
- Scala - Home
- Scala - Overview
- Scala - Features
- Scala - Environment Setup
- Scala - Build Tool (SBT)
- Scala - REPL
- Scala - Dot & Dotty
- Scala - Basic Syntax
- Scala - Hello World Program
- Scala - Identifiers
- Scala - Keywords
- Scala - Comments
- Scala - Code Blocks
- Scala - Semicolon
- Scala - Constructs
- Scala - Expressions
- Scala - Input and Output
- Scala - Optional Braces
- Scala - Underscore (_)
- Data Types and Variables
- Scala - Data Types
- Scala - Type Bounds
- Scala - Context Bound
- Scala - Variances
- Scala - Type Hierarchy
- Scala - Variables
- Scala - Variable Scopes
- Scala - Literals
- Scala - Numeric Types
- Scala - Boolean Types
- Scala - Char Type
- Scala - Unit Types
- Scala - Strings
- Scala - Arrays
- Scala - Null Type
- Scala - Nothing
- Scala - Any Type
- Scala - AnyRef Type
- Scala - Unified Types
- Scala - Dates and Times
- Scala - Ranges
- Scala - Multidimensional Arrays
- Scala - WrappedArray
- Scala - StringBuilder
- Scala - String Interpolation
- Scala - StringContext
- Scala - Type Casting
- Scala var vs val
- Scala Operators
- Scala - Operators
- Scala - Rules for Operators
- Scala - Arithmetic Operators
- Scala - Relational Operators
- Scala - Logical Operators
- Scala - Bitwise Operators
- Scala - Assignment Operators
- Scala - Operators Precedence
- Scala - Symbolic Operators
- Scala - Range Operator
- Scala - String Concatenation Operator
- Scala Conditional Statements
- Scala - IF ELSE
- Scala - IF-ELSE-IF-ELSE Statement
- Scala - Nested IF-ELSE Statement
- Scala Loop Statements
- Scala - Loop Statements
- Scala - while Loop
- Scala - do-while Loop
- Scala - Nested Loops
- Scala - for Loop
- Scala - break Statement
- Scala - yield Keyword
- Scala Classes & Objects
- Scala - Classes & Objects
- Scala - Constructors
- Scala - Auxiliary Constructor
- Scala - Primary Constructor
- Scala - This Keyword
- Scala - Nested Classes
- Scala - Getters and Setters
- Scala - Object Private Fields
- Scala - Singleton Object
- Scala - Companion Objects
- Scala - Creating Executable Programs
- Scala - Stateful Object
- Scala - Enumerations
- Scala - Polymorphism
- Scala - Access Modifiers
- Scala - Apply Method
- Scala - Update Methods
- Scala - UnapplySeq Method
- Scala - Inheritance
- Scala - Extending a Class
- Scala - Method Overloading
- Scala - Method Overriding
- Scala - Generic Classes
- Scala - Generic Functions
- Scala - Superclass Construction
- Scala Methods & Functions
- Scala - Methods
- Scala - Functions
- Scala - Methods vs Functions
- Scala - Main Methods
- Scala - Functions Call-by-Name
- Scala - Functions with Named Arguments
- Scala - Function with Variable Arguments
- Scala - Recursion Functions
- Scala - Default Parameter Values
- Scala - Functions without Parameters
- Scala - Implicit Parameters
- Scala - Higher-Order Functions
- Scala - Nested Functions
- Scala - Extension Methods
- Scala - Anonymous Functions
- Partially Applied Functions
- Scala - Lazy Val
- Scala - Pure Function
- Scala - Currying Functions
- Scala - Control Abstractions
- Scala - Corecursion
- Scala - Unfold
- Scala - Tail Recursion
- Scala - Infinite Sequences
- Scala - Dynamic Invocation
- Scala - Lambda Expressions
- Scala - Polymorphic Functions
- Scala Collections
- Scala - Collections
- Mutable and Immutable Collections
- Scala - Lists
- Scala - Sets
- Scala - Maps
- Scala - TreeMap
- Scala - SortedMap
- Scala - Tuples
- Scala - Iterators
- Scala - Options
- Scala - NumericRange
- Scala - Infinite Streams
- Scala - Parallel Collections
- Scala Advanced Types
- Scala - Union Types
- Scala - Intersection Types
- Scala - Type Aliases
- Scala - Structural Types
- Scala - Match Expression
- Scala - Singleton Type Operator
- Scala - Abstract Types
- Scala - Dependent Types
- Scala - Abstract Type Bounds
- Scala - Higher-Kinded Types
- Scala - Opaque Type Alias
- Scala - Path-Dependent Types
- Scala - Type Lambdas
- Scala - Type Inference
- Scala - Algebraic Data Types
- Scala Pattern Matching
- Scala - Pattern Matching
- Scala - Guards
- Scala - Variables in Patterns
- Scala - Type Patterns
- Scala - The Matchable Trait
- Scala - Matching Arrays
- Scala - Matching Lists
- Scala - Matching Tuples
- Scala - Exception Handling
- Scala - Extractors
- Scala - Pattern Bindings
- Scala - Regular Expressions
- Scala - Case Classes
- Scala - Partial Functions
- Scala - Packaging and Imports
- Scala - Implicit Imports
- Scala - Export Clauses
- Scala - Nested Packages
- Scala - Chained Packages
- Scala - Package Objects
- Scala Files I/O
- Scala - Files I/O
- Scala - Writing Files
- Scala - Listing Files
- Scala - Deleting Directories
- Scala - Check File Exists
- Scala Advanced Concepts
- Scala - Closures
- Scala - Futures
- Scala - Promises
- Scala - Traits
- Scala - Trait Mixins
- Scala - Layered Traits
- Scala - Trait Linearization
- Scala - Sealed Traits
- Scala - Transparent Traits
- Scala - Process Management
- Scala - Scaladoc
- Scala - Literal Type Arithmetic
- Scala - Inline keyword
- Scala - Def, Var & Val
- Scala - Dropped Features
- Scala Unit Testing
- Scala - Unit Testing
- Scala - uTest
- Scala - MUnit
- Scala - ScalaTest Runner
- Scala - ScalaMock
- Scala - JUnit
- Scala - Mocking
- Scala - BDD Testing
Scala - Process Management
You can manage external processes using the scala.sys.process package. You can manage execution, chaining, and management of external commands, just similar to Unix shells. So you can execute commands, pipe outputs, inputs/outputs etc. It is built on Java Process and ProcessBuilder classes.
