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

trait ProcessBuilder extends Source with Sink

Represents a sequence of one or more external processes that can be executed. A ProcessBuilder can be a single external process, or a combination of other ProcessBuilder. One can control where the output of an external process will go to, and where its input will come from, or leave that decision to whoever starts it.

One creates a ProcessBuilder through factories provided in scala.sys.process.Process's companion object, or implicit conversions based on these factories made available in the package object scala.sys.process. Here are some examples:

import scala.sys.process._

// Executes "ls" and sends output to stdout
"ls".!

// Execute "ls" and assign a `Stream[String]` of its output to "contents".
val contents = Process("ls").lineStream

// Here we use a `Seq` to make the parameter whitespace-safe
def contentsOf(dir: String): String = Seq("ls", dir).!!

The methods of ProcessBuilder are divided in three categories: the ones that combine two ProcessBuilder to create a third, the ones that redirect input or output of a ProcessBuilder, and the ones that execute the external processes associated with it.

Combining ProcessBuilder

Two existing ProcessBuilder can be combined in the following ways:

  • They can be executed in parallel, with the output of the first being fed as input to the second, like Unix pipes. This is achieved with the #| method.
  • They can be executed in sequence, with the second starting as soon as the first ends. This is done by the ### method.
  • The execution of the second one can be conditioned by the return code (exit status) of the first, either only when it's zero, or only when it's not zero. The methods #&& and #|| accomplish these tasks.

Redirecting Input/Output

Though control of input and output can be done when executing the process, there's a few methods that create a new ProcessBuilder with a pre-configured input or output. They are #<, #> and #>>, and may take as input either another ProcessBuilder (like the pipe described above), or something else such as a java.io.File or a java.io.InputStream. For example:

new URL("http://databinder.net/dispatch/About") #> "grep JSON" #>> new File("About_JSON") !

Starting Processes

To execute all external commands associated with a ProcessBuilder, one may use one of four groups of methods. Each of these methods have various overloads and variations to enable further control over the I/O. These methods are:

  • run: the most general method, it returns a scala.sys.process.Process immediately, and the external command executes concurrently.
  • !: blocks until all external commands exit, and returns the exit code of the last one in the chain of execution.
  • !!: blocks until all external commands exit, and returns a String with the output generated.
  • lineStream: returns immediately like run, and the output being generated is provided through a Stream[String]. Getting the next element of that Stream may block until it becomes available. This method will throw an exception if the return code is different than zero -- if this is not desired, use the lineStream_! method.

Handling Input and Output

If not specified, the input of the external commands executed with run or ! will not be tied to anything, and the output will be redirected to the stdout and stderr of the Scala process. For the methods !! and lineStream, no input will be provided, and the output will be directed according to the semantics of these methods.

Some methods will cause stdin to be used as input. Output can be controlled with a scala.sys.process.ProcessLogger -- !! and lineStream will only redirect error output when passed a ProcessLogger. If one desires full control over input and output, then a scala.sys.process.ProcessIO can be used with run.

For example, we could silence the error output from lineStream_! like this:

val etcFiles = "find /etc" lineStream_! ProcessLogger(line => ())

Extended Example

Let's examine in detail one example of usage:

import scala.sys.process._
"find src -name *.scala -exec grep null {} ;"  #|  "xargs test -z"  #&&  "echo null-free"  #||  "echo null detected"  !

Note that every String is implicitly converted into a ProcessBuilder through the implicits imported from scala.sys.process. These ProcessBuilder are then combined in three different ways.

  1. #| pipes the output of the first command into the input of the second command. It mirrors a shell pipe (|).
  2. #&& conditionally executes the second command if the previous one finished with exit value 0. It mirrors shell's &&.
  3. #|| conditionally executes the third command if the exit value of the previous command is different than zero. It mirrors shell's ||.

Finally, ! at the end executes the commands, and returns the exit value. Whatever is printed will be sent to the Scala process standard output. If we wanted to capture it, we could run that with !! instead.

Note: though it is not shown above, the equivalent of a shell's ; would be ###. The reason for this name is that ; is a reserved token in Scala.

Note: the lines method, though deprecated, may conflict with the StringLike method of the same name. To avoid this, one may wish to call the builders in Process instead of importing scala.sys.process._. The example above would be

import scala.sys.process.Process
Process("find src -name *.scala -exec grep null {} ;") #| Process("xargs test -z") #&& Process("echo null-free") #|| Process("echo null detected") !
Source
ProcessBuilder.scala
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Abstract Value Members

  1. abstract def !(log: ProcessLogger): Int

    Starts the process represented by this builder, blocks until it exits, and returns the exit code.

