123 Essential Linux Commands Every DevOps Engineer Should Master

Linux is an open-source operating system kernel developed by Linus Torvalds in 1991. It’s the foundation of various Linux distributions like Ubuntu and Fedora. Known for stability, security, and flexibility, it’s widely used in servers, desktops, and embedded systems.

• Open-source software allows users to view, modify, and distribute its source code freely.
• Linux kernel is the core component of the Linux operating system.
• Licensed under the GNU General Public License (GPL), promoting openness and collaboration.

Basics of Navigating the File System (ls, cd, pwd):

• ls: Lists the contents of the current directory.
• cd: Changes the current directory. For example, cd Documents moves to the "Documents" directory.
• pwd: Prints the current working directory, showing you where you are in the file system.

File Manipulation Commands (mkdir, touch, rm, cp, mv):

• mkdir: Creates a new directory. For example, mkdir my_folder create a directory named "my_folder".
• touch: Creates a new empty file or updates the timestamp of an existing file. For example, touch my_file.txt create a new text file named "my_file.txt".
• rm: Removes files or directories. Be cautious with this command, as it permanently deletes files. For example, rm my_file.txt delete the file "my_file.txt".
• cp: Copies files or directories. For example, cp file1.txt file2.txt copy "file1.txt" to "file2.txt".
• mv: Moves or renames files or directories. For example, mv old_file.txt new_file.txt rename "old_file.txt" to "new_file.txt", and mv file.txt directory/ move "file.txt" into the "directory" directory.

Text Editing (nano, vim):

• nano: A simple and user-friendly text editor. Use nano filename to open a file in nano for editing. It provides on-screen instructions for basic operations.
• vim: A powerful and customizable text editor with a steeper learning curve. Use vim filename to open a file in vim. Press i to enter insert mode for editing, Esc to exit insert mode, and :wq to save and exit.

User Management:

Adding Users:

• useradd: Adds a new user account to the system. Syntax: useradd username.
• adduser: A user-friendly interface for adding users, often with additional configuration options.

Deleting Users:

• userdel: Deletes a user account from the system. Syntax: userdel username.
• deluser: A user-friendly interface for deleting users, often handling additional cleanup tasks.

Modifying User Attributes:

• usermod: Modifies user account attributes, such as username, home directory, or group membership. Syntax: usermod options username.

Changing User Passwords:

• passwd: Allows users to change their passwords. As an administrator, you can use it to change another user's password by typing passwd username.

Viewing User Information:

• id: Displays user and group IDs, as well as additional information about a specified user. Syntax: id username.
• finger: Provides detailed user information, including login name, real name, terminal, and more. Syntax: finger username.

Switching Users:

• su: Allows you to switch to another user account or execute commands as another user. Syntax: su username.

Listing Users:

• who: Displays information about users who are currently logged in.
• w: Provides detailed information about currently logged-in users, including what they're doing.

System Information:

Displaying Basic System Information:

• uname: Displays system information such as kernel name, network node hostname, kernel release, kernel version, machine hardware name, and processor type. For example, uname -a displays all available system information.

Viewing System Hardware Information:

• lscpu: Provides information about the CPU architecture and processor details.
• lshw: Lists detailed hardware configuration, including memory, processor, disk, and network information. Requires root privileges or sudo access.
• lspci: Shows information about PCI buses and connected devices.
• lsusb: Displays information about USB buses and connected devices.
• lsblk: Lists block devices, such as hard drives and partitions, along with their mount points.

Monitoring System Performance:

• top: Displays dynamic real-time information about system processes, CPU usage, memory usage, and more.
• htop: An interactive process viewer that provides an overview of system resources and allows for easy process management.

Checking System Memory Usage:

• free: Displays the amount of free and used memory in the system, including total, used, and free memory, as well as buffers and cache.
• vmstat: Reports information about processes, memory, paging, block IO, traps, and CPU activity.

Checking Disk Usage:

• df: Displays disk space usage for all mounted filesystems.
• du: Estimates disk usage for directories and files.

Viewing Network Information:

• ifconfig or ip addr: Shows network interface information, including IP addresses, MAC addresses, and network configuration.
• netstat: Displays network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.
• ss: Another utility to investigate sockets, displaying more detailed information than netstat.

Package Management:

Debian/Ubuntu-based Systems (using APT):

• apt update: Updates the local package index to reflect the latest changes made in the repositories.
• apt upgrade: Upgrades all installed packages to their latest versions.
• apt install <package>: Installs the specified package.
• apt remove <package>: Removes the specified package, along with its configuration files.
• apt purge <package>: Completely removes the specified package, including its configuration files.
• apt search <keyword>: Searches for packages matching the specified keyword.
• apt list --installed: Lists all installed packages.

