How to read from standard input line by line?

The most straight-forward looking approach will just use readLine() which is part of Predef. however that is rather ugly as you need to check for eventual null value:

object ScannerTest {
  def main(args: Array[String]) {
    var ok = true
    while (ok) {
      val ln = readLine()
      ok = ln != null
      if (ok) println(ln)
    }
  }
}

this is so verbose, you'd rather use java.util.Scanner instead.

I think a more pretty approach will use scala.io.Source:

object ScannerTest {
  def main(args: Array[String]) {
    for (ln <- io.Source.stdin.getLines) println(ln)
  }
}

For the console you can use Console.readLine. You can write (if you want to stop on an empty line):

Iterator.continually(Console.readLine).takeWhile(_.nonEmpty).foreach(line => println("read " + line))

If you cat a file to generate the input you may need to stop on either null or empty using:

@inline def defined(line: String) = {
  line != null && line.nonEmpty
}
Iterator.continually(Console.readLine).takeWhile(defined(_)).foreach(line => println("read " + line))