    Starts the process represented by this builder, blocks until it exits, and returns the exit code. Standard output and error are sent to the given ProcessLogger.

  2. abstract def !: Int

    Starts the process represented by this builder, blocks until it exits, and returns the exit code.

    Starts the process represented by this builder, blocks until it exits, and returns the exit code. Standard output and error are sent to the console.

  3. abstract def !!(log: ProcessLogger): String

    Starts the process represented by this builder, blocks until it exits, and returns the output as a String.

    Starts the process represented by this builder, blocks until it exits, and returns the output as a String. Standard error is sent to the provided ProcessLogger. If the exit code is non-zero, an exception is thrown.

  4. abstract def !!: String

    Starts the process represented by this builder, blocks until it exits, and returns the output as a String.

    Starts the process represented by this builder, blocks until it exits, and returns the output as a String. Standard error is sent to the console. If the exit code is non-zero, an exception is thrown.

  5. abstract def !!<(log: ProcessLogger): String

    Starts the process represented by this builder, blocks until it exits, and returns the output as a String.

    Starts the process represented by this builder, blocks until it exits, and returns the output as a String. Standard error is sent to the provided ProcessLogger. If the exit code is non-zero, an exception is thrown. The newly started process reads from standard input of the current process.

  6. abstract def !!<: String

    Starts the process represented by this builder, blocks until it exits, and returns the output as a String.

    Starts the process represented by this builder, blocks until it exits, and returns the output as a String. Standard error is sent to the console. If the exit code is non-zero, an exception is thrown. The newly started process reads from standard input of the current process.

  7. abstract def !<(log: ProcessLogger): Int

    Starts the process represented by this builder, blocks until it exits, and returns the exit code.

    Starts the process represented by this builder, blocks until it exits, and returns the exit code. Standard output and error are sent to the given ProcessLogger. The newly started process reads from standard input of the current process.

  8. abstract def !<: Int

    Starts the process represented by this builder, blocks until it exits, and returns the exit code.

    Starts the process represented by this builder, blocks until it exits, and returns the exit code. Standard output and error are sent to the console. The newly started process reads from standard input of the current process.

  9. abstract def ###(other: ProcessBuilder): ProcessBuilder

    Constructs a command that will run this command and then other.

    Constructs a command that will run this command and then other. The exit code will be the exit code of other.

  10. abstract def #&&(other: ProcessBuilder): ProcessBuilder

    Constructs a command that runs this command first and then other if this command succeeds.

  11. abstract def #|(other: ProcessBuilder): ProcessBuilder

    Constructs a command that will run this command and pipes the output to other.

    Constructs a command that will run this command and pipes the output to other. other must be a simple command.

  12. abstract def #||(other: ProcessBuilder): ProcessBuilder

    Constructs a command that runs this command first and then other if this command does not succeed.

  13. abstract def canPipeTo: Boolean

    True if this command can be the target of a pipe.

  14. abstract def hasExitValue: Boolean

    True if this command has an exit code which should be propagated to the user.

    True if this command has an exit code which should be propagated to the user. Given a pipe between A and B, if B.hasExitValue is true then the exit code will be the one from B; if it is false, the one from A. This exists to prevent output redirections (implemented as pipes) from masking useful process error codes.

  15. abstract def lineStream(log: ProcessLogger): Stream[String]

    Starts the process represented by this builder.

    Starts the process represented by this builder. The output is returned as a Stream that blocks when lines are not available but the process has not completed. Standard error is sent to the provided ProcessLogger. If the process exits with a non-zero value, the Stream will provide all lines up to termination and then throw an exception.

  16. abstract def lineStream: Stream[String]

    Starts the process represented by this builder.

    Starts the process represented by this builder. The output is returned as a Stream that blocks when lines are not available but the process has not completed. Standard error is sent to the console. If the process exits with a non-zero value, the Stream will provide all lines up to termination and then throw an exception.

  17. abstract def lineStream_!(log: ProcessLogger): Stream[String]

    Starts the process represented by this builder.

    Starts the process represented by this builder. The output is returned as a Stream that blocks when lines are not available but the process has not completed. Standard error is sent to the provided ProcessLogger. If the process exits with a non-zero value, the Stream will provide all lines up to termination but will not throw an exception.

