cli-development

Best practices for designing and implementing command-line interface (CLI) applications across programming languages.

CLI Design Principles

Unix Philosophy

Follow the core Unix philosophy for robust, composable CLIs:

• Do one thing well - Each command should have a single, well-defined responsibility
• Make it composable - Design output to work as input to other programs
• Handle text streams - CLIs work with stdin/stdout/stderr
• Exit cleanly - Use appropriate exit codes (0 for success, non-zero for errors)
• Fail fast - Detect and report problems immediately
• Be scriptable - All functionality must be accessible from non-interactive contexts

Command Structure

Verb-Noun Pattern

Organize commands using verb-noun relationships:

# Good: clear action and object
git add <file>           # verb: add, noun: file
docker run <image>       # verb: run, noun: image
npm install <package>    # verb: install, noun: package

# Avoid: unclear structure
git foo                  # ambiguous
docker something         # unclear what happens

Subcommands vs Flags

Use subcommands when:

• Commands have distinct behaviors or workflows
• Different sets of options apply to different operations
• You want clear logical grouping

git branch                    # subcommand
git branch --list             # subcommand with flag
git branch --delete <name>    # subcommand with flag and argument

Use flags when:

• Modifying behavior of a single operation
• Toggling optional features
• Providing configuration

ls --color          # flag modifies ls behavior
grep -r --include   # flags modify search behavior

Never mix deeply:

# Avoid: unclear hierarchy
tool --verbose subcommand --debug --mode strict

# Better: clear structure
tool subcommand --verbose --debug --mode strict

Command Hierarchy

Keep hierarchy shallow (max 2-3 levels):

# Good: clear, discoverable
aws s3 ls
aws ec2 describe-instances

# Avoid: too deep
cloud provider storage list all buckets

Argument Parsing

Argument Types

Positional Arguments

• Required by default
• Appear in specific positions
• Use for primary input/object

cp <source> <destination>   # Two required positional arguments
docker run <image>          # One required positional argument

Optional Arguments (with defaults)

• Used less frequently
• Should have sensible defaults
• Clearly documented

npm install [directory]     # Optional, defaults to current directory
grep [options] <pattern> [file]  # File argument optional

Flags and Options

Short Flags (-f)

• Single letter, used frequently
• For common operations

ls -l          # long format
grep -r        # recursive
tar -xvf       # combined flags

Long Flags (--flag)

• Verbose, self-documenting
• For less-common operations
• Preferred in scripts

docker run --detach --name myapp
npm install --save-dev
grep --recursive --include="*.js"

Flag Styles

# Boolean flags
--verbose              # boolean true/false
--color=always         # explicit value

# Flags with values
--output file.txt      # space-separated
--output=file.txt      # equals-separated

# Accept both styles when possible
--timeout 30 or --timeout=30

Environment Variables

Convention: Use SCREAMING_SNAKE_CASE for environment variables

export DEBUG=1
export LOG_LEVEL=debug
export API_TOKEN=secret

Hierarchy (high to low precedence):

1. Command-line flags (highest priority - most explicit)
2. Environment variables (middle - applies to multiple invocations)
3. Configuration file (lower - shared settings)
4. Built-in defaults (lowest priority - fallback)

Example precedence:

# If user runs with flag, it takes precedence
tool --timeout 10       # Uses 10, ignores TIMEOUT env var

# If no flag, check env var
export TIMEOUT=5
tool                    # Uses 5 from TIMEOUT

# If no flag or env var, use config file default
tool --config config.yaml  # Reads timeout from config.yaml

# If nothing specified, use built-in default
tool                    # Uses hardcoded default (e.g., 30)

Configuration Files

Location convention:

• Linux/macOS: ~/.config/app/config.yaml or ~/.apprc
• Windows: %APPDATA%\App\config.yaml or ~\AppData\Local\App\config.yaml
• All platforms: ./config.yaml (current directory, highest priority)

Format preference: YAML > TOML > JSON > INI

• YAML is human-readable and concise
• TOML is simpler than YAML but supports nested structures
• JSON is universal but verbose
• INI is legacy

Example config file:

