Command-line proficiency separates effective infrastructure engineers from those who struggle with basic server operations. Whether you are managing a single development server or orchestrating hundreds of production instances, these commands form the foundation of daily operations.

1. Navigating the File System

ls - List Directory Contents

The ls command lists files and directories within the current directory.

Usage:

ls

Examples:

ls -l   # Detailed listing with permissions, sizes, and modification dates
ls -a   # List all files including hidden files
ls -lh  # Human-readable file sizes

cd - Change Directory

The cd command is used to change the current directory.

Usage:

cd [directory]

Examples:

cd /home/user/Documents  # Change to Documents directory
cd ..                    # Move up one directory level
cd ~                     # Change to the home directory

pwd - Print Working Directory

The pwd command displays the current directory's full path.

Usage:

pwd

Example:

pwd  # Outputs something like /home/user/Documents

2. File Management

cp - Copy Files and Directories

The cp command copies files or directories from one location to another.

Usage:

cp [source] [destination]

Examples:

cp file.txt /home/user/backup/     # Copy file.txt to the backup directory
cp -r /home/user/dir1 /home/user/dir2  # Recursively copy dir1 to dir2

mv - Move/Rename Files and Directories

The mv command moves files or directories to a new location or renames them.

Usage:

mv [source] [destination]

Examples:

mv file.txt /home/user/backup/     # Move file.txt to the backup directory
mv oldname.txt newname.txt         # Rename oldname.txt to newname.txt

rm - Remove Files and Directories

The rm command deletes files or directories.

Usage:

rm [file]

Examples:

rm file.txt                        # Remove file.txt
rm -r /home/user/old_directory     # Recursively remove old_directory
rm -i file.txt                     # Prompt before every removal

touch - Create Empty Files

The touch command creates an empty file or updates the timestamp of an existing file.

Usage:

touch [file]

Example:

touch newfile.txt  # Create an empty file named newfile.txt

mkdir - Create Directories

The mkdir command creates new directories.

Usage:

mkdir [directory]

Examples:

mkdir new_directory  # Create a directory named new_directory
mkdir -p dir1/dir2   # Create a directory tree with dir1 and dir2

rmdir - Remove Empty Directories

The rmdir command removes empty directories.

Usage:

rmdir [directory]

Example:

rmdir old_directory  # Remove an empty directory named old_directory

3. File Viewing and Editing

cat - Concatenate and Display Files

The cat command displays the contents of a file or concatenates multiple files.

Usage:

cat [file]

Examples:

cat file.txt                        # Display the contents of file.txt
cat file1.txt file2.txt > combined.txt  # Concatenate file1.txt and file2.txt into combined.txt

less - View File Contents Page by Page

The less command allows you to view file contents one page at a time.

Usage:

less [file]

Example:

less longfile.txt  # View the contents of longfile.txt page by page

head - View the Beginning of a File

The head command displays the first few lines of a file.

Usage:

head [file]

Examples:

head file.txt            # Display the first 10 lines of file.txt
head -n 20 file.txt      # Display the first 20 lines of file.txt

tail - View the End of a File

The tail command displays the last few lines of a file.

Usage:

tail [file]

Examples:

tail file.txt            # Display the last 10 lines of file.txt
tail -n 20 file.txt      # Display the last 20 lines of file.txt
tail -f log.txt          # Continuously monitor log.txt for new entries

4. System Information

uname - Display System Information

The uname command displays basic information about the system.

Usage:

uname [options]

Examples:

uname -a  # Display all system information
uname -r  # Display the kernel version

top - Display System Processes

The top command displays a real-time view of system processes.

Usage:

top

Example:

top  # Launch the top utility to view system processes and resource usage

df - Disk Space Usage

The df command reports the amount of disk space used and available on filesystems.

Usage:

df [options]

Examples:

df -h  # Display disk space usage in human-readable format
df -T  # Display filesystem type

du - Directory Space Usage

The du command estimates file and directory space usage.

Usage:

du [options] [directory]

Examples:

du -h            # Display space usage in human-readable format
du -sh directory # Summarize and display total space used by directory

free - Memory Usage

The free command displays the amount of free and used memory in the system.

Usage:

free [options]

Example:

free -h  # Display memory usage in human-readable format

5. Managing Processes

ps - Display Process Information

The ps command provides information about active processes.

Usage:

ps [options]

Examples:

ps -e       # Display all processes
ps aux      # Detailed view of all processes

kill - Terminate Processes

The kill command terminates processes by their process ID (PID).

Usage:

kill [PID]

Examples:

kill 1234           # Terminate the process with PID 1234
kill -9 1234        # Forcefully terminate the process with PID 1234

pkill - Terminate Processes by Name

The pkill command terminates processes based on their name or other attributes.

Usage:

pkill [process_name]

Examples:

pkill firefox       # Terminate all instances of Firefox
pkill -u username   # Terminate all processes owned by the specified user

bg and fg - Background and Foreground Processes

The bg and fg commands are used to manage background and foreground processes.

Usage:

bg [job_spec]
fg [job_spec]

Examples:

ctrl+z        # Suspend the current foreground process
bg            # Resume the suspended process in the background
fg            # Bring the most recent background process to the foreground

6. Networking

ping - Test Network Connectivity

The ping command checks the network connectivity to a host.

Usage:

ping [host]

Example:

ping google.com  # Send ICMP ECHO_REQUEST packets to google.com

ifconfig - Configure Network Interfaces

The ifconfig command displays or configures network interfaces. Note: ifconfig is deprecated and replaced by ip.

