/Scala 2.13 Library

Package 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" lazyLines

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 companion object.

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 LazyList[String] (lazyLines 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 LazyList[String]
def sourceFilesAt(baseDir: String): LazyList[String] = {
  val cmd = Seq("find", baseDir, "-name", "*.scala", "-type", "f")

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
  val p = cat run new ProcessIO(_.close(), byteCounter, _.close())

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

Type Members

class FileProcessLogger extends ProcessLogger with Closeable with Flushable

trait Process extends AnyRef

Represents a process that is running or has finished running. It may be a compound process with several underlying native processes (such as a #&& b).

This trait is often not used directly, though its companion object contains factories for scala.sys.process.ProcessBuilder, the main component of this package.

It is used directly when calling the method run on a ProcessBuilder, which makes the process run in the background. The methods provided on Process make it possible for one to block until the process exits and get the exit value, or destroy the process altogether.

See also


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

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

// 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. lazyLines: returns immediately like run, and the output being generated is provided through a LazyList[String]. Getting the next element of that LazyList 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 lazyLines_! 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 lazyLines, 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 lazyLines 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 lazyLines_! like this:

val etcFiles = "find /etc" lazyLines_! 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.

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

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.

final class ProcessIO extends AnyRef

This class is used to control the I/O of every scala.sys.process.Process. The functions used to create it will be called with the process streams once it has been started. It might not be necessary to use ProcessIO directly -- scala.sys.process.ProcessBuilder can return the process output to the caller, or use a scala.sys.process.ProcessLogger which avoids direct interaction with a stream. One can even use the factories at BasicIO to create a ProcessIO, or use its helper methods when creating one's own ProcessIO.

When creating a ProcessIO, it is important to close all streams when finished, since the JVM might use system resources to capture the process input and output, and will not release them unless the streams are explicitly closed.

ProcessBuilder will call writeInput, processOutput and processError in separate threads, and if daemonizeThreads is true, they will all be marked as daemon threads.


Failure to close the passed streams may result in resource leakage.

trait ProcessImplicits extends AnyRef

Provide implicit conversions for the factories offered by scala.sys.process.Process's companion object. These implicits can then be used to decrease the noise in a pipeline of commands, making it look more shell-like. They are available through the package object scala.sys.process.

trait ProcessLogger extends AnyRef

Encapsulates the output and error streams of a running process. This is used by scala.sys.process.ProcessBuilder when starting a process, as an alternative to scala.sys.process.ProcessIO, which can be more difficult to use. Note that a ProcessLogger will be used to create a ProcessIO anyway. The object BasicIO has some functions to do that.

Here is an example that counts the number of lines in the normal and error output of a process:

import scala.sys.process._

var normalLines = 0
var errorLines = 0
val countLogger = ProcessLogger(line => normalLines += 1,
                                line => errorLines += 1)
"find /etc" ! countLogger
See also


Value Members

implicit def builderToProcess(builder: JProcessBuilder): ProcessBuilder

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

Definition Classes

implicit def buildersToProcess[T](builders: collection.Seq[T])(implicit convert: (T) => Source): collection.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.

Definition Classes

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. For example:

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

def stderr: PrintStream

def stdin: InputStream

def stdout: PrintStream

implicit def stringSeqToProcess(command: collection.Seq[String]): 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

implicit def stringToProcess(command: String): ProcessBuilder

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

Definition Classes

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. 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

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.

object Process extends ProcessImpl with ProcessCreation

object ProcessBuilder extends ProcessBuilderImpl

object ProcessLogger

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Licensed under the Apache License, Version 2.0.