scala.sys

# process

#### package process

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" lines

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]` (`lines` 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.lines
    }

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.OutpuStream` 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:

- scala.sys.process.ProcessIO: provides the low level abstraction.
- scala.sys.process.ProcessLogger: provides a higher level abstraction for output, and can be created through its object companion
- scala.sys.process.BasicIO: a library of helper methods for the creation of `ProcessIO`.
- This package object itself, with a few implicit conversions.

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()
      }
      cat ! new ProcessIO(_.close(), byteCounter, _.close())
      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 lines = cmd lines_! ProcessLogger(buffer append _)
      (lines, 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 looked at 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.

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### Type Members

1. #### class FileProcessLogger extends ProcessLogger with Closeable with Flushable

   A scala.sys.process.ProcessLogger that writes output to a file.

2. #### trait Process extends AnyRef

   Represents a process that is running or has finished running.

3. #### trait ProcessBuilder extends Source with Sink

   Represents a sequence of one or more external processes that can be executed.

4. #### trait ProcessCreation extends AnyRef

   Factories for creating scala.sys.process.ProcessBuilder.

5. #### final class ProcessIO extends AnyRef

   This class is used to control the I/O of every scala.sys.process.Process.

6. #### trait ProcessImplicits extends AnyRef

   Provide implicit conversions for the factories offered by scala.sys.process.Process's companion object.

7. #### trait ProcessLogger extends AnyRef

   Encapsulates the output and error streams of a running process.

### Value Members

1. #### 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.

2. #### object Process extends ProcessImpl with ProcessCreation

   Methods for constructing simple commands that can then be combined.

3. #### object ProcessBuilder extends ProcessBuilderImpl

   This object contains traits used to describe input and output sources.

4. #### object ProcessLogger

   Provides factories to create scala.sys.process.ProcessLogger, which are used to capture output of scala.sys.process.ProcessBuilder commands when run.

5. #### implicit def builderToProcess(builder: JProcessBuilder): ProcessBuilder

   Implicitly convert a `java.lang.ProcessBuilder` into a Scala one.

   Implicitly convert a `java.lang.ProcessBuilder` into a Scala one.

   Definition Classes
     ProcessImplicits

6. #### implicit def buildersToProcess[T](builders: Seq[T])(implicit convert: (T) ⇒ Source): Seq[Source]

   Return a sequence of scala.sys.process.ProcessBuilder.Source from a sequence of values for which an implicit conversion to `Source` is available.

   Return a sequence of scala.sys.process.ProcessBuilder.Source from a sequence of values for which an implicit conversion to `Source` is available.

   Definition Classes
     ProcessImplicits

7. #### implicit def fileToProcess(file: File): FileBuilder

   Implicitly convert a `java.io.File` into a scala.sys.process.ProcessBuilder.FileBuilder, which can be used as either input or output of a process.

   Implicitly convert a `java.io.File` into a scala.sys.process.ProcessBuilder.FileBuilder, which can be used as either input or output of a process. For example:

       import scala.sys.process._
       "ls" #> new java.io.File("dirContents.txt") !

   Definition Classes
     ProcessImplicits

8. #### def javaVmArguments: List[String]

   The arguments passed to `java` when creating this process

9. #### def stderr: PrintStream

   The error stream of this process

10. #### def stdin: InputStream

    The input stream of this process

11. #### def stdout: PrintStream

    The output stream of this process

12. #### implicit def stringSeqToProcess(command: Seq[String]): ProcessBuilder

    Implicitly convert a sequence of `String` into a scala.sys.process.ProcessBuilder.

    Implicitly convert a sequence of `String` into a scala.sys.process.ProcessBuilder. The first argument will be taken to be the command to be executed, and the remaining will be its arguments. When using this, arguments may contain spaces.

    Definition Classes
      ProcessImplicits

13. #### implicit def stringToProcess(command: String): ProcessBuilder

    Implicitly convert a `String` into a scala.sys.process.ProcessBuilder.

    Implicitly convert a `String` into a scala.sys.process.ProcessBuilder.

    Definition Classes
      ProcessImplicits

14. #### implicit def urlToProcess(url: URL): URLBuilder

    Implicitly convert a `java.net.URL` into a scala.sys.process.ProcessBuilder.URLBuilder , which can be used as input to a process.

    Implicitly convert a `java.net.URL` into a scala.sys.process.ProcessBuilder.URLBuilder , which can be used as input to a process. For example:

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

    Definition Classes
      ProcessImplicits

### Inherited from ProcessImplicits

### Inherited from AnyRef

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