Usage:

ifconfig [interface]

Example:

ifconfig eth0  # Display configuration of the eth0 interface

ip - Network Interface Management

The ip command is used for network interface management.

Usage:

ip [options] [command]

Examples:

ip addr show          # Display IP addresses assigned to all interfaces
ip link set eth0 up   # Bring the eth0 interface up
ip route show         # Display the routing table

netstat - Network Statistics

The netstat command displays various network statistics.

Usage:

netstat [options]

Examples:

netstat -a      # Display all active connections and listening ports
netstat -r      # Display routing table

Advanced Linux Commands and Examples

For those who are already familiar with basic Linux commands, delving into more advanced commands can significantly enhance your efficiency and capabilities. Here are 10 advanced Linux commands with examples to help you take your Linux skills to the next level.

1. grep - Search Text Using Patterns

The grep command searches for patterns within files. It's a powerful tool for text processing and pattern matching.

Usage:

grep [options] pattern [file]

Examples:

grep "error" /var/log/syslog  # Search for "error" in the syslog file
grep -r "TODO" ~/projects     # Recursively search for "TODO" in the projects directory
grep -i "hello" file.txt      # Case-insensitive search for "hello" in file.txt

2. find - Search for Files and Directories

The find command searches for files and directories in a directory hierarchy.

Usage:

find [path] [options] [expression]

Examples:

find /home/user -name "*.txt"  # Find all .txt files in the home directory
find / -type d -name "backup"  # Find directories named "backup" anywhere in the filesystem
find /var/log -mtime -7        # Find files in /var/log modified in the last 7 days

3. awk - Pattern Scanning and Processing Language

The awk command is used for pattern scanning and processing. It's a powerful programming language for working with text files.

Usage:

awk 'pattern {action}' [file]

Examples:

awk '{print $1}' file.txt               # Print the first column of each line in file.txt
awk '/error/ {print $0}' /var/log/syslog # Print lines containing "error" in syslog
awk -F: '{print $1, $3}' /etc/passwd    # Print the first and third fields (colon-separated) from /etc/passwd

4. sed - Stream Editor for Filtering and Transforming Text

The sed command is used to perform basic text transformations on an input stream (a file or input from a pipeline).

Usage:

sed [options] 'script' [file]

Examples:

sed 's/old/new/g' file.txt        # Replace all occurrences of "old" with "new" in file.txt
sed -n '5,10p' file.txt           # Print lines 5 to 10 from file.txt
sed '/^#/d' file.txt              # Delete lines starting with a hash (#) in file.txt

5. rsync - Remote Sync

The rsync command is used to synchronize files and directories between two locations over a network or locally.

Usage:

rsync [options] source destination

Examples:

rsync -avz /home/user/docs/ remote:/backup/docs/   # Synchronize docs directory to remote server with compression
rsync -av --delete /source/ /destination/          # Synchronize and delete files not present in source
rsync -azP /home/user/backup user@remote:/backup   # Synchronize with progress and partial transfers

6. tar - Archive Files

The tar command is used to create and extract archive files.

Usage:

tar [options] [archive-file] [file/directory]

Examples:

tar -cvf archive.tar /path/to/directory       # Create an archive from a directory
tar -xvf archive.tar                          # Extract files from an archive
tar -czvf archive.tar.gz /path/to/directory   # Create a compressed archive
tar -xzvf archive.tar.gz                      # Extract a compressed archive

7. curl - Transfer Data from or to a Server

The curl command is used to transfer data from or to a server using various protocols.

Usage:

curl [options] [URL]

Examples:

curl -O     # Download a file from a URL
curl -u user:password   # Download a file from an FTP server with authentication
curl -X POST -d "name=value"   # Send a POST request with data

8. iptables - Configure Network Packet Filtering Rules

The iptables command is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel.

Usage:

iptables [options]

Examples:

iptables -L                            # List all current rules
iptables -A INPUT -p tcp --dport 22 -j ACCEPT  # Allow incoming SSH connections
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT  # Allow established connections
iptables -A INPUT -p tcp --dport 80 -j DROP  # Drop all incoming HTTP connections

9. cron - Schedule Jobs

The cron command is used to schedule jobs to run at specific times or intervals.

Usage:

crontab [options]

Examples:

crontab -e                      # Edit the crontab file for the current user
crontab -l                      # List the current user's crontab
# Example cron job to run a script every day at midnight
0 0 * * * /path/to/script.sh

10. ssh - Secure Shell

The ssh command is used to securely connect to a remote machine over the network.

Usage:

ssh [options] [user@hostname]

Examples:

ssh user@remote-server.com            # Connect to a remote server
ssh -i /path/to/private_key user@remote-server.com  # Connect using a private key
ssh -L 8080:localhost:80 user@remote-server.com     # Set up local port forwarding

Key Takeaways

• Master the basics first: Commands like ls, cd, cp, mv, and rm form the foundation of all file system operations. Invest time in understanding their flags and options.
• Use tail -f for real-time log monitoring: This is indispensable for debugging production issues and monitoring application behavior.
• Prefer ip over ifconfig: The ifconfig command is deprecated on modern distributions. Migrate your muscle memory to ip addr and ip link.
• Combine grep, awk, and sed for powerful text processing: These three commands together can handle virtually any log parsing or data extraction task.
• Automate repetitive tasks with cron: Schedule backups, log rotation, and maintenance scripts to run automatically.
• Use rsync for reliable file transfers: Unlike scp, rsync supports incremental transfers and can resume interrupted operations.
• Understand process management: Knowing how to use ps, kill, and background/foreground job control is essential for managing long-running operations.