# ~/.config/myapp/config.yaml
debug: false
log_level: info
timeout: 30
output_format: json
color: true

api:
  endpoint: https://api.example.com
  timeout: 10

Load with proper precedence:

# Python example
import os
from pathlib import Path
import yaml

def load_config():
    # Built-in defaults
    config = {
        'debug': False,
        'timeout': 30,
        'color': True
    }

    # Load from config file
    config_path = Path.home() / '.config' / 'myapp' / 'config.yaml'
    if config_path.exists():
        with open(config_path) as f:
            file_config = yaml.safe_load(f)
            config.update(file_config)

    # Override with environment variables
    if os.getenv('DEBUG'):
        config['debug'] = os.getenv('DEBUG').lower() == 'true'
    if os.getenv('TIMEOUT'):
        config['timeout'] = int(os.getenv('TIMEOUT'))

    # Note: Command-line args handled by argument parser, applied after
    return config

User Interface

Help Text

Provide comprehensive help:

$ tool --help
Usage: tool [OPTIONS] COMMAND [ARGS]...

A brief description of what this tool does.

Options:
  -v, --verbose         Increase output verbosity
  -q, --quiet          Suppress non-error output
  -h, --help           Show this message and exit
  --version            Show version and exit

Commands:
  add                  Add a new item
  list                 List all items
  delete              Delete an item
  config              Manage configuration

Examples:
  tool add myitem
  tool list --format json
  tool delete --force

For more information: https://docs.example.com

Help text structure:

• Usage line: Usage: tool [OPTIONS] COMMAND [ARGS]...
• Brief description (1-2 lines)
• Options section (sorted: common first)
• Commands section (if subcommands exist)
• Examples section (show common patterns)
• Additional resources (docs link, contact info)

Per-command help:

$ tool add --help
Usage: tool add [OPTIONS] NAME

Add a new item to the collection.

Options:
  --description TEXT  Item description
  --tags TEXT        Comma-separated tags
  --priority INT     Priority level (1-10)
  -h, --help        Show this message and exit

Examples:
  tool add "My Item"
  tool add "Task" --priority 5 --tags work,urgent

Version Information

Provide version with --version / -v:

$ tool --version
tool 1.2.3

# For complex tools, show component versions
$ tool --version
tool 1.2.3
  dependency-a: 2.1.0
  dependency-b: 5.4.3

Store version in configuration:

# __version__.py or __init__.py
__version__ = "1.2.3"

# In main CLI file
import sys
from . import __version__

@click.command()
@click.option('--version', is_flag=True, help='Show version and exit')
def main(version):
    if version:
        click.echo(f"tool {__version__}")
        sys.exit(0)

Progress Indicators

For long operations, show progress:

# Python: using rich library
from rich.progress import track
import time

for i in track(range(100), description="Processing..."):
    time.sleep(0.1)  # Long operation

Output format:

Processing... [████████░░] 80%
or
Processing... ⠏ (spinning indicator)

Spinners and Progress Bars

Use spinners for indeterminate progress:

Connecting to server... ⠋
Uploading file... ⠙
Waiting for response... ⠹

Use progress bars for determinate progress:

Downloading [████████░░░░░░] 40% (2.1 MB / 5.3 MB)
Processing  [██████████████████░░] 90%

Implement responsibly:

• Disable in non-interactive environments (detect piped output)
• Respect --no-progress flag
• Never output progress to stdout (use stderr)

Color Output

Support color with disable option:

# Python: using rich or click
import click

@click.command()
@click.option('--color', type=click.Choice(['always', 'auto', 'never']),
              default='auto', help='Color output mode')
def main(color):
    # auto: color if terminal supports it
    # always: force color (for piping to less -R)
    # never: disable color
    use_color = color == 'always' or (color == 'auto' and is_terminal())

Environment variable override:

# User can disable via environment
NO_COLOR=1 tool           # Disable color
FORCE_COLOR=1 tool        # Force color

Terminal detection:

import sys
import os

def supports_color():
    # Check NO_COLOR env var
    if os.getenv('NO_COLOR'):
        return False

    # Check FORCE_COLOR env var
    if os.getenv('FORCE_COLOR'):
        return True

    # Check if stdout is a TTY
    return sys.stdout.isatty()

Interactive Prompts

Use for confirmations and user input:

