CODEWITHSUNDEEP
node.jsunixterminaltty
Alexander Mills
Alexander Mills

Reputation: 100020

Capturing secondary / tertiary stream to terminal / TTY

See picture below. I run a command from the terminal which starts process A, which starts a server (process B), and the server in turn will start workers (processes C). I want to stream the stdout/stderr of the server to some log file, but I want the stdout and stderr of the workers to stream back to the terminal. All processes here are Node.js processes.

No idea how to do this or if it's possible. My only guess as to how it might work is if the terminal session has some sort of handle or id which I can use and tell the worker processes to stream to that handle or id. I don't know enough about *nix to know how this works. Any explanation would help.

Here's a visual:

enter image description here

Upvotes: 3

Views: 569

Answers (2)

Jason Livesay
Jason Livesay

Reputation: 6377

Nice job figuring out the tty thing. Not sure if there is an easier way to do exactly that, but I have a few ideas that involve cheating on your question in a way but are probably better in the long run anyway.

  1. Use a logging framework like winston or bunyan. Or a structured logging system like timequerylog. The logging frameworks will allow for multiple loggers, one can go to a file and another can go to stdout.

  2. Use the Cluster built-in Node module to create workers and have the workers send messages to the master with events/data. The master could then log those stdout.

Upvotes: 0

Alexander Mills
Alexander Mills

Reputation: 100020

Using Node.js (since all the processes from the OP are Node.js processes), here is one solution I have discovered.

step 1, in process A, get the tty identity of the current terminal/tty

const tty = String(cp.execSync('tty', {stdio:['inherit','pipe','pipe']})).trim();

step 2, pass the tty value from process A to process B => I pass that dynamic value (a string) to the child process using socket.io (you could also use IPC)

this.emit('tty-value', tty);

step 3 In the child process B, I use fd = fs.openSync(tty) to get the right file descriptor.

const fd = fs.openSync(tty)

step 4 Then I can write to the terminal that I want to write to with the following

const strm = fs.createWriteStream(null, {fd: fd});

So when process B creates child process C, it can make the calls necessary to pipe the stdout/stderr from process C to the above stream.

...this took me all day to figure out, so leaving this here for anyone to see

Upvotes: 5

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