[rush] Rush should support the ${NAME:-fallback} syntax in .npmrc

[fzxen]

Summary

I want to set the .npmrc based on environment variables, so I added the following content to the .npmrc:

store-dir=${PNPM_STORE_DIR:-~/.pnpm/store}

This configuration means that when the environment variable PNPM_SOTRE_DIR exists, the store-dir takes its value. If it does not exist, then it takes ~/.pnpm/store

Pnpm supports this feature

Values in the .npmrc files may contain env variables using the ${NAME} syntax. The env variables may also be specified with default values. Using ${NAME-fallback} will return fallback if NAME isn't set. ${NAME:-fallback} will return fallback if NAME isn't set, or is an empty string. —— https://pnpm.io/npmrc

However, After executing "rush install", I found that rush modified the content of the npmrc file. The line "store-dir" was added with the prefix ; MISSING ENVIRONMENT VARIABLE. This causes this configuration to be invalid.

; MISSING ENVIRONMENT VARIABLE: store-dir=${PNPM_STORE_DIR:-~/.pnpm/store}

Repro steps

Add store-dir=${PNPM_STORE_DIR:-~/.pnpm/store} to the common/config/rush/.npmrc file.
Then, execute rush install command.

Expected result:

The content of common/temp/.npmrc displays store-dir=${PNPM_STORE_DIR:-~/.pnpm/store}

Actual result:

The content of common/temp/.npmrc displays ; MISSING ENVIRONMENT VARIABLE: store-dir=${PNPM_STORE_DIR:-~/.pnpm/store}

Details

I located the logical code causing the problem. This code seems to have not considered the fallback syntax.
Actually, I don't understand why the prefix MISSING ENVIRONMENT VARIABLE is added here.

rushstack/libraries/rush-lib/src/utilities/npmrcUtilities.ts

Lines 55 to 89 in ca5a3a9

	for (let line of npmrcFileLines) {
	let lineShouldBeTrimmed: boolean = false;
	//remove spaces before or after key and value
	line = line
	.split('=')
	.map((lineToTrim) => lineToTrim.trim())
	.join('=');
	// Ignore comment lines
	if (!commentRegExp.test(line)) {
	const environmentVariables: string[] | null = line.match(expansionRegExp);
	if (environmentVariables) {
	for (const token of environmentVariables) {
	// Remove the leading "${" and the trailing "}" from the token
	const environmentVariableName: string = token.substring(2, token.length - 1);
	// Is the environment variable defined?
	if (!process.env[environmentVariableName]) {
	// No, so trim this line
	lineShouldBeTrimmed = true;
	break;
	}
	}
	}
	}
	if (lineShouldBeTrimmed) {
	// Example output:
	// "; MISSING ENVIRONMENT VARIABLE: //my-registry.com/npm/:_authToken=${MY_AUTH_TOKEN}"
	resultLines.push('; MISSING ENVIRONMENT VARIABLE: ' + line);
	} else {
	resultLines.push(line);
	}
	}

Standard questions

Please answer these questions to help us investigate your issue more quickly:

Question	Answer
@microsoft/rush globally installed version?	5.142.0
rushVersion from rush.json?	5.142.0
Operating system?	Mac
Would you consider contributing a PR?	Yes
Node.js version (node -v)?	20.16.0

[iclanton]

I suspect the fallback feature didn't exist when this was first implemented.

There is(/used to be) a behavior where a missing env variable that a .npmrc depends on would cause an error, so Rush automatically just filters those lines out. Supporting the fallback syntax with a regexp would be pretty easy.