# Python: using click
import click

@click.command()
@click.option('--force', is_flag=True, help='Skip confirmations')
def delete_item(force):
    if not force:
        if not click.confirm('Are you sure you want to delete?'):
            click.echo("Cancelled")
            return

    # Proceed with deletion
    click.echo("Deleted")

# Interactive selection
choice = click.prompt(
    'Choose an option',
    type=click.Choice(['a', 'b', 'c']),
    default='a'
)

Guidelines:

• Ask for confirmation before destructive operations
• Provide sensible defaults
• Allow bypassing with --force flag
• Never prompt in non-interactive contexts (detect piped input)

Output Formatting

Human-Readable Output

Default format for interactive use:

$ tool list
Name          Status    Modified
────────────────────────────────
Project A     Active    2 hours ago
Project B     Inactive  3 days ago
Project C     Active    1 week ago

Characteristics:

• Table format with clear columns
• Natural language (e.g., "2 hours ago")
• Colors for emphasis (when supported)
• Sorted intelligently

Machine-Readable Output

JSON format for scripting:

$ tool list --format json
[
  {
    "name": "Project A",
    "status": "active",
    "modified": "2024-01-15T14:30:00Z"
  },
  {
    "name": "Project B",
    "status": "inactive",
    "modified": "2024-01-12T09:15:00Z"
  }
]

YAML format (compact, readable):

$ tool list --format yaml
- name: Project A
  status: active
  modified: 2024-01-15T14:30:00Z
- name: Project B
  status: inactive
  modified: 2024-01-12T09:15:00Z

CSV format (for spreadsheets):

$ tool list --format csv
name,status,modified
"Project A",active,2024-01-15T14:30:00Z
"Project B",inactive,2024-01-12T09:15:00Z

Implementation:

import click
import json
import csv
import io

@click.command()
@click.option('--format', type=click.Choice(['table', 'json', 'yaml', 'csv']),
              default='table', help='Output format')
def list_items(format):
    items = get_items()

    if format == 'json':
        click.echo(json.dumps(items, indent=2))
    elif format == 'csv':
        output = io.StringIO()
        writer = csv.DictWriter(output, fieldnames=items[0].keys())
        writer.writeheader()
        writer.writerows(items)
        click.echo(output.getvalue())
    elif format == 'yaml':
        import yaml
        click.echo(yaml.dump(items, default_flow_style=False))
    else:  # table
        display_table(items)

Exit Codes

Standard exit codes:

0   - Success, no errors
1   - General error
2   - Misuse of shell command (invalid arguments)
3-125 - Application-specific errors
126 - Command cannot execute
127 - Command not found
128+130 - Fatal signal "N"
130 - Script terminated by Ctrl+C

Common application codes:

0   - Success
1   - General/unspecified error
2   - Misuse of command syntax
64  - Bad input data
65  - Data format error
66  - No input file
69  - Service unavailable
70  - Internal software error
71  - System error
77  - Permission denied
78  - Configuration error

Implementation:

import click
import sys

@click.command()
def main():
    try:
        result = do_work()
        if not result:
            sys.exit(1)  # Error condition
    except ValueError as e:
        click.echo(f"Error: {e}", err=True)
        sys.exit(2)  # Bad input
    except PermissionError as e:
        click.echo(f"Permission denied: {e}", err=True)
        sys.exit(77)
    except Exception as e:
        click.echo(f"Internal error: {e}", err=True)
        sys.exit(70)

    sys.exit(0)  # Success

Always exit explicitly:

# Good: script can detect success/failure
tool && echo "Success" || echo "Failed"

# Bad: unclear exit status
tool
echo "Done"  # Prints regardless of success

Error Handling

Clear Error Messages

Good error messages:

• Concise (one line if possible)
• Specific about what went wrong
• Suggest how to fix it

# Good
$ tool add --priority invalid
Error: --priority must be a number (1-10), got 'invalid'

# Bad
$ tool add --priority invalid
Error: Invalid argument

# Good
$ tool delete nonexistent
Error: Item 'nonexistent' not found. Use 'tool list' to see available items.

# Bad
$ tool delete nonexistent
File not found

Error format:

Error: [what happened]. [How to fix it or where to learn more].