Red Hat/CentOS-based Systems (using YUM or DNF):

• yum update or dnf update: Updates all installed packages to their latest versions.
• yum install <package> or dnf install <package>: Installs the specified package.
• yum remove <package> or dnf remove <package>: Removes the specified package.
• yum search <keyword> or dnf search <keyword>: Searches for packages matching the specified keyword.
• yum list installed or dnf list installed: Lists all installed packages.

Common Package Management Commands for All Systems:

• apt-cache search <keyword>: Searches for packages matching the specified keyword in Debian/Ubuntu-based systems.
• rpm -qa: Lists all installed packages in Red Hat/CentOS-based systems.
• dpkg -l: Lists all installed packages in Debian/Ubuntu-based systems.

File Management:

• find: Searches for files and directories in a directory hierarchy based on various criteria such as name, size, or permissions.
• grep: Searches for patterns in files or standard input. It's commonly used to filter lines containing a specific pattern.
• awk: A versatile text processing tool that operates on lines of input and can perform actions based on patterns.
• sed: A stream editor used to perform text transformations on an input stream. It's often used for search and replace operations.
• tar: Archives files into a single file (often called a "tarball") and optionally compresses them.
• gzip: Compresses files using the gzip compression algorithm. It replaces the original file with a compressed version.
• zip: Compresses files into a zip archive, which can include multiple files and directories.

Networking:

Network Configuration:

• ifconfig or ip addr: Displays or configures network interfaces, including IP addresses, MAC addresses, and network configuration.
• iwconfig: Configures wireless network interfaces.

Network Connectivity:

• ping: Tests connectivity to a remote host by sending ICMP echo request packets.
• traceroute or traceroute6: Traces the route packets take to reach a destination host.
• mtr: Combines the functionality of ping and traceroute to provide real-time network diagnostics.
• netcat or nc: Reads and writes data across network connections, often used for debugging and network exploration.
• telnet: Allows users to communicate with remote systems using the Telnet protocol.
• ssh: Securely connects to a remote system using the SSH protocol.

DNS Configuration and Resolution:

• dig: A versatile DNS lookup utility for querying DNS servers and retrieving DNS records.
• nslookup: Another DNS lookup utility for querying DNS servers and resolving domain names.

Network Services and Ports:

• netstat: Displays network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.
• ss: Another utility to investigate sockets, providing more detailed information than netstat.
• nmap: A powerful network scanning tool for discovering hosts and services on a network.

Network Diagnostics:

• arp: Displays or manipulates the IP-to-MAC address translation tables.
• tcpdump: Captures and analyzes network packets in real-time.
• wireshark: A graphical network protocol analyzer that allows for deep inspection of network traffic.

Firewall and Security:

• iptables: Controls packet filtering, network address translation (NAT), and other firewall-related tasks.
• ufw: A user-friendly interface for managing iptables firewall rules.
• fail2ban: Monitors system logs and dynamically blocks IP addresses exhibiting malicious behavior.

Other Utilities:

• hostname: Displays or sets the system's hostname.
• route: Displays or manipulates the IP routing table.
• ifup and ifdown: Brings network interfaces up or down, respectively.

Process Management:

Viewing Processes:

• ps: Displays information about active processes. Common options include -aux for a detailed list of all processes and -ef for a full listing of processes.
• top: Provides a dynamic real-time view of system processes, CPU usage, and memory usage.
• htop: An interactive process viewer that provides an overview of system resources and allows for easy process management.
• pgrep: Searches for processes based on name or other attributes and prints their process IDs.
• pstree: Displays a tree diagram of processes, showing their hierarchical relationship.

Killing Processes:

• kill: Terminates a process by sending a signal to it. By default, it sends the TERM signal, but other signals like KILL or HUP can be specified.
• killall: Terminates processes by name rather than PID.
• pkill: Similar to killall, but more versatile as it allows specifying processes by name or other attributes using regular expressions.
• xkill: A graphical utility that allows users to kill a window or process by clicking on it.

Background and Foreground Processes:

• bg: Puts a stopped or backgrounded process into the background.
• fg: Brings a backgrounded process to the foreground.
• jobs: Lists active jobs (background processes) associated with the current shell.

