update package          # Update specific package
dnf upgrade                 # Upgrade packages (CentOS 8+)

# Package removal
yum remove package          # Remove package
yum autoremove              # Remove unneeded dependencies

# Package search and info
yum search keyword          # Search packages by keyword
yum info package            # Display package information
yum provides /path/to/file  # Find which package provides a file
rpm -qa                     # List all installed packages
rpm -ql package             # List files installed by package

Repository Management

# Repository configuration
yum repolist                # List enabled repositories
yum-config-manager --enable repo  # Enable repository
yum-config-manager --disable repo # Disable repository

# Add a new repository
yum-config-manager --add-repo URL

# EPEL repository installation
yum install epel-release    # Install EPEL repository

Service Management

Systemd (CentOS 7 and 8+)

# Service control
systemctl start service     # Start a service
systemctl stop service      # Stop a service
systemctl restart service   # Restart a service
systemctl reload service    # Reload configuration without restart
systemctl status service    # Check service status
systemctl enable service    # Enable service at boot
systemctl disable service   # Disable service at boot

# System control
systemctl poweroff          # Shut down the system
systemctl reboot            # Reboot the system
systemctl suspend           # Suspend the system
systemctl hibernate         # Hibernate the system

Important System Services

Network Administration

Network Configuration

# Interface information
ip addr show               # Show IP addresses
ip link show               # Show network interfaces
nmcli device show          # NetworkManager device details
nmcli connection show      # Show network connections

# Configure network with NetworkManager
nmcli con add type ethernet con-name "My Connection" ifname eth0
nmcli con mod "My Connection" ipv4.addresses 192.168.1.100/24
nmcli con mod "My Connection" ipv4.gateway 192.168.1.1
nmcli con mod "My Connection" ipv4.dns "8.8.8.8 8.8.4.4"
nmcli con mod "My Connection" ipv4.method manual
nmcli con up "My Connection"

# Network testing
ping host                  # Test connectivity to host
traceroute host            # Trace route to host
mtr host                   # Combination of ping and traceroute
dig domain                 # DNS lookup
nslookup domain            # DNS lookup (alternative)
whois domain               # Get WHOIS information
netstat -tulpn             # Show listening ports and associated processes
ss -tulpn                  # Modern alternative to netstat

Firewall Management

# firewalld management
firewall-cmd --state                          # Check firewall state
firewall-cmd --get-default-zone               # Get default zone
firewall-cmd --get-active-zones               # Get active zones
firewall-cmd --list-all                       # List all firewall rules
firewall-cmd --permanent --add-service=http   # Add HTTP service
firewall-cmd --permanent --add-port=8080/tcp  # Add specific port
firewall-cmd --reload                         # Reload firewall configuration

Storage Management

Disk and Filesystem Operations

# Partition management
fdisk -l                   # List disk partitions
fdisk /dev/sda             # Partition a disk
gdisk /dev/sda             # GPT partition tool
parted -l                  # List partitions (alternative)

# Filesystem operations
mkfs.ext4 /dev/sda1        # Create ext4 filesystem
mkfs.xfs /dev/sda1         # Create XFS filesystem
mount /dev/sda1 /mnt       # Mount filesystem
umount /mnt                # Unmount filesystem

# Persistent mounts
blkid                      # Show block device attributes
# Add to /etc/fstab:
# UUID=xxxx-xxxx /mount_point filesystem defaults 0 0

Logical Volume Management (LVM)

# Physical volumes
pvcreate /dev/sdb          # Create physical volume
pvs                        # List physical volumes
pvdisplay                  # Display physical volume details

# Volume groups
vgcreate vg_name /dev/sdb  # Create volume group
vgs                        # List volume groups
vgextend vg_name /dev/sdc  # Extend volume group

# Logical volumes
lvcreate -L 10G -n lv_name vg_name  # Create logical volume
lvs                        # List logical volumes
lvextend -L +5G /dev/vg_name/lv_name  # Extend logical volume
lvreduce -L -5G /dev/vg_name/lv_name  # Reduce logical volume
xfs_growfs /mount_point    # Resize XFS filesystem after extending
resize2fs /dev/vg_name/lv_name  # Resize ext4 after extending/reducing

System Monitoring and Logs

System Monitoring

# Resource monitoring
top                        # Process activity, CPU usage
htop                       # Enhanced interactive process viewer
glances                    # Advanced system monitoring tool
vmstat 1                   # Virtual memory statistics
iostat -x 1                # IO statistics
mpstat -P ALL 1            # CPU statistics
sar -u 1 10                # CPU utilization (1 second intervals, 10 times)