Suggestions for Fixes

Provide actionable suggestions:

import click

@click.command()
@click.argument('command')
def main(command):
    valid_commands = ['add', 'list', 'delete']

    if command not in valid_commands:
        # Suggest closest match
        from difflib import get_close_matches
        suggestions = get_close_matches(command, valid_commands, n=1)

        msg = f"Unknown command '{command}'."
        if suggestions:
            msg += f" Did you mean '{suggestions[0]}'?"
        else:
            msg += f" Available commands: {', '.join(valid_commands)}"

        click.echo(f"Error: {msg}", err=True)
        raise SystemExit(1)

Debug Mode

Provide --debug flag for detailed output:

import click
import sys
import traceback

@click.command()
@click.option('--debug', is_flag=True, help='Show detailed error information')
def main(debug):
    try:
        do_work()
    except Exception as e:
        if debug:
            # Show full traceback
            traceback.print_exc(file=sys.stderr)
        else:
            # Show user-friendly message
            click.echo(f"Error: {str(e)}", err=True)
            click.echo("Use --debug to see details", err=True)
        sys.exit(1)

Example output:

$ tool --debug
Error: Connection failed
Traceback (most recent call last):
  File "tool.py", line 42, in main
    connect_to_server()
  File "tool.py", line 15, in connect_to_server
    raise ConnectionError("Timeout after 30s")
ConnectionError: Timeout after 30s

Cross-Platform Considerations

Path Separators

Always use proper path handling:

# Good: use pathlib (cross-platform)
from pathlib import Path

config_path = Path.home() / '.config' / 'app' / 'config.yaml'
output_path = Path('output') / 'result.txt'

# Also good: use os.path
import os
config_path = os.path.join(os.path.expanduser('~'), '.config', 'app', 'config.yaml')

# Avoid: hardcoded separators
config_path = '~/.config/app/config.yaml'  # Wrong on Windows

Node.js example:

const path = require('path');
const os = require('os');

const configPath = path.join(os.homedir(), '.config', 'app', 'config.yaml');
const outputPath = path.join('output', 'result.txt');

Line Endings

Normalize line endings:

# Python: open files with universal newline support (default)
with open('file.txt', 'r') as f:  # Automatically handles \r\n, \n, \r
    content = f.read()

# When writing, use default newline handling
with open('file.txt', 'w') as f:
    f.write(content)  # Uses platform default newline

Git configuration:

# Set in .gitattributes to normalize line endings
* text=auto
*.py text eol=lf
*.json text eol=lf

Terminal Capabilities

Detect terminal capabilities:

import sys
import os

def get_terminal_width():
    """Get terminal width, default to 80."""
    try:
        return os.get_terminal_size().columns
    except (AttributeError, ValueError):
        return 80

def supports_unicode():
    """Check if terminal supports Unicode."""
    encoding = sys.stdout.encoding or 'utf-8'
    return encoding.lower() in ('utf-8', 'utf8')

def supports_color():
    """Check if terminal supports colors."""
    # Already covered above
    return True

# Use capabilities to adjust output
if supports_unicode():
    symbol = '✓'  # Check mark
else:
    symbol = '✔'  # Alternative

Avoid assumptions:

# Bad: assumes capabilities
output = "✓ Success\n"

# Good: checks capabilities
symbol = '✓' if supports_unicode() else '✔'
output = f"{symbol} Success\n"

Testing CLI Applications

Integration Tests

Test the complete CLI, not just functions:

import subprocess
import json

def test_list_command():
    """Test list command output."""
    result = subprocess.run(
        ['tool', 'list', '--format', 'json'],
        capture_output=True,
        text=True
    )

    assert result.returncode == 0
    output = json.loads(result.stdout)
    assert isinstance(output, list)

def test_list_command_human_readable():
    """Test list command with default format."""
    result = subprocess.run(
        ['tool', 'list'],
        capture_output=True,
        text=True
    )

    assert result.returncode == 0
    assert 'Name' in result.stdout  # Table header

Test with Click (Python):

from click.testing import CliRunner
from tool.cli import main

def test_list_command():
    runner = CliRunner()
    result = runner.invoke(main, ['list', '--format', 'json'])

    assert result.exit_code == 0
    output = json.loads(result.output)
    assert isinstance(output, list)

