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
t

scala.sys.process

ProcessCreation

trait ProcessCreation extends AnyRef

Factories for creating scala.sys.process.ProcessBuilder. They can be found on and used through scala.sys.process.Process's companion object.

Source
Process.scala
Linear Supertypes
AnyRef, Any
Known Subclasses
Process
Ordering
  1. Alphabetic
  2. By Inheritance
Inherited
  1. ProcessCreation
  2. AnyRef
  3. Any
Implicitly
  1. by any2stringadd
  2. by StringFormat
  3. by Ensuring
  4. by ArrowAssoc
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Visibility
  1. Public
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Value Members

  1. def apply(name: String, exitValue: ⇒ Int): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder from a String name and a Boolean.

    Creates a scala.sys.process.ProcessBuilder from a String name and a Boolean. This can be used to force an exit value, with the name being used for toString.

  2. def apply(value: Boolean): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder from a Boolean.

    Creates a scala.sys.process.ProcessBuilder from a Boolean. This can be to force an exit value.

  3. def apply(url: URL): URLBuilder

    Creates a scala.sys.process.ProcessBuilder from a java.net.URL.

    Creates a scala.sys.process.ProcessBuilder from a java.net.URL. This ProcessBuilder can then be used as a Source, so that one can pipe things from it.

  4. def apply(file: File): FileBuilder

    Creates a scala.sys.process.ProcessBuilder from a java.io.File.

    Creates a scala.sys.process.ProcessBuilder from a java.io.File. This ProcessBuilder can then be used as a Source or a Sink, so one can pipe things from and to it.

  5. def apply(builder: JProcessBuilder): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder from a java.lang.ProcessBuilder.

    Creates a scala.sys.process.ProcessBuilder from a java.lang.ProcessBuilder.

    Example: 

    1. apply((new java.lang.ProcessBuilder("ls", "-l")) directory new java.io.File(System.getProperty("user.home")))
  6. def apply(command: Seq[String], cwd: Option[File], extraEnv: (String, String)*): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder with working dir optionally set to File and extra environment variables.

    Creates a scala.sys.process.ProcessBuilder with working dir optionally set to File and extra environment variables.

    Example: 

    1. apply("java" :: javaArgs, params.get("cwd"), "CLASSPATH" -> "library.jar")
  7. def apply(command: String, cwd: Option[File], extraEnv: (String, String)*): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder with working dir optionally set to File and extra environment variables.

    Creates a scala.sys.process.ProcessBuilder with working dir optionally set to File and extra environment variables.

    Example: 

    1. apply("java", params.get("cwd"), "CLASSPATH" -> "library.jar")
  8. def apply(command: Seq[String], cwd: File, extraEnv: (String, String)*): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder with working dir set to File and extra environment variables.

    Creates a scala.sys.process.ProcessBuilder with working dir set to File and extra environment variables.

    Example: 

    1. apply("java" :: javaArgs, new java.io.File("/opt/app"), "CLASSPATH" -> "library.jar")
  9. def apply(command: String, cwd: File, extraEnv: (String, String)*): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder with working dir set to File and extra environment variables.

    Creates a scala.sys.process.ProcessBuilder with working dir set to File and extra environment variables.

    Example: 

    1. apply("java", new java.io.File("/opt/app"), "CLASSPATH" -> "library.jar")
  10. def apply(command: String, arguments: Seq[String]): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder from a command represented by a String, and a sequence of String representing the arguments.

    Creates a scala.sys.process.ProcessBuilder from a command represented by a String, and a sequence of String representing the arguments.

    Example: 

    1. apply("cat", files)
  11. def apply(command: Seq[String]): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder from a sequence of String, where the head is the command and each element of the tail is a parameter.

    Creates a scala.sys.process.ProcessBuilder from a sequence of String, where the head is the command and each element of the tail is a parameter.

    Example: 

    1. apply("cat" :: files)
  12. def apply(command: String): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder from a String, including the parameters.

    Creates a scala.sys.process.ProcessBuilder from a String, including the parameters.

    Example: 

    1. apply("cat file.txt")
  13. def applySeq[T](builders: Seq[T])(implicit convert: (T) ⇒ Source): Seq[Source]

    Creates a sequence of scala.sys.process.ProcessBuilder.Source from a sequence of something else for which there's an implicit conversion to Source.

  14. def cat(files: Seq[Source]): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder from a non-empty sequence of scala.sys.process.ProcessBuilder.Source, which can then be piped to something else.

    Creates a scala.sys.process.ProcessBuilder from a non-empty sequence of scala.sys.process.ProcessBuilder.Source, which can then be piped to something else.

    This will concatenate the output of all sources.

  15. def cat(file: Source, files: Source*): ProcessBuilder

    Creates a scala.sys.process.ProcessBuilder from one or more scala.sys.process.ProcessBuilder.Source, which can then be piped to something else.

    Creates a scala.sys.process.ProcessBuilder from one or more scala.sys.process.ProcessBuilder.Source, which can then be piped to something else.

    This will concatenate the output of all sources. For example:

    import scala.sys.process._
import scala.sys.process.Process.cat
import java.net.URL
import java.io.File

val spde = new URL("http://technically.us/spde.html")
val dispatch = new URL("https://dispatchhttp.org/Dispatch.html")
val build = new File("project/build.properties")
cat(spde, dispatch, build) #| "grep -i scala" !