Process Priority and Control:

• nice: Launches a process with a specified priority level.
• renice: Changes the priority of an existing process.
• ionice: Sets the I/O scheduling priority for a process.

Monitoring and Debugging:

• strace: Traces system calls and signals made by a process, helpful for debugging.
• lsof: Lists open files and the processes that opened them, useful for troubleshooting.
• pidof: Returns the process ID of a running program.
• pgrep: Searches for processes based on name and other attributes and prints their process IDs.
• killall: Terminates processes by name rather than PID.

System Resource Usage:

• free: Displays the amount of free and used memory in the system.
• vmstat: Reports information about processes, memory, paging, block IO, traps, and CPU activity.
• sar: Collects, reports, and saves system activity information, including CPU, memory, disk, and network usage.

System Administration:

Managing System Services:

systemctl: Controls systemd services, including starting, stopping, restarting, enabling, disabling, and viewing service status. For example:

• Start a service: sudo systemctl start serviceName
• Stop a service: sudo systemctl stop serviceName
• Restart a service: sudo systemctl restart serviceName
• Enable a service: sudo systemctl enable serviceName
• Disable a service: sudo systemctl disable serviceName
• Check service status: sudo systemctl status serviceName

service: A command-line tool for managing system services, commonly used in traditional SysVinit systems. It can start, stop, restart, reload, enable, and disable services. For example:

• Start a service: sudo service serviceName start
• Stop a service: sudo service serviceName stop
• Restart a service: sudo service serviceName restart
• Reload a service: sudo service serviceName reload
• Enable a service: sudo service serviceName enable
• Disable a service: sudo service serviceName disable

System Backup and Restore:

Backup:

• tar: Archives files into a single file (tarball) and optionally compresses them. For example:
• Create a tarball: tar -cvf backup.tar /path/to/backup
• rsync: Syncs files and directories between different locations, often used for backup purposes. For example:

Backup files: rsync -av /source/directory/ /destination/directory/

Restore:

• tar: Extracts files from a tarball. For example:
• Extract files: tar -xvf backup.tar -C /path/to/restore
• rsync: Restores files from a backup location. For example:

Restore files: rsync -av /backup/source/ /restore/destination/

Disk Management:

• fdisk: A command-line utility for disk partitioning. It allows creating, deleting, and managing disk partitions. For example:

Start fdisk: sudo fdisk /dev/sdX

• mkfs: Creates a filesystem on a disk partition. It is used to format partitions with various filesystem types. For example:

Format partition with ext4: sudo mkfs.ext4 /dev/sdX1

• mount: Mounts filesystems to the directory tree, making them accessible. For example:

Mount a filesystem: sudo mount /dev/sdX1 /mnt

• df: Displays disk space usage for all mounted filesystems. For example:

Show disk space usage: df -h

Advanced Topics:

Shell Scripting (Bash Scripting Basics):

• Bash scripting allows users to automate tasks by writing scripts that execute commands and perform operations.
• Basic elements include variables, loops, conditionals, functions, and command substitution.
• Example:

#!/bin/bash
# This is a simple Bash script

# Define variables
greeting="Hello, world!"

# Print greeting
echo $greeting

Remote Administration (SSH, rsync):

• Secure Shell (SSH) allows users to securely access and manage remotessh username@hostname systems over a network.
• Example:

ssh username@hostname

• Rsync is a utility for efficiently syncing files and directories between systems.
• Example:

rsync -av /local/path/ username@remotehost:/remote/path/

System Security (Firewall Configuration, SELinux, AppArmor):

Firewall Configuration:

• Tools like iptables (legacy) or firewalld (modern) are used to configure firewall rules.

SELinux (Security-Enhanced Linux):

• Provides access control security policies, including mandatory access controls (MAC).
• Commands include sestatus for status and setenforce for changing the enforcement mode.

AppArmor:

• Mandatory access control framework for restricting programs’ capabilities.
• Commands include apparmor_status for status and aa-enforce for enforcing policies.

Virtualization and Containerization (VirtualBox, Docker):

VirtualBox:

• Allows users to create and manage virtual machines (VMs) on a host system.
• Provides a graphical user interface (GUI) and command-line interface (CLI) tools.

Docker:

• Platform for developing, shipping, and running applications in containers.
• Commands include docker run to run containers, docker build to build images, and docker-compose for multi-container applications.

These advanced topics cover essential concepts and commands for tasks like scripting, remote administration, system security, and virtualization/containerization in Linux environments. They enable users to automate tasks, manage remote systems, enhance security, and deploy applications efficiently.

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