1. What is CLI

CLI (Command Line Interface) is a way of interacting with a computer through plain-text commands. Unlike using a mouse to click on graphical interfaces (GUI), CLI requires users to type text commands in a terminal, and the computer returns text results after execution.

Core Components

Historical Background

In the 1960s-1980s, CLI was the only way to interact with computers. After graphical interfaces (GUI) became widespread, CLI gradually moved behind the scenes, but it remains indispensable in the following areas:

• Software Development — Code compilation, testing, version control, dependency management
• Server Administration — Remote management of headless servers
• Data Science — Python virtual environments, pip package management, Jupyter startup
• DevOps / CI/CD — Automated deployment scripts, Docker operations

2. CLI vs GUI

3. Common CLI Programs (Shells)

A Shell is a “command interpreter” that reads user-input commands and passes them to the operating system for execution. Different operating systems provide different Shells:

Recommendations for Windows Users

Windows offers three commonly used terminals:

4. How to Open CLI

5. Basic Command Structure

Every CLI command follows a unified syntax format:

Meaning of each part:

Practical Breakdown Example

More Examples

# View help (supported by almost all CLI tools)
git --help
python --help
pip --help

# View version
python --version
node --version

# Multiple short options can be combined
ls -a -l -h     # equivalent to
ls -alh

# Long and short options are equivalent
pip install -r requirements.txt
pip install --requirement requirements.txt

6. File System Navigation

The first step in CLI is learning to navigate the file system. The following commands work across all platforms (PowerShell also supports them):

6.1 Check Current Path

/home/user/projects/my-app

6.2 List Directory Contents

file1.txt  folder_a  README.md  src/

Common options:

6.3 Change Directory

# Enter subdirectory
cd src

# Go up one level
cd ..

# Go up two levels
cd ../..

# Return to home directory
cd ~          # Linux/macOS
cd %USERPROFILE%    # Windows (cmd)
cd $HOME     # PowerShell

# Return to previous directory
cd -

7. File & Directory Operations

7.1 Create Directories

# Linux / macOS / PowerShell
mkdir new_folder

# Create nested directories (auto-create parent directories if they don't exist)
mkdir -p project/src/utils     # Linux/macOS
New-Item -ItemType Directory -Force -Path project/src/utils  # PowerShell

7.2 Create / Write Files

# Create empty file with touch (works on Linux/macOS/PowerShell)
touch config.json

# Write content with echo (> overwrite, >> append)
echo "Hello" > hello.txt
echo "World" >> hello.txt

7.3 Copy Files / Directories

# Linux / macOS
cp file1.txt file2.txt          # Copy file
cp -r folder_a folder_b          # Recursively copy directory

# Windows (cmd)
copy file1.txt file2.txt
xcopy folder_a folder_b /E /I

# PowerShell
Copy-Item file1.txt file2.txt
Copy-Item -Recurse folder_a folder_b

7.4 Move / Rename

# Linux / macOS
mv old_name.txt new_name.txt     # Rename
mv file.txt folder/              # Move to directory

# Windows (cmd)
move old_name.txt new_name.txt

# PowerShell
Move-Item old_name.txt new_name.txt
Move-Item file.txt folder/

7.5 Delete Files / Directories

# Linux / macOS
rm file.txt                      # Delete file
rm -r folder/                    # Recursively delete directory and contents
rm -rf folder/                   # Force recursive delete (no confirmation)

# Windows (cmd)
del file.txt
rmdir /s /q folder

# PowerShell
Remove-Item file.txt
Remove-Item -Recurse -Force folder

8. File Viewing & Editing

8.1 View File Contents

# View entire content (small files)
cat file.txt      # Linux/macOS/PowerShell
type file.txt     # Windows cmd
Get-Content file.txt  # PowerShell full syntax

# Paginated view (large files)
less file.txt     # Linux/macOS, press q to exit
more file.txt     # Windows cmd

# View first N lines
head -20 file.txt
Get-Content file.txt -Head 20   # PowerShell

# View last N lines
tail -20 file.txt
Get-Content file.txt -Tail 20   # PowerShell

# Real-time tail (commonly used for log viewing)
tail -f app.log

8.2 Search File Contents

# Search for a keyword in a file
grep "error" log.txt
Select-String "error" log.txt    # PowerShell

# Recursively search all files in a directory
grep -r "TODO" src/
Select-String -Recurse "TODO" src/   # PowerShell

# Search by filename
find . -name "*.py"
Get-ChildItem -Recurse -Filter "*.py"   # PowerShell

8.3 File Information

# Check file sizes and disk usage
du -sh folder/                       # Linux/macOS
du -h --max-depth=1                  # View first-level subdirectory sizes

# Check total disk space
df -h                                # Linux/macOS
Get-PSDrive C                        # PowerShell

9. Process Management

9.1 View Running Processes

# Linux / macOS
ps aux                  # View all processes
ps aux | grep python    # Filter Python-related processes
top                     # Real-time process monitoring (press q to exit)

# Windows
tasklist
tasklist | findstr python

# PowerShell
Get-Process
Get-Process | Where-Object {$_.Name -like "*python*"}

9.2 Terminate Processes

# Linux / macOS
kill 12345               # PID is the process ID
kill -9 12345             # Force terminate
pkill -f "python app.py"  # Terminate by name

# Windows
taskkill /PID 12345 /F

# PowerShell
Stop-Process -Id 12345 -Force
Stop-Process -Name python -Force

10. Pipes & Redirection

Pipes and redirection are core CLI capabilities that allow the output of one command to be used as input for another, or to write output to files.

