Robust Commands

Robust Command Execution Skill

This skill provides patterns for executing commands with automatic error recovery, fallback mechanisms, and installation of missing dependencies. Never give up when a command fails - try alternatives!

When to Use This Skill

• When a command might not be installed
• When working in different environments (containers, CI, local)
• When tool availability is uncertain
• When debugging command failures
• When you need to ensure commands succeed

Core Principle

If a command doesn't work, don't just report failure - fix it!

Process for handling command failures:

1. Check if the command exists
2. Try to install if missing
3. Try alternative commands
4. Verify prerequisites (permissions, paths)
5. Only report as blocked if all options exhausted

Command Existence Checking

Check if Command Exists

# Method 1: which
which command_name

# Method 2: command -v (POSIX-compliant, more reliable)
command -v command_name

# Method 3: type
type command_name

# Best practice - silent check
if command -v jq &> /dev/null; then
  echo "jq is available"
else
  echo "jq is not installed"
fi

Check Multiple Alternative Commands

# Find first available command from list
for cmd in jq python3 python; do
  if command -v "$cmd" &> /dev/null; then
    echo "Using $cmd"
    break
  fi
done

Installing Missing Commands

Debian/Ubuntu Systems

# Update package lists first
apt-get update

# Install single package
apt-get install -y package-name

# Search for package
apt-cache search keyword

# Install with error handling
if ! command -v jq &> /dev/null; then
  echo "Installing jq..."
  apt-get update && apt-get install -y jq
fi

Common Package Names

# JSON processing
apt-get install -y jq

# YAML processing
apt-get install -y yq

# HTTP requests
apt-get install -y curl wget

# Text processing
apt-get install -y sed gawk grep

# Build tools
apt-get install -y build-essential

# Python tools
apt-get install -y python3 python3-pip

# Node.js tools
apt-get install -y nodejs npm

Python Packages

# Install with pip
pip install package-name

# Install specific version
pip install package-name==1.2.3

# Install from requirements
pip install -r requirements.txt

# Common Python tools
pip install pyyaml requests black pylint

Command Fallback Patterns

JSON Processing

# Try jq first, fallback to python
if command -v jq &> /dev/null; then
  jq . file.json
else
  python3 -m json.tool < file.json
fi

# Or one-liner with ||
jq . file.json 2>/dev/null || python3 -m json.tool < file.json

YAML Processing

# Try yq, fallback to python
if command -v yq &> /dev/null; then
  yq eval file.yaml
else
  python3 -c "import yaml, sys; print(yaml.safe_load(open('file.yaml')))"
fi

HTTP Requests

# Try curl, fallback to wget
if command -v curl &> /dev/null; then
  curl -s https://example.com
else
  wget -q -O - https://example.com
fi

# Or with Python as final fallback
curl -s https://example.com 2>/dev/null || \
wget -q -O - https://example.com 2>/dev/null || \
python3 -c "import urllib.request; print(urllib.request.urlopen('https://example.com').read().decode())"

Text Processing

# Line counting alternatives
wc -l file.txt 2>/dev/null || \
cat file.txt | wc -l 2>/dev/null || \
awk 'END {print NR}' file.txt

# Grep alternatives
grep "pattern" file.txt 2>/dev/null || \
awk '/pattern/' file.txt 2>/dev/null || \
python3 -c "import sys; [print(line, end='') for line in open('file.txt') if 'pattern' in line]"

Permission Handling

Check File Permissions

# Check if file is readable
if [ -r file.txt ]; then
  cat file.txt
else
  echo "File not readable"
fi

# Check if file is writable
if [ -w file.txt ]; then
  echo "data" > file.txt
else
  echo "File not writable"
fi

# Check if file is executable
if [ -x script.sh ]; then
  ./script.sh
else
  echo "File not executable"
fi

Fix Permissions

# Make file readable
chmod +r file.txt

# Make file writable
chmod +w file.txt

# Make file executable
chmod +x script.sh

# Common permission patterns
chmod 644 file.txt    # rw-r--r--
chmod 755 script.sh   # rwxr-xr-x
chmod 600 secret.key  # rw-------

Handle Permission Denied

# Try normal command, fall back to sudo if needed
cat /etc/secret 2>/dev/null || sudo cat /etc/secret

# Check if sudo is available
if command -v sudo &> /dev/null && sudo -n true 2>/dev/null; then
  sudo command
else
  echo "Cannot elevate privileges"
fi

Path Verification

Check if File/Directory Exists

# Check file exists
if [ -f file.txt ]; then
  echo "File exists"
fi

# Check directory exists
if [ -d /path/to/dir ]; then
  echo "Directory exists"
fi

# Check path exists (file or directory)
if [ -e /path/to/something ]; then
  echo "Path exists"
fi

Create Missing Paths

# Create directory if missing
mkdir -p /path/to/dir

# Create file if missing
touch file.txt

# Create with error handling
if [ ! -d /path/to/dir ]; then
  mkdir -p /path/to/dir || {
    echo "Failed to create directory"
    exit 1
  }
fi

Complete Robust Command Pattern

Template for Any Command

#!/bin/bash

# Function to run command with fallbacks
run_robust() {
  local cmd=$1
  shift
  local args="$@"

  # Check if command exists
  if ! command -v "$cmd" &> /dev/null; then
    echo "Command '$cmd' not found. Attempting to install..."

