Packages

  • package root

    This is the documentation for the Scala standard library.

    This is the documentation for the Scala standard library.

    Package structure

    The scala package contains core types like Int, Float, Array or Option which are accessible in all Scala compilation units without explicit qualification or imports.

    Notable packages include:

    Other packages exist. See the complete list on the right.

    Additional parts of the standard library are shipped as separate libraries. These include:

    • scala.reflect - Scala's reflection API (scala-reflect.jar)
    • scala.xml - XML parsing, manipulation, and serialization (scala-xml.jar)
    • scala.swing - A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)
    • scala.util.parsing - Parser combinators (scala-parser-combinators.jar)

    Automatic imports

    Identifiers in the scala package and the scala.Predef object are always in scope by default.

    Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example, List is an alias for scala.collection.immutable.List.

    Other aliases refer to classes provided by the underlying platform. For example, on the JVM, String is an alias for java.lang.String.

    Definition Classes
    root
  • package scala

    Core Scala types.

    Core Scala types. They are always available without an explicit import.

    Definition Classes
    root
  • package sys

    The package object scala.sys contains methods for reading and altering core aspects of the virtual machine as well as the world outside of it.

    The package object scala.sys contains methods for reading and altering core aspects of the virtual machine as well as the world outside of it.

    Definition Classes
    scala
    Since

    2.9

  • package process

    This package handles the execution of external processes.

    This package handles the execution of external processes. The contents of this package can be divided in three groups, according to their responsibilities:

    • Indicating what to run and how to run it.
    • Handling a process input and output.
    • Running the process.

    For simple uses, the only group that matters is the first one. Running an external command can be as simple as "ls".!, or as complex as building a pipeline of commands such as this:

    import scala.sys.process._
"ls" #| "grep .scala" #&& Seq("sh", "-c", "scalac *.scala") #|| "echo nothing found" lineStream

    We describe below the general concepts and architecture of the package, and then take a closer look at each of the categories mentioned above.

    Concepts and Architecture

    The underlying basis for the whole package is Java's Process and ProcessBuilder classes. While there's no need to use these Java classes, they impose boundaries on what is possible. One cannot, for instance, retrieve a process id for whatever is executing.

    When executing an external process, one can provide a command's name, arguments to it, the directory in which it will be executed and what environment variables will be set. For each executing process, one can feed its standard input through a java.io.OutputStream, and read from its standard output and standard error through a pair of java.io.InputStream. One can wait until a process finishes execution and then retrieve its return value, or one can kill an executing process. Everything else must be built on those features.

    This package provides a DSL for running and chaining such processes, mimicking Unix shells ability to pipe output from one process to the input of another, or control the execution of further processes based on the return status of the previous one.

    In addition to this DSL, this package also provides a few ways of controlling input and output of these processes, going from simple and easy to use to complex and flexible.

    When processes are composed, a new ProcessBuilder is created which, when run, will execute the ProcessBuilder instances it is composed of according to the manner of the composition. If piping one process to another, they'll be executed simultaneously, and each will be passed a ProcessIO that will copy the output of one to the input of the other.

    What to Run and How

    The central component of the process execution DSL is the scala.sys.process.ProcessBuilder trait. It is ProcessBuilder that implements the process execution DSL, that creates the scala.sys.process.Process that will handle the execution, and return the results of such execution to the caller. We can see that DSL in the introductory example: #|, #&& and #!! are methods on ProcessBuilder used to create a new ProcessBuilder through composition.

    One creates a ProcessBuilder either through factories on the scala.sys.process.Process's companion object, or through implicit conversions available in this package object itself. Implicitly, each process is created either out of a String, with arguments separated by spaces -- no escaping of spaces is possible -- or out of a scala.collection.Seq, where the first element represents the command name, and the remaining elements are arguments to it. In this latter case, arguments may contain spaces.

    To further control what how the process will be run, such as specifying the directory in which it will be run, see the factories on scala.sys.process.Process's object companion.

    Once the desired ProcessBuilder is available, it can be executed in different ways, depending on how one desires to control its I/O, and what kind of result one wishes for:

    • Return status of the process (! methods)
    • Output of the process as a String (!! methods)
    • Continuous output of the process as a Stream[String] (lineStream methods)
    • The Process representing it (run methods)

    Some simple examples of these methods:

    import scala.sys.process._

// This uses ! to get the exit code
def fileExists(name: String) = Seq("test", "-f", name).! == 0

// This uses !! to get the whole result as a string
val dirContents = "ls".!!

