Cross-Platform npm Scripts: Why Your Windows Commands Fail on macOS/Linux (and How to Fix Them)
As developers, we often work in teams with mixed operating systems—some on Windows, others on macOS or Linux. When you're building a React application or any Node.js project, you'll inevitably encounter a frustrating problem: npm scripts that work perfectly on your Windows machine suddenly break when your teammate on macOS tries to run them. The culprit? Platform-specific environment variable syntax.
While working on PixelPerfect Screenshot API, I encountered this exact issue. My npm scripts used Windows-style set VARIABLE=value && commands, which worked flawlessly in my development environment. However, this approach is fundamentally incompatible with Unix-based systems (macOS and Linux), where the set command doesn't exist in the same form.
This article explores why this happens, how to fix it using cross-env, and best practices for writing truly cross-platform npm scripts that work seamlessly across all operating systems.
Windows Command Prompt understands the
set command for environment variables.
Unix shells don't recognize
set as a variable assignment command. They expect export VAR=value.
The Core Issue: Different operating systems use different shell syntaxes for setting environment variables. Windows uses set VAR=value, while Unix systems use VAR=value or export VAR=value.
Understanding the Problem: Shell Syntax Differences
The root cause of this incompatibility lies in how different shells handle environment variables:
Windows Command Prompt / PowerShell
On Windows, the traditional way to set environment variables before running a command is using the set keyword:
set VARIABLE=value && npm startThis syntax tells Windows Command Prompt to create a temporary environment variable that exists only for the duration of that command execution. Once the command completes, the variable disappears.
Unix Shells (Bash, Zsh, etc.)
On macOS and Linux systems, which typically use Bash or Zsh shells, environment variables are set differently:
VARIABLE=value npm start
# or
export VARIABLE=value && npm startWhen you try to run set VARIABLE=value on a Unix system, the shell interprets set as a command (which doesn't exist in this context), resulting in a "command not found" error.
Real-World Impact: In a typical development team, this means that a developer on Windows might commit perfectly functional npm scripts to the repository, only to have macOS and Linux developers unable to run the project. This breaks the fundamental principle of write-once, run-anywhere code.
The Solution: cross-env
The cross-env package solves this problem elegantly by providing a cross-platform way to set environment variables. It's a tiny npm package that acts as a universal translator between different shell syntaxes.
How cross-env Works
Instead of using platform-specific syntax, you prefix your environment variables with cross-env:
cross-env VARIABLE=value npm startcross-env detects the current operating system and shell, then translates your variable assignment into the appropriate syntax for that platform. On Windows, it uses set. On Unix systems, it uses the standard variable assignment. The result: one command that works everywhere.
VAR=value
npm start
&
Translates Syntax
macOS/Linux: VAR=value
Key Benefit: You write the syntax once, and cross-env handles the platform-specific translation automatically. No conditional logic needed, no separate scripts for different operating systems.
Installation and Setup
Installing cross-env is straightforward. Since it's a development dependency that only affects your npm scripts, you should install it as a devDependency:
npm install --save-dev cross-envOnce installed, you can use it in any npm script in your package.json file.
Real-World Example: PixelPerfect Screenshot API
Let me share the exact transformation I made in the PixelPerfect Screenshot API project. The issue became apparent when I needed to set multiple environment variables for different deployment scenarios—local development, LAN testing with mobile devices, and production builds.
The Problem: Windows-Only Scripts
Here's what my original package.json scripts looked like:
{
"scripts": {
"start": "set GENERATE_SOURCEMAP=false && react-scripts start",
"build": "set GENERATE_SOURCEMAP=false && set ESLINT_NO_DEV_ERRORS=true && react-scripts build"
}
}These scripts worked perfectly on Windows, but they had critical flaws:
- The
setcommand would fail immediately on macOS and Linux - Any developer not using Windows couldn't run the development server
- CI/CD pipelines running on Linux containers would fail
- The team couldn't collaborate effectively across platforms
Warning: This type of platform-specific code is particularly dangerous because it often goes unnoticed during development. If your entire team uses Windows, you might never discover the issue until you try to deploy or onboard a developer using a different OS.
• Uses Windows-only
set command• Fails on macOS and Linux
• Breaks team collaboration
• CI/CD incompatible
• Works on Windows, macOS, Linux
• Clean, readable syntax
• Team-friendly
• CI/CD compatible
The Solution: Cross-Platform Scripts
After implementing cross-env, here's the transformed scripts section:
{
"scripts": {
"start": "cross-env GENERATE_SOURCEMAP=false react-scripts start",
"start:lan": "cross-env GENERATE_SOURCEMAP=false REACT_APP_API_URL=http://192.168.1.185:8000 REACT_APP_API_BASE_URL=http://192.168.1.185:8000 react-scripts start",
"build": "cross-env GENERATE_SOURCEMAP=false ESLINT_NO_DEV_ERRORS=true react-scripts build"
}
}Notice the key improvements:
- No platform-specific commands: The
setkeyword is completely removed - Cleaner syntax: Variables are defined inline without
&&separators - Multiple variables: You can define multiple environment variables in a single
cross-envcall - Universal compatibility: Works identically on Windows, macOS, and Linux
Understanding Each Script
Let's break down what each script does and why it's configured this way:
1. Development Server (start)
"start": "cross-env GENERATE_SOURCEMAP=false react-scripts start"This script starts the React development server with source map generation disabled. Source maps are useful for debugging, but they can significantly slow down compilation in development. For a production-focused workflow, disabling them speeds up the development experience.
