1. Basics: Navigating the Command Line

Code Example: Navigation

# Print the current working directory
pwd

# List files and directories
ls            # Basic listing
ls -l         # Detailed listing
ls -a         # Show hidden files

# Change directory
cd /path/to/directory    # Absolute path
cd ..                    # Move up one directory
cd ~                     # Move to home directory

# Create and remove directories
mkdir new_folder         # Create a directory
rmdir empty_folder       # Remove an empty directory

Key Concepts

• pwd: Shows your current location in the file system.
• ls: Lists files and directories.
• cd: Changes directories.
• Use absolute paths (/) or relative paths (../).

2. File Management

Code Example: Creating and Managing Files

# Create an empty file
touch newfile.txt

# View file content
cat file.txt             # Print the content of a file
less file.txt            # View file content page by page
head file.txt            # Show the first 10 lines
tail file.txt            # Show the last 10 lines

# Copy, move, and delete files
cp source.txt destination.txt    # Copy a file
mv oldname.txt newname.txt       # Rename or move a file
rm file.txt                      # Delete a file
rm -r folder                     # Delete a folder and its contents

Key Concepts

• touch: Creates new files.
• cp, mv, rm: For file copy, move/rename, and delete.
• Use rm -r cautiously to remove directories recursively.

3. Viewing System Information

Code Example: Checking System Stats

# Display disk usage
df -h           # Disk space in human-readable format
du -sh folder   # Check size of a specific folder

# Show running processes
top             # Real-time system process monitor
ps              # Display currently running processes
kill <PID>      # Kill a process by its ID

# Check memory usage
free -h         # Free and used memory stats

Key Concepts

• top: Interactive view of processes.
• df and du: Disk usage stats.
• kill: Terminate processes.

4. Searching and Finding Files

Code Example: Find Files and Search Content

# Search for files
find /path -name "filename.txt"      # Find a file by name
find . -type f -name "*.log"         # Find files with a specific extension

# Search within files
grep "search_term" file.txt          # Search for a term in a file
grep -r "search_term" /path          # Recursive search in a directory
grep -i "search_term" file.txt       # Case-insensitive search

# Search file content with line numbers
grep -n "search_term" file.txt

Key Concepts

• find: Search for files and directories.
• grep: Search within files.

5. Working with Archives

Code Example: Compress and Extract Files

# Create tar archives
tar -cvf archive.tar folder/         # Create a tar archive
tar -czvf archive.tar.gz folder/     # Create a compressed tar.gz archive

# Extract tar archives
tar -xvf archive.tar                 # Extract a tar archive
tar -xzvf archive.tar.gz             # Extract a tar.gz archive

# ZIP and Unzip
zip archive.zip file.txt             # Compress to zip
unzip archive.zip                    # Extract zip archive

Key Concepts

• tar: Archive and compress files.
• zip/unzip: For .zip archives.

6. File Permissions

Code Example: Changing Permissions

# Show file permissions
ls -l file.txt

# Change file permissions
chmod 644 file.txt       # Owner: read/write, Group/others: read
chmod +x script.sh       # Add execute permission

# Change file ownership
chown user:group file.txt

Key Concepts

• chmod: Modify file permissions (r = read, w = write, x = execute).
• chown: Change file ownership.

7. Networking Commands

Code Example: Check Network Status

# Test connectivity
ping google.com

# Download files
wget https://example.com/file.zip    # Download a file
curl -O https://example.com/file.zip # Download using curl

# Check open ports
netstat -tuln

# Display IP address
ifconfig           # For Unix/Linux
ip addr            # Modern alternative

Key Concepts

• ping: Test connectivity to a server.
• wget/curl: Download files.
• netstat: Show active network connections.

8. Process Management

Code Example: Manage Processes

# View processes
ps aux                      # Show all processes
top                         # Real-time process monitoring

# Kill a process
kill <PID>                  # Kill by process ID
killall process_name        # Kill all processes by name

# Run a process in the background
command &                   # Run in background
jobs                        # Show background jobs
fg %1                       # Bring job #1 to the foreground

Key Concepts

• ps: Shows running processes.
• kill/killall: Terminate processes.
• &: Run commands in the background.

9. Text Manipulation

Code Example: Working with Text Files

# View and edit files
cat file.txt                # Display file content
nano file.txt               # Edit file in terminal
vim file.txt                # Edit with Vim

# Sort and unique
sort file.txt               # Sort lines alphabetically
uniq file.txt               # Remove duplicate lines

# Replace text with sed
sed 's/old/new/g' file.txt  # Replace "old" with "new"

# Combine commands
cat file.txt | grep "term" | sort | uniq

Key Concepts

• sort, uniq: Sort and deduplicate text.
• sed: Stream editor for text replacement.

10. Shortcuts and Tricks

Code Example: Terminal Shortcuts

# Re-run previous commands
!!                 # Run last command
!n                 # Run command n from history
!<keyword>         # Run last command starting with keyword

# Search through command history
Ctrl + R           # Reverse search in history

# Navigate more efficiently
Ctrl + A           # Move cursor to start of line
Ctrl + E           # Move cursor to end of line
Ctrl + C           # Cancel current command
Ctrl + L           # Clear terminal

Key Concepts

• Use shortcuts to save time.
• Reverse search (Ctrl + R) helps quickly find past commands.