# Service and login monitoring
systemctl list-units --state=running  # List running services
journalctl -f              # Follow system logs
journalctl -u service      # View logs for specific service
who                        # Show logged-in users
w                          # Show logged-in users and activity
last                       # Show last logged-in users

Log Management

# Important log files
/var/log/messages          # General system logs
/var/log/secure            # Authentication logs
/var/log/audit/audit.log   # Audit logs
/var/log/httpd/            # Apache logs
/var/log/boot.log          # Boot logs

# Log tools
tail -f /var/log/messages  # Follow log file in real-time
grep "error" /var/log/messages  # Search for errors in log
journalctl --since "1 hour ago"  # View logs from last hour
journalctl -p err          # View error-level logs

Security and Hardening

SELinux Management

# SELinux status
getenforce                 # Get current SELinux mode
sestatus                   # Detailed SELinux status

# SELinux configuration
setenforce 1               # Set Enforcing mode
setenforce 0               # Set Permissive mode (not recommended for production)
# Edit /etc/selinux/config for permanent change

# SELinux troubleshooting
ausearch -m avc -ts recent # Search for recent AVC denials
sealert -a /var/log/audit/audit.log  # Analyze SELinux issues
getsebool -a               # List all SELinux booleans
setsebool -P http_can_network_connect on  # Set boolean permanently

SSH Hardening

# Important settings in /etc/ssh/sshd_config
PermitRootLogin no         # Disable root login
PasswordAuthentication no  # Disable password authentication
PubkeyAuthentication yes   # Enable key-based authentication
AllowUsers user1 user2     # Allow only specific users
Port 2222                  # Change SSH port (not 22)

# After changing config:
systemctl restart sshd     # Restart SSH server

Password Policies

# Configuration in /etc/login.defs
# For existing users:
chage -M 90 username       # Set maximum password age
chage -m 7 username        # Set minimum password age
chage -W 14 username       # Set password expiration warning
chage -l username          # List password aging information

Backup and Recovery

Backup Commands

# File backup tools
tar -czvf backup.tar.gz /path/to/backup  # Create compressed archive
tar -xzvf backup.tar.gz                  # Extract archive
rsync -avz --progress source/ destination/  # Sync directories

# System backup
dd if=/dev/sda of=/path/to/disk.img      # Disk image backup

System Snapshots

# LVM snapshots
lvcreate -L 1G -s -n snap_name /dev/vg_name/lv_name  # Create snapshot
lvconvert --merge /dev/vg_name/snap_name  # Restore from snapshot

Automation and Scripting

Cron Jobs

# Cron configuration
crontab -e                 # Edit user's crontab
crontab -l                 # List user's crontab jobs

# Crontab format
# minute hour day month day-of-week command
# Examples:
# 0 5 * * * /path/to/script.sh  # Run at 5:00 AM daily
# */10 * * * * command          # Run every 10 minutes

Basic Bash Scripting

#!/bin/bash

# Variables
NAME="CentOS"
echo "Hello, $NAME!"

# Conditionals
if [ -f /etc/centos-release ]; then
    echo "This is CentOS"
else
    echo "This is not CentOS"
fi

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

# Functions
function check_service() {
    systemctl is-active $1 >/dev/null 2>&1
    if [ $? -eq 0 ]; then
        echo "$1 is running"
    else
        echo "$1 is not running"
    fi
}

check_service httpd

Comparison: CentOS 7 vs. CentOS 8+ vs. CentOS Stream

Common Administration Challenges and Solutions

Best Practices for CentOS Administration

• Regular Updates: Maintain system security by regularly applying security updates
• Configuration Management: Use tools like Ansible, Puppet, or Chef for consistent configurations
• Documentation: Maintain detailed documentation of system configurations and changes
• Monitoring: Implement system monitoring and alerting with tools like Nagios, Zabbix, or Prometheus
• Backup Strategy: Implement regular automated backups with testing of restore procedures
• Security Hardening: Follow CIS benchmarks for CentOS security hardening
• Change Control: Implement change management procedures for production systems
• Performance Tuning: Optimize system performance based on workload requirements
• Log Management: Centralize and monitor logs with tools like ELK stack or Graylog

Resources for Further Learning

Official Documentation

• CentOS Project Website
• Red Hat Enterprise Linux Documentation

Books

• “RHEL/CentOS 7 Certified System Administrator Study Guide” by Asghar Ghori
• “Mastering CentOS 7 Linux Server” by Mohamed Alibi
• “Pro Linux System Administration” by James Turnbull, et al.

Online Resources

• CentOS Wiki
• DigitalOcean CentOS Tutorials
• Linux Journal

Training and Certification

• Red Hat Certified System Administrator (RHCSA)
• Red Hat Certified Engineer (RHCE)
• Linux Professional Institute Certification (LPIC)