    # Try to install
    case "$cmd" in
      jq)
        apt-get update && apt-get install -y jq
        ;;
      yq)
        apt-get update && apt-get install -y yq
        ;;
      curl)
        apt-get update && apt-get install -y curl
        ;;
      *)
        echo "Don't know how to install '$cmd'"
        return 1
        ;;
    esac

    # Check again after install
    if ! command -v "$cmd" &> /dev/null; then
      echo "Failed to install '$cmd'"
      return 1
    fi
  fi

  # Run the command
  "$cmd" $args
}

# Usage
run_robust jq . file.json

Error Recovery Strategies

Retry with Backoff

# Retry command up to N times
retry_command() {
  local max_attempts=3
  local timeout=2
  local attempt=1

  while [ $attempt -le $max_attempts ]; do
    echo "Attempt $attempt of $max_attempts..."

    if "$@"; then
      return 0
    fi

    echo "Command failed. Retrying in ${timeout}s..."
    sleep $timeout
    timeout=$((timeout * 2))
    attempt=$((attempt + 1))
  done

  echo "Command failed after $max_attempts attempts"
  return 1
}

# Usage
retry_command curl -f https://api.example.com

Timeout Protection

# Run command with timeout
timeout 30s long_running_command || {
  echo "Command timed out after 30 seconds"
}

# Or with fallback
timeout 10s risky_command || echo "Command failed or timed out"

Capture and Handle Errors

# Capture both stdout and stderr
output=$(command 2>&1) || {
  echo "Command failed with output:"
  echo "$output"
  # Try alternative
  alternative_command
}

# Check exit code explicitly
command
exit_code=$?
if [ $exit_code -ne 0 ]; then
  echo "Command failed with exit code $exit_code"
  # Handle error
fi

Environment Detection

Detect Operating System

# Detect OS
if [ -f /etc/os-release ]; then
  . /etc/os-release
  echo "OS: $NAME"
  echo "Version: $VERSION"
fi

# Or use uname
case "$(uname -s)" in
  Linux*)
    echo "Linux system"
    ;;
  Darwin*)
    echo "macOS system"
    ;;
  *)
    echo "Unknown system"
    ;;
esac

Detect Package Manager

# Find available package manager
if command -v apt-get &> /dev/null; then
  PKG_MANAGER="apt-get"
elif command -v yum &> /dev/null; then
  PKG_MANAGER="yum"
elif command -v brew &> /dev/null; then
  PKG_MANAGER="brew"
else
  echo "No known package manager found"
fi

Detect Container/CI Environment

# Check if running in Docker
if [ -f /.dockerenv ]; then
  echo "Running in Docker"
fi

# Check if running in GitHub Actions
if [ -n "$GITHUB_ACTIONS" ]; then
  echo "Running in GitHub Actions"
fi

# Check if running in GitLab CI
if [ -n "$GITLAB_CI" ]; then
  echo "Running in GitLab CI"
fi

Common Command Alternatives

File Operations

# Read file
cat file.txt || head -n 99999 file.txt || python3 -c "print(open('file.txt').read())"

# Write file
echo "data" > file.txt || python3 -c "open('file.txt', 'w').write('data')"

# Append to file
echo "data" >> file.txt || python3 -c "open('file.txt', 'a').write('data\n')"

Network Operations

# Download file
curl -O url || wget url || python3 -c "import urllib.request; urllib.request.urlretrieve('url', 'file')"

# Check if URL is reachable
curl -I url || wget --spider url || python3 -c "import urllib.request; urllib.request.urlopen('url')"

Archive Operations

# Extract tar.gz
tar -xzf file.tar.gz || python3 -m tarfile -e file.tar.gz

# Create tar.gz
tar -czf archive.tar.gz files/ || python3 -m tarfile -c archive.tar.gz files/

# Extract zip
unzip file.zip || python3 -m zipfile -e file.zip .

Best Practices

1. Always check first: Use command -v before executing
2. Silent checks: Redirect stderr to /dev/null for checks
3. Prefer POSIX: Use command -v over which
4. Install when missing: Don't ask user if you can install
5. Try alternatives: Have fallbacks for common commands
6. Handle permissions: Check and fix permissions as needed
7. Verify paths: Ensure files/directories exist before using
8. Graceful degradation: Try best option first, fall back progressively
9. Clear feedback: Tell user what you're trying and why
10. Document workarounds: Note unusual solutions for future reference

Remember

• Never give up: If one approach fails, try another
• Be resourceful: Many tools can accomplish the same task
• Think creatively: Python/shell scripts can replace missing tools
• Install proactively: Don't wait for failures to install tools
• Test assumptions: Verify command exists before using it
• Handle errors: Always have a fallback plan

Commands fail for many reasons - most are fixable!

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