10.1 Pipes |

Pass the output of the left command to the right command for processing:

# Count Python files in a directory
ls | grep ".py" | wc -l

# Find the process using port 8080
netstat -ano | findstr :8080

# Paginate long output
ps aux | less

# PowerShell pipe example
Get-Process | Sort-Object CPU -Descending | Select-Object -First 5

10.2 Redirection

# > Overwrite to file
echo "log message" > output.log

# >> Append to file
echo "another line" >> output.log

# 2> Redirect error output
python script.py 2> error.log

# &> Redirect both stdout and stderr
python script.py &> all_output.log

10.3 Practical Combinations

# Save pip install log to file
pip install numpy > install.log 2>&1

# Count lines of code
find . -name "*.py" | xargs wc -l | tail -1

# Quick backup of multiple files
cp *.py backup/

11. Environment Variables

Environment variables are global configurations at the operating system level that affect the runtime behavior of all programs.

11.1 View Environment Variables

# Linux / macOS
echo $PATH
echo $HOME

# Windows (cmd)
echo %PATH%

# PowerShell
echo $env:PATH
echo $env:USERPROFILE

11.2 Set Environment Variables

# Temporary setting (only valid for current terminal session)
export API_KEY="abc123"             # Linux/macOS
$env:API_KEY = "abc123"             # PowerShell
set API_KEY=abc123                  # Windows cmd

# Permanent setting (write to config file)
# Linux/macOS: add to ~/.bashrc or ~/.zshrc
echo 'export API_KEY="abc123"' >> ~/.bashrc

# Windows: System Properties → Advanced → Environment Variables, or use PowerShell
[Environment]::SetEnvironmentVariable("API_KEY", "abc123", "User")

11.3 The PATH Variable

12. Package Managers

Package managers are one of the most commonly used tool categories in CLI, used for installing, upgrading, and uninstalling software.

pip Quick Reference

pip install package_name         # Install package
pip install -r requirements.txt   # Batch install from file
pip install -e .                  # Install local project in editable mode
pip uninstall package_name       # Uninstall package
pip list                          # View installed packages
pip show package_name             # View package details
pip install --upgrade package    # Upgrade package
pip freeze > requirements.txt     # Export current environment dependencies

13. Common Development Commands

######## Python Development ########
python --version                 # Check Python version
python -m venv venv               # Create virtual environment
venv\Scripts\activate             # Activate virtual environment (Windows)
source venv/bin/activate          # Activate virtual environment (macOS/Linux)
python main.py                    # Run Python script
python -m pytest tests/           # Run tests
jupyter notebook                  # Start Jupyter

######## Git Version Control ########
git clone https://github.com/xxx/project.git   # Clone project
git status                                   # Check status
git add .                                    # Stage all changes
git commit -m "commit message"               # Commit
git push origin main                         # Push
git pull origin main                         # Pull updates
git log --oneline                            # View commit history
git branch feature-xxx                        # Create branch
git checkout feature-xxx                      # Switch branch

######## Node.js Development ########
node --version                    # Check Node version
npm init -y                       # Initialize project
npm install express               # Install dependency
npm run dev                       # Run script

######## Docker ########
docker ps                         # View running containers
docker run -it ubuntu bash        # Start container
docker build -t myapp .           # Build image

14. Advanced Tips

14.1 Command History

# View command history
history                              # Linux/macOS
history | Select-String "git"        # PowerShell search history

# Shortcuts
↑ / ↓                                # Navigate up/down through history
Ctrl + R                             # Reverse search command history
!!                                   # Repeat last command
!$                                   # Last argument of previous command

14.2 Tab Auto-Completion

After typing the first few characters of a command or path, press Tab to auto-complete:

# Type partial command then press Tab
pyth[TAB]        # → python
cd doc[TAB]      # → cd Documents/
pip ins[TAB]     # → pip install

14.3 Keyboard Shortcuts Reference

14.4 Wildcards

# * matches any characters
ls *.py                  # List all .py files
rm *.log                  # Delete all .log files

# ? matches a single character
ls file?.txt              # Matches file1.txt, fileA.txt, etc.

14.5 Writing Shell Scripts

Combine frequently used commands into a script file for one-click execution:

#!/bin/bash
# deploy.sh — One-click deployment script

echo "=== Updating code ==="
git pull origin main

echo "=== Installing dependencies ==="
pip install -r requirements.txt

echo "=== Running tests ==="
python -m pytest tests/ -v

echo "=== Deployment complete ==="

15. Command Quick Reference