Scala 3/scala.sys.process/ProcessBuilder

ProcessBuilder

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 `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.

#| 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.

Companion:: object
Source:: ProcessBuilder.scala

trait Sink

trait Source

class Object

trait Matchable

class Any

Value members

Abstract methods

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.

Source:: ProcessBuilder.scala

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.

Source:: ProcessBuilder.scala

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.

Source:: ProcessBuilder.scala

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.

Source:: ProcessBuilder.scala

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.

Source:: ProcessBuilder.scala

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

Source:: ProcessBuilder.scala

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.

Source:: ProcessBuilder.scala

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.

Source:: ProcessBuilder.scala

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.

Source:: ProcessBuilder.scala

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

Source:: ProcessBuilder.scala

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.

Source:: ProcessBuilder.scala

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

Source:: ProcessBuilder.scala

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

Source:: ProcessBuilder.scala

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.

Source:: ProcessBuilder.scala