Support using stdout

[mzhubail]

📑 Summary

Support writing to stdout if the user explicitly specifies -. In case of no output format, it is assumed to be svg and a warning is displayed.

Resolves #683

📏 Design Decisions

For the sake of making the code more readable, I'm using explicit '/dev/stdout'. This is just a personal preference and doesn't have anything to do with the platform.

I have also disabled writing to stdout if the input is markdown.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added unit/e2e tests (if appropriate)
• 🔖 targeted master branch

[aloisklink]

Thanks for implementing this. I've got one minor comment that could potentially slightly simplify the code, but other than this, it looks great to me!

I'm not 100% sure, since it's been a while since I last programmed for Windows, but I think this should even work on Windows too!

[mzhubail]

Forgot to mention that this feature is blocked by mermaid-js/zenuml-core#169, because the warning is written to stdout not stderr.

[mzhubail]

Now that I think about it, maybe we shouldn't check the directory if we're writing to stdout.

Suggested change

	const outputDir = path.dirname(output)
	if (!fs.existsSync(outputDir)) {
	error(`Output directory "${outputDir}/" doesn't exist`)
	if (output !== '/dev/stdout') {
	const outputDir = path.dirname(output)
	if (!fs.existsSync(outputDir)) {
	error(`Output directory "${outputDir}/" doesn't exist`)

[aloisklink]

That's a good idea.

Non-Linux operating systems probably won't have a /dev folder.

If you want to avoid having an extra level of indentation, we could also change the code to the following:

Suggested change

	const outputDir = path.dirname(output)
	if (!fs.existsSync(outputDir)) {
	error(`Output directory "${outputDir}/" doesn't exist`)
	const outputDir = path.dirname(output)
	if (output !== '/dev/stdout' && !fs.existsSync(outputDir)) {
	error(`Output directory "${outputDir}/" doesn't exist`)

[mzhubail]

Noted.

[aloisklink]

Forgot to mention that this feature is blocked by mermaid-js/zenuml-core#169, because the warning is written to stdout not stderr.

Unfortunately, that's a pretty major issue! As a quick fix, I think we could instead write this message to stderr instead, by changing the following to use console.warn instead:

mermaid-cli/src/index.js

Lines 243 to 245 in 967768c

	page.on('console', (msg) => {
	console.log(msg.text())
	})

I've also noticed that the recently released Mermaid v11 seems to be logging a lot more information, so it's probably worth having that change anyway!

[aloisklink]

Awesome! Thanks for making those changes!

It should be released soon with mermaid-cli v11 once #701 is reviewed and merged!

[mzhubail]

Thanks!