scala.sys.process

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.

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.

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 `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("https://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.

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.

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.

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

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") !

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("https://www.scala-lang.org") #> new java.io.File("fixed.html") !