// This "fire-and-forgets" the method, which can be lazily read through
// a Stream[String]
def sourceFilesAt(baseDir: String): Stream[String] = {
  val cmd = Seq("find", baseDir, "-name", "*.scala", "-type", "f")
  cmd.lineStream
}

    We'll see more details about controlling I/O of the process in the next section.

    Handling Input and Output

    In the underlying Java model, once a Process has been started, one can get java.io.InputStream and java.io.OutputStream representing its output and input respectively. That is, what one writes to an OutputStream is turned into input to the process, and the output of a process can be read from an InputStream -- of which there are two, one representing normal output, and the other representing error output.

    This model creates a difficulty, which is that the code responsible for actually running the external processes is the one that has to take decisions about how to handle its I/O.

    This package presents an alternative model: the I/O of a running process is controlled by a scala.sys.process.ProcessIO object, which can be passed _to_ the code that runs the external process. A ProcessIO will have direct access to the java streams associated with the process I/O. It must, however, close these streams afterwards.

    Simpler abstractions are available, however. The components of this package that handle I/O are:

    Some examples of I/O handling:

    import scala.sys.process._

// An overly complex way of computing size of a compressed file
def gzFileSize(name: String) = {
  val cat = Seq("zcat", name)
  var count = 0
  def byteCounter(input: java.io.InputStream) = {
    while(input.read() != -1) count += 1
    input.close()
  }
  val p = cat run new ProcessIO(_.close(), byteCounter, _.close())
  p.exitValue()
  count
}

// This "fire-and-forgets" the method, which can be lazily read through
// a Stream[String], and accumulates all errors on a StringBuffer
def sourceFilesAt(baseDir: String): (Stream[String], StringBuffer) = {
  val buffer = new StringBuffer()
  val cmd = Seq("find", baseDir, "-name", "*.scala", "-type", "f")
  val lineStream = cmd lineStream_! ProcessLogger(buffer append _)
  (lineStream, buffer)
}

    Instances of the java classes java.io.File and java.net.URL can both be used directly as input to other processes, and java.io.File can be used as output as well. One can even pipe one to the other directly without any intervening process, though that's not a design goal or recommended usage. For example, the following code will copy a web page to a file:

    import java.io.File
import java.net.URL
import scala.sys.process._
new URL("http://www.scala-lang.org/") #> new File("scala-lang.html") !

    More information about the other ways of controlling I/O can be found in the Scaladoc for the associated objects, traits and classes.

    Running the Process

    Paradoxically, this is the simplest component of all, and the one least likely to be interacted with. It consists solely of scala.sys.process.Process, and it provides only two methods:

    • exitValue(): blocks until the process exit, and then returns the exit value. This is what happens when one uses the ! method of ProcessBuilder.
    • destroy(): this will kill the external process and close the streams associated with it.
    Definition Classes
    sys
  • BasicIO
  • FileProcessLogger
  • Process
  • ProcessBuilder
  • ProcessCreation
  • ProcessIO
  • ProcessImplicits
  • ProcessLogger

object BasicIO

This object contains factories for scala.sys.process.ProcessIO, which can be used to control the I/O of a scala.sys.process.Process when a scala.sys.process.ProcessBuilder is started with the run command.

It also contains some helper methods that can be used to in the creation of ProcessIO.

It is used by other classes in the package in the implementation of various features, but can also be used by client code.

Source
BasicIO.scala
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Value Members

  1. final val BufferSize: Int(8192)

    Size of the buffer used in all the functions that copy data

  2. final val Newline: String

    Used to separate lines in the processFully function that takes Appendable.

  3. def apply(withIn: Boolean, log: ProcessLogger): ProcessIO

    Creates a ProcessIO from a ProcessLogger .

    Creates a ProcessIO from a ProcessLogger . It can attach the process input to stdin.

    withIn

    True if the process input should be attached to stdin.

    log

    A ProcessLogger to receive all output, normal and error.

    returns

    A ProcessIO with the characteristics above.

  4. def apply(withIn: Boolean, buffer: StringBuffer, log: Option[ProcessLogger]): ProcessIO

    Creates a ProcessIO that appends its output to a StringBuffer.

    Creates a ProcessIO that appends its output to a StringBuffer. It can attach the process input to stdin, and it will either send the error stream to stderr, or to a ProcessLogger.