  18. abstract def lineStream_!: Stream[String]

    Starts the process represented by this builder.

    Starts the process represented by this builder. The output is returned as a Stream that blocks when lines are not available but the process has not completed. Standard error is sent to the console. If the process exits with a non-zero value, the Stream will provide all lines up to termination but will not throw an exception.

  19. abstract def run(log: ProcessLogger, connectInput: Boolean): Process

    Starts the process represented by this builder.

    Starts the process represented by this builder. Standard output and error are sent to the given ProcessLogger. The newly started process reads from standard input of the current process if connectInput is true.

  20. abstract def run(connectInput: Boolean): Process

    Starts the process represented by this builder.

    Starts the process represented by this builder. Standard output and error are sent to the console. The newly started process reads from standard input of the current process if connectInput is true.

  21. abstract def run(io: ProcessIO): Process

    Starts the process represented by this builder.

    Starts the process represented by this builder. I/O is handled by the given ProcessIO instance.

  22. abstract def run(log: ProcessLogger): Process

    Starts the process represented by this builder.

    Starts the process represented by this builder. Standard output and error are sent to the given ProcessLogger.

  23. abstract def run(): Process

    Starts the process represented by this builder.

    Starts the process represented by this builder. Standard output and error are sent to the console.

Concrete Value Members

  1. def #<(b: ProcessBuilder): ProcessBuilder

    Reads the output of a scala.sys.process.ProcessBuilder into the input stream of this process.

    Reads the output of a scala.sys.process.ProcessBuilder into the input stream of this process.

    Definition Classes
    Sink
  2. def #<(in: ⇒ InputStream): ProcessBuilder

    Reads the given InputStream into the input stream of this process.

    Reads the given InputStream into the input stream of this process. The argument is call-by-name, so the stream is recreated, read, and closed each time this process is executed.

    Definition Classes
    Sink
  3. def #<(f: URL): ProcessBuilder

    Reads the given URL into the input stream of this process.

    Reads the given URL into the input stream of this process.

    Definition Classes
    Sink
  4. def #<(f: File): ProcessBuilder

    Reads the given file into the input stream of this process.

    Reads the given file into the input stream of this process.

    Definition Classes
    Sink
  5. def #>(b: ProcessBuilder): ProcessBuilder

    Writes the output stream of this process to a scala.sys.process.ProcessBuilder.

    Writes the output stream of this process to a scala.sys.process.ProcessBuilder.

    Definition Classes
    Source
  6. def #>(out: ⇒ OutputStream): ProcessBuilder

    Writes the output stream of this process to the given OutputStream.

    Writes the output stream of this process to the given OutputStream. The argument is call-by-name, so the stream is recreated, written, and closed each time this process is executed.

    Definition Classes
    Source
  7. def #>(f: File): ProcessBuilder

    Writes the output stream of this process to the given file.

    Writes the output stream of this process to the given file.

    Definition Classes
    Source
  8. def #>>(f: File): ProcessBuilder

    Appends the output stream of this process to the given file.

    Appends the output stream of this process to the given file.

    Definition Classes
    Source
  9. def cat: ProcessBuilder

    Returns a scala.sys.process.ProcessBuilder representing this Source.

    Returns a scala.sys.process.ProcessBuilder representing this Source.

    Definition Classes
    Source

Deprecated Value Members

  1. def lines(log: ProcessLogger): Stream[String]

    Deprecated (renamed).

    Deprecated (renamed). Use lineStream(log: ProcessLogger) instead.

    Annotations
    @deprecated
    Deprecated

    (Since version 2.11.0) use lineStream instead

  2. def lines: Stream[String]

    Deprecated (renamed).

    Deprecated (renamed). Use lineStream instead.

    Annotations
    @deprecated
    Deprecated

    (Since version 2.11.0) use lineStream instead

  3. def lines_!(log: ProcessLogger): Stream[String]

    Deprecated (renamed).

    Deprecated (renamed). Use lineStream_!(log: ProcessLogger) instead.

    Annotations
    @deprecated
    Deprecated

    (Since version 2.11.0) use lineStream_! instead

  4. def lines_!: Stream[String]

    Deprecated (renamed).

    Deprecated (renamed). Use lineStream_! instead.

    Annotations
    @deprecated
    Deprecated

    (Since version 2.11.0) use lineStream_! instead