CentOS (Community Enterprise Operating System) is a Linux distribution that historically provided a free, community-supported, and functionally compatible alternative to Red Hat Enterprise Linux (RHEL). It was widely used for servers due to its stability, long-term support, and robustness.
While CentOS Linux (the traditional rebuild of RHEL) has ceased, it has been succeeded by **CentOS Stream**, which is an upstream development branch for RHEL. Despite this shift, understanding CentOS principles is crucial for anyone working with RHEL-based systems in enterprise environments.
It's important to clarify the relationship between these distributions:
CentOS (or CentOS Stream) is primarily used as a server OS, but can also be installed with a GUI for desktop use or learning.
The command-line interface (CLI) is fundamental for managing CentOS. Open a "Terminal" application (if using GUI) or log in via SSH (if headless server).
pwdls -l # Long format (details)
ls -a # Show hidden files
ls -lh # Human-readable sizescd /etc
cd ~ # Home directorymkdir my_new_folder
mkdir -p /opt/my_app/configsrmdir empty_dirtouch document.txtcp source.txt destination.txt
cp -r my_dir /tmp/mv old_name.txt new_name.txt
mv file.txt /archive/rm unwanted.txt
rm -rf my_dangerous_folder # Force remove recursively (EXTREME CAUTION!)cat /etc/os-release # Show OS informationless /var/log/messagestail -f /var/log/audit/audit.log # Follow a log file in real-timeecho "Hello, CentOS!"man dnfsudo dnf update # Update package listsCentOS adheres to the Filesystem Hierarchy Standard (FHS), which defines a standardized directory structure for Linux systems.
Linux security relies heavily on file permissions and ownership to control access.
ls -l my_script.sh
# Example Output: -rwxr-xr-- 1 user group 1234 Jul 19 10:00 my_script.shCan use symbolic mode or octal (numeric) mode.
chmod 755 my_script.sh # Owner: rwx, Group: r-x, Others: r-x (-rwxr-xr-x)
chmod 644 my_document.txt # Owner: rw-, Group: r--, Others: r-- (-rw-r--r--)
chmod 700 my_private_key.pem # Only owner has full accesschmod u+x my_script.sh # Add execute permission for owner
chmod go-w my_document.txt # Remove write for group and othersRequires `sudo`.
sudo chown newuser:newgroup /opt/app/config.confsudo chgrp webadmin /var/www/html/index.htmlCentOS, like other Linux systems, supports multi-user environments.
sudo adduser newuser
sudo passwd newuser # Set password for newusersudo useradd -m -s /bin/bash newuser2 # Create home dir, set Bash shellsudo usermod -aG wheel newuser # Add newuser to 'wheel' group (grants sudo privileges on CentOS)
sudo usermod -l new_name old_name # Change usernamesudo userdel newuser # Delete user, leave home directory
sudo userdel -r newuser2 # Delete user and remove home directorysudo groupadd webdevssudo groupdel oldgroupsudo gpasswd -a newuser webdevs # Add newuser to webdevs group
sudo gpasswd -d olduser webdevs # Remove olduser from webdevs groupCentOS (and RHEL-based systems) uses the RPM Package Manager, with `dnf` (Dandified YUM) being the modern front-end for installing, updating, and removing software packages. Older CentOS 7 and earlier used `yum`.
sudo dnf check-update # Check for available updates
sudo dnf update # Updates all installed packagessudo dnf install httpd # Install Apache HTTP Server
sudo dnf install git nano vim # Install multiple packagessudo dnf remove httpd
sudo dnf erase httpd # Same as removednf search phpdnf info httpddnf list installeddnf list availablesudo dnf group list # List available groups
sudo dnf groupinstall "Development Tools" # Install a group of related packagessudo dnf autoremovesudo dnf clean allManaging running programs and tasks on your system.
ps aux # Show all running processes (user, CPU, memory)
ps -ef # Show all processes in full format
ps -p <PID> # Show info for a specific Process IDtophtopkill <PID> # Sends SIGTERM (graceful termination request)
kill -9 <PID> # Sends SIGKILL (forceful termination, cannot be ignored)pkill firefox # Kills all processes named firefox
killall nginx # Kills all processes named nginxCentOS (and other RHEL-based systems) uses `systemd` to manage system services (daemons) and boot processes.