11. Summary of Key Commands

12. Additional Resources

1. Linux Command Cheat Sheet: Linux Command Cheat Sheet
2. Advanced Bash Guide: The Linux Documentation Project
3. Interactive Command Line Practice: https://cmdchallenge.com/

13. Environment Variables

What Are Environment Variables?

Environment variables are system-wide variables accessible to applications and the command line. They provide dynamic values like system paths and configurations.

Code Example: Access and Set Environment Variables

# View all environment variables
printenv

# Access a specific variable
echo $HOME

# Set an environment variable (current session only)
export MY_VAR="Hello World"

# Make it permanent (add to .bashrc or .zshrc)
echo 'export MY_VAR="Hello World"' >> ~/.bashrc
source ~/.bashrc

Key Concepts

• printenv: Prints all environment variables.
• export: Temporarily sets a variable.
• Add variables to ~/.bashrc or ~/.zshrc for persistence.

14. Redirection and Pipelines

Code Example: Input/Output Redirection

# Redirect output to a file
ls > output.txt       # Overwrite file
ls >> output.txt      # Append to file

# Redirect input from a file
cat < input.txt

# Redirect error messages
ls nonexistentfile 2> error.log   # Redirect errors only
ls nonexistentfile > output.log 2>&1  # Redirect all output and errors

Code Example: Pipelines

# Pipe output to another command
ls | grep ".txt"       # List files with ".txt" in their names
ps aux | sort -k 3     # Sort processes by CPU usage

Key Concepts

• >: Redirects standard output to a file.
• 2>: Redirects errors to a file.
• |: Pipes the output of one command to another.

15. Advanced File Manipulation

Code Example: Linking Files

# Create a symbolic link
ln -s target_file symlink

# Create a hard link
ln target_file hardlink

# View the target of a symbolic link
ls -l symlink

Working with Timestamps

# Update file timestamp
touch file.txt

# View file timestamps
stat file.txt

Key Concepts

• Symbolic Links: Point to a file or directory.
• Hard Links: Duplicate the file entry in the filesystem.

16. Disk and Partition Management

Code Example: Disk Information

# Show disk usage
df -h              # Human-readable format
du -sh folder/     # Size of a folder

# Manage partitions
lsblk              # List block devices
fdisk /dev/sdX     # Partition a disk (requires sudo)

Key Concepts

• df: Disk free space.
• du: Directory usage.
• lsblk: List information about block devices.

17. Job Control

Code Example: Managing Background Jobs

# Run a command in the background
command &

# View running jobs
jobs

# Bring a job to the foreground
fg %1

# Send a running job to the background
Ctrl + Z
bg %1

Key Concepts

• jobs: Lists all background jobs.
• fg and bg: Resume jobs in the foreground or background.

18. SSH (Secure Shell)

Code Example: Connect to a Remote Server

# Connect to a remote server
ssh user@remote_host

# Copy files between local and remote
scp file.txt user@remote_host:/remote/path
scp user@remote_host:/remote/path/file.txt ./local/path

# Generate SSH keys
ssh-keygen -t rsa -b 4096
ssh-copy-id user@remote_host

Key Concepts

• ssh: Securely access remote systems.
• scp: Secure file copy.
• Use SSH keys for passwordless authentication.

19. Scripting Basics

Code Example: Bash Scripting

#!/bin/bash
# A simple script

echo "Hello, World!"
NAME="Alice"
echo "Hello, $NAME!"

# Conditional logic
if [ $NAME == "Alice" ]; then
  echo "Hi, Alice!"
else
  echo "Who are you?"
fi

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

Key Concepts

• Scripts start with a shebang: #!/bin/bash.
• Use variables, conditionals, and loops for automation.

20. File Compression Formats

Code Example: Common Formats

# Compress to .gz
gzip file.txt
gunzip file.txt.gz

# Compress to .bz2
bzip2 file.txt
bunzip2 file.txt.bz2

# Compress to .xz
xz file.txt
unxz file.txt.xz

Key Concepts

• gzip, bzip2, xz: Compress files efficiently.
• Use .tar to combine multiple files before compression.

21. Process Priority

Code Example: Adjust Priority

# Run a command with lower priority
nice -n 10 command

# Change priority of a running process
renice -n 5 -p <PID>

Key Concepts

• nice: Sets the priority of a command.
• renice: Adjusts the priority of a running process.

22. Remote File Systems

Code Example: Mount Remote Directories

# Mount a remote directory using SSHFS
sshfs user@remote_host:/remote/path /local/mountpoint

# Unmount the directory
fusermount -u /local/mountpoint

Key Concepts

• SSHFS: Mount remote file systems over SSH.
• Use for easy access to remote files locally.

23. Practical Tips

Tips to Speed Up Command Line Workflows

1. Auto-Completion:
   • Use Tab to autocomplete file and directory names.
2. Aliases:

   alias ll="ls -la"
   alias gs="git status"
   

   Add aliases to ~/.bashrc or ~/.zshrc.
3. Command Substitution:

   current_date=$(date)
   echo "Today is $current_date"
   

4. Create Shortcuts for Frequently Used Directories:

   alias project="cd /path/to/project"
   

24. Summary of Key Concepts

25. Additional Resources

1. Explainshell: https://explainshell.com (Explains commands in detail).
2. Linux Journey: https://linuxjourney.com/ (Interactive tutorials).
3. Cheat Sheet: https://www.cheatography.com/ (Find quick command references).