What is the best way to stream stdout from a Process, in Dart?

Question

I would like to run a Process, and stream the process's stdout to the console. What is the best (most effective, easiest to understand, least lines of code) way to do this?

For example:

var process = await Process.start(exec, args);

I'd like to see any stdout contents as soon as they are available.

Thanks!

score 16 · Answer 1 · edited Sep 19 '20 at 21:16

16

import 'dart:io';
void main() async {
   var process = await Process.start(exec, args);
   process.stdout.pipe(stdout);
}

Or using then:

import 'dart:io';
void main() {
   Process.start(exec, args).then(
      (process) => process.stdout.pipe(stdout)
   );
}

https://api.dart.dev/dart-async/Stream/pipe.html

edited Sep 19 '20 at 21:16

Zombo

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answered Oct 21 '15 at 05:26

Günter Zöchbauer

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I wonder what the pros and cons are between this and addStream are? – Seth Ladd Oct 22 '15 at 02:07
1

`.pipe()` itself just calls `addStream()` ;-), but in addition, when the source stream is closed without an error, it also closes the target stream automatically. – Günter Zöchbauer Oct 24 '15 at 18:05
Wouldn't this do some unexpected things if it closed your process's stdout stream before your process is done? – Dan Field Apr 12 '21 at 20:54
You can't close it, because the controller is private to the process. What you can do is stop listening and that won't have any effect except not getting further output on the stream. – Günter Zöchbauer Apr 13 '21 at 06:55
1

Thanks, dude :) it's help me for async processes – Tretyakov Pavel Feb 05 '23 at 23:26

score 4 · Answer 2 · answered Oct 21 '15 at 04:56

4

Here's one way:

var process = await Process.start(exec, args);
stdout.addStream(process.stdout);

Notice that I add the process.stdout stream to the normal stdout stream, which comes from dart:io.

answered Oct 21 '15 at 04:56

Seth Ladd

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score 1 · Answer 3 · answered Dec 02 '20 at 17:36

For completeness, you could use the mode argument in Process.start and pass a ProcessStartMode.inheritStdio

var process = await Process.start(
  command,
  args,
  mode: ProcessStartMode.inheritStdio
);

Be careful though, this will, as the mode's name implies, pass on all stdio from the process (stdin, stdout and stderr) to the default stdout which might cause unexpected results as stuff like sigterms are passed on too.

What is the best way to stream stdout from a Process, in Dart?

3 Answers3