Output Verification

Verify output format and content:

import subprocess

def test_add_command_success():
    """Test successful add operation."""
    result = subprocess.run(
        ['tool', 'add', 'Test Item', '--priority', '5'],
        capture_output=True,
        text=True
    )

    assert result.returncode == 0
    assert 'Added' in result.stdout or 'Success' in result.stdout

def test_add_command_invalid_priority():
    """Test validation of priority argument."""
    result = subprocess.run(
        ['tool', 'add', 'Test Item', '--priority', 'invalid'],
        capture_output=True,
        text=True
    )

    assert result.returncode != 0
    assert 'Error' in result.stderr
    assert 'priority' in result.stderr.lower()

Exit Code Checks

Verify proper exit codes:

import subprocess

def test_help_flag_exit_code():
    """Test --help returns success."""
    result = subprocess.run(['tool', '--help'], capture_output=True)
    assert result.returncode == 0

def test_invalid_command_exit_code():
    """Test invalid command returns error code."""
    result = subprocess.run(['tool', 'invalid'], capture_output=True)
    assert result.returncode != 0

def test_missing_required_arg():
    """Test missing required argument returns error."""
    result = subprocess.run(['tool', 'delete'], capture_output=True)
    assert result.returncode == 2  # Misuse of command syntax

Documentation

README

CLI tools need clear README documentation:

# Tool Name

Brief description of what the tool does.

## Installation

Installation instructions (package manager, build from source, etc.)

## Usage

### Basic Usage

```bash
tool [COMMAND] [OPTIONS] [ARGUMENTS]

Examples

# List all items
tool list

# Add new item
tool add "My Item" --priority 5

# Delete with confirmation
tool delete "My Item"

# Suppress confirmation
tool delete "My Item" --force

Commands

• add [NAME] - Add a new item
• list - List all items
• delete [NAME] - Delete an item
• config - Manage configuration

Options

• -v, --verbose - Increase output verbosity
• --format [json|yaml|csv] - Output format
• --debug - Show debug information
• -h, --help - Show help message
• --version - Show version

Configuration

Settings can be configured via:

1. Command-line flags (highest priority)
2. Environment variables
3. Config file at ~/.config/tool/config.yaml
4. Built-in defaults

Environment Variables

• DEBUG - Enable debug mode
• TOOL_CONFIG - Path to config file
• NO_COLOR - Disable colored output

Configuration File

Create ~/.config/tool/config.yaml:

debug: false
format: table
color: true

Troubleshooting

Common Issues

Issue: "command not found"

• Ensure tool is installed and in PATH
• Check: which tool

Issue: Permission denied

• Make script executable: chmod +x tool

Development

Build and test instructions.

### Man Pages

**Create man pages for Unix systems:**

TOOL(1) User Commands TOOL(1)

NAME tool - A tool that does one thing well

SYNOPSIS tool [OPTIONS] COMMAND [ARGS]...

DESCRIPTION Detailed description of what the tool does.

COMMANDS add NAME Add a new item list List all items delete NAME Delete an item

OPTIONS -v, --verbose Increase output verbosity --format FMT Output format (json, yaml, csv) -h, --help Show help message --version Show version

EXAMPLES Add a new item: tool add "My Item"

List items as JSON:
    tool list --format json

EXIT STATUS 0 Success 1 General error 2 Invalid arguments

FILES ~/.config/tool/config.yaml Configuration file

SEE ALSO tool-add(1), tool-delete(1)

### Built-in Help

**Make help accessible and comprehensive:**

```python
@click.group()
def main():
    """Main CLI tool."""
    pass

@main.command()
def help():
    """Show detailed help information."""
    click.echo(click.get_current_context().get_help())

Common CLI Libraries

Python

Click - Most popular, decorator-based

import click

@click.command()
@click.option('--name', prompt='Your name', help='Name of person')
@click.option('--count', default=1, help='Number of greetings')
def hello(name, count):
    """Simple program that 

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