    For example, the ProcessIO created by the function below will store the normal output on the buffer provided, and print all error on stderr. The input will be read from stdin.

    import scala.sys.process.{BasicIO, ProcessLogger}
val printer = ProcessLogger(println(_))
def appendToBuffer(b: StringBuffer) = BasicIO(true, b, Some(printer))
    withIn

    True if the process input should be attached to stdin.

    buffer

    A StringBuffer which will receive the process normal output.

    log

    An optional ProcessLogger to which the output should be sent. If None, output will be sent to stderr.

    returns

    A ProcessIO with the characteristics above.

  5. def apply(withIn: Boolean, output: (String) ⇒ Unit, log: Option[ProcessLogger]): ProcessIO

    Creates a ProcessIO from a function String => Unit.

    Creates a ProcessIO from a function String => Unit. It can attach the process input to stdin, and it will either send the error stream to stderr, or to a ProcessLogger.

    For example, the ProcessIO created below will print all normal output while ignoring all error output. No input will be provided.

    import scala.sys.process.BasicIO
val errToDevNull = BasicIO(false, println(_), None)
    withIn

    True if the process input should be attached to stdin.

    output

    A function that will be called with the process output.

    log

    An optional ProcessLogger to which the output should be sent. If None, output will be sent to stderr.

    returns

    A ProcessIO with the characteristics above.

  6. def close(c: Closeable): Unit

    Closes a Closeable without throwing an exception

  7. def connectToIn(o: OutputStream): Unit

    Copy contents of stdin to the OutputStream.

  8. def getErr(log: Option[ProcessLogger]): (InputStream) ⇒ Unit

    Returns a function InputStream => Unit given an optional ProcessLogger.

    Returns a function InputStream => Unit given an optional ProcessLogger. If no logger is passed, the function will send the output to stderr. This function can be used to create a scala.sys.process.ProcessIO.

    log

    An optional ProcessLogger to which the contents of the InputStream will be sent.

    returns

    A function InputStream => Unit (used by scala.sys.process.ProcessIO) which will send the data to either the provided ProcessLogger or, if None, to stderr.

  9. def input(connect: Boolean): (OutputStream) ⇒ Unit

    Returns a function OutputStream => Unit that either reads the content from stdin or does nothing.

    Returns a function OutputStream => Unit that either reads the content from stdin or does nothing. This function can be used by scala.sys.process.ProcessIO.

  10. def processFully(processLine: (String) ⇒ Unit): (InputStream) ⇒ Unit

    Returns a function InputStream => Unit that will call the passed function with all data read.

    Returns a function InputStream => Unit that will call the passed function with all data read. This function can be used to create a scala.sys.process.ProcessIO. The processLine function will be called with each line read, and Newline will be appended after each line.

    processLine

    A function that will be called with all data read from the stream.

    returns

    A function InputStream => Unit (used by scala.sys.process.ProcessIO which will call processLine with all data read from the stream.

  11. def processFully(buffer: Appendable): (InputStream) ⇒ Unit

    Returns a function InputStream => Unit that appends all data read to the provided Appendable.

    Returns a function InputStream => Unit that appends all data read to the provided Appendable. This function can be used to create a scala.sys.process.ProcessIO. The buffer will be appended line by line.

    buffer

    An Appendable such as StringBuilder or StringBuffer.

    returns

    A function InputStream => Unit (used by scala.sys.process.ProcessIO which will append all data read from the stream to the buffer.

  12. def processLinesFully(processLine: (String) ⇒ Unit)(readLine: () ⇒ String): Unit

    Calls processLine with the result of readLine until the latter returns null or the current thread is interrupted.

  13. def standard(in: (OutputStream) ⇒ Unit): ProcessIO

    Returns a ProcessIO connected to stdout, stderr and the provided in

  14. def standard(connectInput: Boolean): ProcessIO

    Returns a ProcessIO connected to stdout and stderr, and, optionally, stdin.

  15. def toStdErr: (InputStream) ⇒ Unit

    Send all the input from the stream to stderr, and closes the input stream afterwards.

  16. def toStdOut: (InputStream) ⇒ Unit

    Send all the input from the stream to stdout, and closes the input stream afterwards.

  17. def transferFully(in: InputStream, out: OutputStream): Unit

    Copy all input from the input stream to the output stream.

    Copy all input from the input stream to the output stream. Closes the input stream once it's all read.