env

Overview

The env command displays environment variables or runs a program in a modified environment. It’s essential for managing environment variables and running commands with specific environmental settings.

Syntax

env [options] [name=value...] [command [args...]]

Common Options

Option Description
-i Start with empty environment
-u name Remove variable from environment
-0 End output lines with null character
--help Display help
--version Display version

Key Use Cases

  1. Display environment variables
  2. Run programs with modified environment
  3. Script environment management
  4. Debugging environment issues
  5. Portable script execution

Examples with Explanations

Example 1: Display All Variables

env

Shows all environment variables

Example 2: Run with Clean Environment

env -i /bin/bash

Starts bash with empty environment

Example 3: Set Variable for Command

env PATH=/usr/bin:/bin ls

Runs ls with modified PATH

Example 4: Remove Variable

env -u HOME pwd

Runs pwd without HOME variable

Common Usage Patterns

  1. Check specific variable:

    env | grep PATH

  2. Run with additional variable:

    env EDITOR=vim crontab -e

  3. Clean environment execution:

    env -i PATH=/usr/bin:/bin command

Environment Variables

Common variables: - PATH: Executable search path - HOME: User home directory - USER: Current username - SHELL: Default shell - LANG: Locale setting - PWD: Current directory - EDITOR: Default editor

Performance Analysis

  • Very fast operation
  • No file system access
  • Minimal memory usage
  • Good for environment debugging
  • Efficient for script execution

Best Practices

  1. Use for portable scripts
  2. Clean environment for security
  3. Document required variables
  4. Validate environment in scripts
  5. Use specific paths when needed

Scripting Applications

  1. Portable shebang:

    #!/usr/bin/env bash

  2. Environment validation:

    if ! env | grep -q "REQUIRED_VAR"; then
    echo "Missing required environment variable"
    exit 1
fi

  3. Clean execution:

    env -i PATH=/usr/bin:/bin HOME=/tmp command

Security Considerations

  1. Clean environment for security
  2. Validate environment variables
  3. Avoid exposing sensitive data
  4. Use minimal required environment
  5. Check for environment injection

Integration Examples

  1. With cron jobs:

    0 2 * * * env PATH=/usr/local/bin:/usr/bin:/bin backup.sh

  2. Service execution:

    env -i PATH=/usr/bin USER=service /usr/local/bin/service

  3. Development environment:

    env NODE_ENV=development npm start

Troubleshooting

  1. Missing environment variables
  2. Path resolution issues
  3. Locale problems
  4. Permission errors
  5. Variable inheritance issues