systemctl status httpd.service # Check Apache status
# Output shows Active: (running), Loaded: (enabled/disabled), etc.sudo systemctl start httpd.servicesudo systemctl stop httpd.servicesudo systemctl restart httpd.servicesudo systemctl enable httpd.servicesudo systemctl disable httpd.servicesudo systemctl reload httpd.serviceCentOS uses `NetworkManager` or `systemd-networkd` for network configuration. Configuration files are typically in `/etc/sysconfig/network-scripts/`.
ip a # Shows interfaces like lo, eth0, ens33, etc.ip route showping google.comss -tulnp # Show TCP/UDP listening ports with process infonmcli device status # Show network device status
nmcli connection show # Show active network connections
nmcli device wifi list # Scan for Wi-Fi networks
nmcli device wifi connect "SSID" password "PASSWD" # Connect to Wi-Fisudo systemctl status firewalld # Check firewall status
sudo systemctl start firewalld # Start firewall
sudo systemctl enable firewalld # Enable firewall on boot
sudo firewall-cmd --permanent --add-service=http # Allow HTTP traffic permanently
sudo firewall-cmd --reload # Apply changes# Example ifcfg-eth0 for static IP:
# Use 'sudo nano /etc/sysconfig/network-scripts/ifcfg-eth0' or 'sudo vi /etc/sysconfig/network-scripts/ifcfg-eth0'
# DEVICE=eth0
# BOOTPROTO=static
# ONBOOT=yes
# IPADDR=192.168.1.100
# NETMASK=255.255.255.0
# GATEWAY=1992.168.1.1
# DNS1=8.8.8.8
# DNS2=8.8.4.4
# After editing:
# sudo systemctl restart NetworkManager # Or network.service on older versionsManaging disk space, partitions, and file systems on CentOS.
df -h # Human-readable outputdu -sh /var/log # Summarize size of /var/loglsblksudo fdisk /dev/sda # Interactive partitioning toolsudo mkfs.xfs /dev/sdb1 # Format partition sdb1 with XFS (common on CentOS)sudo mount /dev/sdb1 /mnt/data # Mount sdb1 to /mnt/data
sudo umount /mnt/data # Unmount# Basic LVM commands (conceptual):
sudo pvcreate /dev/sdb1 # Create Physical Volume
sudo vgcreate myvg /dev/sdb1 # Create Volume Group
sudo lvcreate -L 10G -n mylv myvg # Create Logical Volume
sudo mkfs.xfs /dev/myvg/mylv # Format LV
sudo mount /dev/myvg/mylv /data # Mount LVAutomate tasks by writing a series of commands in a file.
# Create a file named 'hello_centos.sh'
nano hello_centos.sh
# Add the following content:
#!/bin/bash
# This is a basic CentOS shell script
echo "Hello from CentOS!"
echo "Current date and time: $(date)"
echo "Disk usage of /home: $(df -h /home | awk 'NR==2{print $5}')"chmod +x hello_centos.sh
./hello_centos.shYou'll frequently use command-line text editors in CentOS, especially for configuration files.
nano /etc/nginx/nginx.confvim /etc/ssh/sshd_configCentOS integrates robust security features. Proper configuration is key.
sudo dnf update -ysestatussudo setenforce 0 # Switch to Permissive
sudo setenforce 1 # Switch to Enforcingsudo nano /etc/selinux/config
# Change SELINUX=enforcing to SELINUX=permissive or SELINUX=disabled
# Reboot after changing this file for permanent effect.sudo ausearch -c 'httpd' --raw | audit2allow -M mywebserver
sudo semodule -i mywebserver.pp # Create and apply custom SELinux policyjournalctl -f # Follow all logs
journalctl -u sshd.service # View SSH service logsIf you installed CentOS Stream as "Server with GUI", it comes with the GNOME desktop environment. If you installed a minimal version and want to add a GUI:
sudo dnf update -ysudo dnf groupinstall "Server with GUI" -ysudo systemctl set-default graphical.targetsudo rebootsudo dnf install epel-release -y
sudo dnf --enablerepo=epel group install "Xfce" -y
echo "exec /usr/bin/xfce4-session" >> ~/.xinitrc
sudo systemctl set-default graphical.target
startx # Or rebootCentOS (and its successor CentOS Stream) provides a robust, stable, and secure foundation for server environments. Mastering its command-line tools, package management, service control, and security features like Firewalld and SELinux is crucial for any system administrator or DevOps professional. Practice regularly in a VM or cloud environment to solidify your skills.