2. LAN Testing Server (start:lan)
"start:lan": "cross-env GENERATE_SOURCEMAP=false REACT_APP_API_URL=http://192.168.1.185:8000 REACT_APP_API_BASE_URL=http://192.168.1.185:8000 react-scripts start"This script is particularly valuable for testing your React application on actual mobile devices. By setting the API URLs to your computer's LAN IP address (192.168.1.185 in this example), mobile devices on the same network can connect to your development server and backend API.
Use Case: When developing PixelPerfect Screenshot API, I needed to test the screenshot functionality on various mobile browsers (Safari on iOS, Chrome on Android). The start:lan script allowed my phone to access http://192.168.1.185:3000 and interact with the backend running on http://192.168.1.185:8000, all while maintaining hot-reload capabilities.
3. Production Build (build)
"build": "cross-env GENERATE_SOURCEMAP=false ESLINT_NO_DEV_ERRORS=true react-scripts build"The production build script disables source maps (to keep the bundle size small and avoid exposing source code) and treats ESLint warnings as non-blocking. This is crucial for production deployments where you want the build to succeed even if there are minor linting warnings.
Only cross-env provides true cross-platform compatibility across all three major operating systems.
Additional Benefits of cross-env
Beyond solving the platform compatibility issue, cross-env offers several additional advantages:
1. Simplified Syntax
Notice how you don't need && between each variable assignment when using cross-env. You can define multiple variables in a single command:
cross-env VAR1=value1 VAR2=value2 VAR3=value3 npm startThis is cleaner and more readable than chaining multiple set or export commands.
2. No Escaping Issues
When dealing with values that contain special characters (like URLs with query parameters), cross-env handles escaping consistently across platforms. You don't need to worry about whether to use quotes, double quotes, or escape characters differently on different systems.
3. CI/CD Compatibility
Most CI/CD systems (GitHub Actions, GitLab CI, CircleCI, Jenkins) run on Linux containers. Using cross-env ensures your npm scripts work seamlessly in these environments without requiring separate CI-specific script variations.
Production Benefit: When deploying PixelPerfect Screenshot API to Render.com (which uses Linux containers), the cross-env-based build script worked immediately without any modifications. This eliminated an entire class of deployment issues.
Best Practices for Cross-Platform npm Scripts
Common Pitfalls and How to Avoid Them
1. Forgetting to Install cross-env in Production
If cross-env is only in your devDependencies and you run npm install --production on your server, your scripts will fail. For most use cases, this is fine since you typically run build scripts during development or in a CI pipeline. However, if you need to run npm scripts in production, move cross-env to dependencies.
2. Mixing Platform-Specific and Cross-Platform Syntax
Don't mix approaches in the same project. If you use cross-env for some scripts and set for others, you'll still have compatibility issues. Be consistent.
3. Not Updating LAN IP Addresses
If you're using the start:lan pattern, remember that your computer's LAN IP address can change, especially on DHCP networks. You'll need to update the script when your IP changes. Consider using environment variables or a configuration file to make this more maintainable:
// In a config file
const API_BASE_URL = process.env.REACT_APP_API_BASE_URL || 'http://localhost:8000';
// Then in your script:
"start:lan": "cross-env REACT_APP_API_BASE_URL=http://$(ipconfig getifaddr en0):8000 react-scripts start"
Beyond cross-env: Other Cross-Platform Tools
While cross-env solves environment variable issues, you might encounter other cross-platform challenges in npm scripts:
- rimraf: Cross-platform file deletion (replaces
rm -rfon Unix anddel /s /qon Windows) - cross-var: Cross-platform environment variable expansion
- npm-run-all: Run multiple npm scripts in parallel or sequence, cross-platform
- copyfiles: Cross-platform file copying
For PixelPerfect, I also use rimraf in the cleanup scripts:
"clean": "rimraf build coverage"This works identically on all platforms, whereas rm -rf build coverage would fail on Windows.
Conclusion
Cross-platform compatibility in npm scripts is not just a nice-to-have—it's essential for modern JavaScript development. Teams work across different operating systems, CI/CD pipelines run on Linux, and production environments may differ from development machines.
The cross-env package provides an elegant, zero-configuration solution to the environment variable problem. By adopting it early and consistently, you eliminate an entire class of platform-specific bugs and ensure that your project can be developed, built, and deployed on any system.
The transformation I made in PixelPerfect Screenshot API—from Windows-only scripts to cross-platform compatibility—took less than five minutes but eliminated potential hours of debugging for future contributors. It's a small investment that pays dividends in team productivity and deployment reliability.
Key Takeaway: Make cross-env a standard part of your development toolchain. Install it in every new project, use it for all environment variable assignments in npm scripts, and enjoy the peace of mind that comes from true cross-platform compatibility.
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