Assign a random number (UUID) value to a environment variable in npm script

I'd like to add a UUID argument when calling my npm script. Each time the script is called, I'd like to generate a new number. It should look like:

"build": "cross-env UUID=unique_number ng build"

The only thing I need is generating the unique_number here. I tried to use the uuid package but I don't know how to fire it in the script and pass the number as the argument.

Solution

tl;dr As you're question shows the use of cross-var I've assumed a cross-platform solution is required. In which case refer to the Solution A. However refer to either Solution B or C if my assumption is incorrect.

Solution A: Cross Platform (Windows/Linux/macOS...)

Fo a cross platform solution, (i.e. one that runs successfully on Windows, Linux, and macOS...), you'll need to utilize nodejs to achieve your requirement. There are a couple of different ways to approach this as described in the following two sub-sections titled:

Using an external nodejs (.js) file
Inlining your JavaScript in package.json.

_{Note both approaches are effectively the same}

Using an external nodejs (.js) file

Create a nodejs utility script. Let's name the file run-ng-build.js and save it in the root of your project directory, i.e. in the same directory where package.json currently resides:

run-ng-build.js
```
const uuid = require('uuid/v1');
const execSync = require('child_process').execSync;

process.env.UUID = uuid();

execSync('ng build', { stdio: [0, 1, 2]} );
```
In the scripts section of your package.json replace your current build script with the following:

package.json
```
"scripts": {
  "build": "node run-ng-build"
}
```

Explanation:

In run-ng-build.js we require the uuid package and the nodejs built-in execSync().
To create the environment variable named UUID we utilize the nodejs builtin process.env, and assign a uuid value to it by invoking uuid().
We then invoke the ng build command using execSync.
The options.stdio option configures the pipes between the parent and child process - [0, 1, 2] effectively inherit's stdin, stdout, and stderr.

Inlining your JavaScript in package.json.

Alternatively, you can inline your nodejs/JavaScript code in the scripts section of your package.json.

In the scripts section of your package.json replace your current build script with the following instead:

package.json

"scripts": {
  "build": "node -e \"process.env.UUID = require('uuid/v1')(); require('child_process').execSync('ng build', { stdio: [0, 1, 2]} );\""
}

Explanation:

This is effectively the same as the aforementioned solution that utilized a separate .js file, however the use of a separate nodejs script/file is now redundant.
The nodejs command line option -e is utilized to evaluate the inline JavaScript.

Important The cross-env package is redundant utilizing either of the two aforementioned solutions. To uninstall it run: npm un -D cross-env via your CLI.

Solution B: *Nix platforms only (Linux/MacOS...)

For *nix platforms only it becomes very terse, you can just define your build script in package.json as follows:

package.json

"scripts": {
  "build": "cross-env UUID=$(uuid) ng build"
}

This utilizes a Bash feature known as command substitution, i.e. $(uuid). However, if *nix is the only platform you need to support, then cross-env is really not necessary. Use the built-in export feature instead. For instance:

package.json

"scripts": {
  "build": "export UUID=$(uuid) && ng build"
}

Solution C: Windows only (cmd.exe)

On Windows (only) running via Command Prompt or PowerShell you can do the following:

package.json

"scripts": {
  "build": "FOR /F %U IN ('uuid') DO cross-env UUID=%~U node -e \"process.env.UUID = require('uuid/v1')(); require('child_process').execSync('ng buuld', { stdio: [0, 1, 2] });\""
}

This is similar to the first example shown in Solution B however command substitution is achieved (very) differently in cmd.exe. See this answer for further explanation.