Pipe a javascript variable into shell command with nodejs

Question

I'm working on a nodejs application and I need to pipe a multi-line string into a shell command. I'm not a pro at shell scripting but if I run this command in my terminal it works just fine:

$((cat $filePath) | dayone new)

Here's what I've got for the nodejs side. The dayone command does work but there is nothing piped into it.

const cp = require('child_process');
const terminal = cp.spawn('bash');

var multiLineVariable = 'Multi\nline\nstring';

terminal.stdin.write('mul');
cp.exec('dayone new', (error, stdout, stderr) => {
    console.log(error, stdout, stderr);
});
terminal.stdin.end();

Thanks for any help!

Lee Marshall · Accepted Answer · 2016-09-04 08:35:20Z

5

Here, you're starting up bash using spawn, but then you're using exec to start your dayone program. They are separate child processes and aren't connected in any way.

'cp' is just a reference to the child_process module, and spawn and exec are just two different ways of starting child processes.

You could use bash and write your dayone command to stdin in order to invoke dayone (as your snippet seems to be trying to do), or you could just invoke dayone directly with exec (bear in mind exec still runs the command in a shell):

var multiLineVariable = 'Multi\nline\nstring';

// get the child_process module
const cp = require('child_process');

// open a child process
var process = cp.exec('dayone new', (error, stdout, stderr) => {
    console.log(error, stdout, stderr);
});

// write your multiline variable to the child process
process.stdin.write(multiLineVariable);
process.stdin.end();

answered Sep 4, 2016 at 8:35

Lee Marshall

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3 Comments

danseethaler Over a year ago

This works perfectly but I'm not sure why. It seems like calling the cp.exec before we write to the stdin would execute the dayone command before there's anything to read from. Can you explain how the dayone command gets the input? Thanks!

Lee Marshall Over a year ago

The exec command won't wait for the dayone to complete/exit, as nothing in node.js blocks. Exec just starts the process up and the process then tries to read from stdin. If there's nothing available, it will block until there is something, which there will be as we are providing it as soon as we have started the process. Just like if you had run it from the command line by hand, it will block until you have typed something into its stdin.

danseethaler Over a year ago

Perfect! Thanks so much!

Sebastian Tilch · Accepted Answer · 2016-09-04 08:36:00Z

2

With Readable Streams it's really easy to listen to the input

const chunks = [];
process.stdin.on('readable', () => {
  const chunk = process.stdin.read()
  chunks.push(chunk);
  if (chunk !== null) {
    const result = Buffer.concat(chunks);
    console.log(result.toString());
  }
});

With Writable Streams you can write to the stdout

 process.stdout.write('Multi\nline\nstring');

Hopefully I could help you

answered Sep 4, 2016 at 8:36

Sebastian Tilch

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