Question

193

How to pipe Node.js scripts together using the Unix | pipe (on the command line)?

rated 0 times [ 198] [ 5] / answers: 1 / hits: 26371 / 11 Years ago, fri, may 3, 2013, 12:00:00

I see how to pipe stuff together using Node.js streams, but how do you pipe multiple scripts together using the Unix |, given that some of these scripts can be async?

$ ./a.js | ./b.js

Example:

a.js (chmod 0755)

#!/usr/bin/env node



setTimeout(function(){

  console.log(JSON.stringify({ foo: 'bar' }));

}, 10);

b.js (chmod 0755)

#!/usr/bin/env node



console.log(process.argv);

This is the output:

$ ./a.js | ./b.js

[ 'node', '/Users/viatropos/tests/b.js' ]



events.js:72

        throw er; // Unhandled 'error' event

              ^

Error: write EPIPE

    at errnoException (net.js:883:11)

    at Object.afterWrite (net.js:700:19)

At first glance it seems like there's a lot going wrong, so not really sure where to start. Is there a way to get this to work? The end goal is to be able to take the console.log output from ./a.js and use it in ./b.js. The reason is, most of the time these scripts will be run one at a time, but sometimes it would be nice to be able to pipe them together, so ideally the system should be able to handle both cases.

Answers

Only authorized users can answer the question. Please sign in first, or register a free account.

margaritakristinak

Add To Favorites

Follow

Total Points: 502

Total Questions: 127

Total Answers: 98

Location: England

Member since Mon, May 17, 2021

3 Years ago

margaritakristinak questions

1 Parcel.js says to add the type=module attribute to the < script> tag but I've already added it

Thu, Mar 31, 22, 00:00, 2 Years ago

1 Error [ERR_MODULE_NOT_FOUND]: Cannot find package '@babel/plugin-preset-react' imported from

Sun, Feb 20, 22, 00:00, 2 Years ago

1 Why is local storage deleted when I refresh the page?

Tue, May 11, 21, 00:00, 3 Years ago

1 Click event not firing on touchscreen when finger moves a bit

Tue, Oct 29, 19, 00:00, 5 Years ago

1 Nuxt.js: How to use the same page with different URLs and switch the components to be displayed according to the URL

Thu, Mar 28, 19, 00:00, 5 Years ago

View All

answered 11 Years ago sam · Accepted Answer

The problem is that your b.js immediately ends and closes its standard in, which causes an error in a.js, because its standard out got shut off and you didn't handle that possibility. You have two options: handle stdout closing in a.js or accept input in b.js.

Fixing a.js:

process.on(SIGPIPE, process.exit);

If you add that line, it'll just give up when there's no one reading its output anymore. There are probably better things to do on SIGPIPE depending on what your program is doing, but the key is to stop console.loging.

Fixing b.js:

#!/usr/bin/env node



var stdin = process.openStdin();



var data = ;



stdin.on('data', function(chunk) {

  data += chunk;

});



stdin.on('end', function() {

  console.log(DATA:n + data + nEND DATA);

});

Of course, you don't have to do anything with that data. They key is to have something that keeps the process running; if you're piping to it, stdin.on('data', fx) seems like a useful thing to do.

Remember, either one of those will prevent that error. I expect the second to be most useful if you're planning on piping between programs.