Interact (i/o) with an external process in Scala - scala

Question

I'm looking for a simple way to start an external process and then write strings to its input and read its output.
In Python, this works:
mosesProcess = subprocess.Popen([mosesBinPath, '-f', mosesModelPath], stdin = subprocess.PIPE, stdout = subprocess.PIPE, stderr = subprocess.PIPE);
# ...
mosesProcess.stdin.write(aRequest);
mosesAnswer = mosesProcess.stdout.readline().rstrip();
# ...
mosesProcess.stdin.write(anotherRequest);
mosesAnswer = mosesProcess.stdout.readline().rstrip();
# ...
mosesProcess.stdin.close();
I think in Scala this should be done with scala.sys.process.ProcessBuilder and scala.sys.process.ProcessIO but I don't get how they work (especially the latter).
EDIT:
I have tried things like:
val inputStream = new scala.concurrent.SyncVar[java.io.OutputStream];
val outputStream = new scala.concurrent.SyncVar[java.io.InputStream];
val errStream = new scala.concurrent.SyncVar[java.io.InputStream];
val cmd = "myProc";
val pb = process.Process(cmd);
val pio = new process.ProcessIO(stdin => inputStream.put(stdin),
stdout => outputStream.put(stdout),
stderr => errStream.put(stderr));
pb.run(pio);
inputStream.get.write(("request1" + "\n").getBytes);
println(outputStream.get.read); // It is blocked here
inputStream.get.write(("request2" + "\n").getBytes);
println(outputStream.get.read);
inputStream.get.close()
But the execution gets stuck.

Answer 1

Granted, attrib below is not a great example on the write side of things. I have an EchoServer that would input/output
import scala.sys.process._
import java.io._
object EchoClient{
def main(args: Array[String]) {
var bContinue=true
var cmd="C:\\\\windows\\system32\\attrib.exe"
println(cmd)
val process = Process (cmd)
val io = new ProcessIO (
writer,
out => {scala.io.Source.fromInputStream(out).getLines.foreach(println)},
err => {scala.io.Source.fromInputStream(err).getLines.foreach(println)})
while (bContinue) {
process run io
var answer = readLine("Run again? (y/n)? ")
if (answer=="n" || answer=="N")
bContinue=false
}
}
def reader(input: java.io.InputStream) = {
// read here
}
def writer(output: java.io.OutputStream) = {
// write here
//
}
// TODO: implement an error logger
}
output below :
C:\\windows\system32\attrib.exe
A C:\dev\EchoClient$$anonfun$1.class
A C:\dev\EchoClient$$anonfun$2$$anonfun$apply$1.class
A C:\dev\EchoClient$$anonfun$2.class
A C:\dev\EchoClient$$anonfun$3$$anonfun$apply$2.class
A C:\dev\EchoClient$$anonfun$3.class
A C:\dev\EchoClient$.class
A C:\dev\EchoClient.class
A C:\dev\EchoClient.scala
A C:\dev\echoServer.bat
A C:\dev\EchoServerChg$$anonfun$main$1.class
A C:\dev\EchoServerChg$.class
A C:\dev\EchoServerChg.class
A C:\dev\EchoServerChg.scala
A C:\dev\ScannerTest$$anonfun$main$1.class
A C:\dev\ScannerTest$.class
A C:\dev\ScannerTest.class
A C:\dev\ScannerTest.scala
Run again? (y/n)?

Answer 2

Scala API for ProcessIO:
new ProcessIO(in: (OutputStream) ⇒ Unit, out: (InputStream) ⇒ Unit, err: (InputStream) ⇒ Unit)
I suppose you should provide at least two arguments, 1 outputStream function (writing to the process), 1 inputStream function (reading from the process).
For instance:
def readJob(in: InputStream) {
// do smthing with in
}
def writeJob(out: OutputStream) {
// do somthing with out
}
def errJob(err: InputStream) {
// do smthing with err
}
val process = new ProcessIO(writeJob, readJob, errJob)
Please keep in mind that the streams are Java streams so you will have to check Java API.
Edit: the package page provides examples, maybe you could take a look at them.

Answer 3

ProcessIO is the way to go for low level control and input and output interaction. There even is an often overlooked helper object BasicIO that assists with creating common ProcessIO instances for reading, connecting in/out streams with utility functions. You can look at the source for BasicIO.scala to see what it is doing internally in creating the ProcessIO Instances.
You can sometimes find inspiration from test cases or tools created for the class itself by the project. In the case of Scala, have a look at the source on GitHub. We are fortunate in that there is a detailed example of ProcessIO being used for the scala GraphViz Dot process runner DotRunner.scala!