There are various tasks, like automating system operations, integrating with other software, performing various utility functions, etc., that need to be performed. So we use Scala concurrency tools (i.e., Futures and the Akka actor system) for concurrent process execution. You can ensure reliable error handling, robust logging, and scalable concurrent programming.
Core Concepts
ProcessBuilder
ProcessBuilder trait is the cornerstone of process control in Scala. It represents at least one external process that can be executed. There are various methods to create instances of ProcessBuilder using Process objects and implicit conversions.
Process Execution
You can execute a process in various ways. Some of these given as follows -
- !: It blocks until the process exits and returns the exit code.
- !!: It blocks until the process exits and returns the output as a String.
- lazyLines: It returns a LazyList[String] that allows lazy reading of the process output.
- run: It executes the process concurrently and returns a Process object for control.
Process Composition
You can compose processes similarly to Unix shell commands. Some of these given as follows -
- #|: It pipes the output of one process as the input to another.
- ###: It executes processes sequentially.
- #&&: It executes the second process only if the first succeeds.
- #||: It executes the second process only if the first fails.
Example
import scala.sys.process._ // Execute `ls` and pipe its output to `grep` val command = Seq("ls") #| Seq("grep", ".scala") val outputBuffer = new StringBuffer() val exitCode = command ! ProcessLogger(outputBuffer.append(_)) // Check the exit code if (exitCode == 0) { println(outputBuffer.toString) } else { println(s"Command failed with exit code $exitCode") }
Save the above program in Demo.scala. Use the following commands to compile and execute this program.
Command
> scalac Demo.scala > scala Demo
Output
Command failed with exit code 1
Handling Input/Output
You can handle input and output effectively. It is important for process control. Scala provides several abstractions -
- ProcessIO: It offers low-level control over process input and output streams.
- ProcessLogger: It is a higher-level abstraction to capture and log process output.
- BasicIO: There are helper methods for creating ProcessIO.
Example
import java.io.{File, InputStream} import scala.sys.process._ def customProcessHandling(name: String): Int = { val cat = Seq("cat", name) var count = 0 def byteCounter(input: InputStream): Unit = { while (input.read() != -1) count += 1 input.close() } val p = cat run new ProcessIO(_.close(), byteCounter, _.close()) p.exitValue() } println(s"Byte count: ${customProcessHandling("example.txt")}")
Save the above program in Demo.scala. Use the following commands to compile and execute this program.
Command
> scalac Demo.scala > scala Demo
Output
Byte count:
Advanced I/O Handling
You can control process I/O using files and URLs directly.
Example
import java.io.File import java.net.URL import scala.sys.process._ // Define the URL and the output file val url = new URL("http://www.scala-lang.org/") val file = new File("scala-lang.html") // Download the webpage and save it to the file val process = url #> file // Execute the process process.!
Save the above program in Demo.scala. Use the following commands to compile and execute this program.
Command
> scalac Demo.scala > scala Demo
It will execute successfully.
You can find concurrency an important aspect of process control. Scala has various tools for managing concurrent tasks.
Controlling Concurrency with Futures
You can control concurrency with Scala Future API and ExecutionContext.
Example
import scala.concurrent.{ExecutionContext, Future} import java.util.concurrent.Executors // Execution context with a fixed thread pool implicit val ec: ExecutionContext = ExecutionContext.fromExecutor(Executors.newFixedThreadPool(10)) // Futures for concurrent tasks val futures = (1 to 10).map { i => Future { // Simulate processing Thread.sleep(10) i } } // Handle the results of the futures Future.sequence(futures).foreach(results => println(s"Processed results: $results"))
Save the above program in Demo.scala. Use the following commands to compile and execute this program.
Command
> scalac Demo.scala > scala Demo
Output
Processed results: Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
Actors for Concurrent Processing
You can use actors to provide a lightweight model for concurrent processing. You can use Akka library for actor-based concurrency in Scala.
Note that you must have this dependency in your build.sbt file to execute below example -
import Dependencies._ ThisBuild / scalaVersion := "2.13.14" lazy val root = (project in file(".")) .settings( name := "Demo", version := "0.1", libraryDependencies ++= Seq( "com.typesafe.akka" %% "akka-actor" % "2.6.14" ) )
Example
import akka.actor.{Actor, ActorSystem, Props} import scala.concurrent.duration._ import scala.concurrent.Await case class ProcessEvent(data: String) class EventProcessor extends Actor { def receive = { case ProcessEvent(data) => // Simulate processing println(s"Processing: $data") } } object Main extends App { // Set up the actor system val system = ActorSystem("ProcessSystem") val processor = system.actorOf(Props[EventProcessor], "processor") // Send a message to the actor processor ! ProcessEvent("Sample data") // Terminate the system after a short delay to ensure the message is processed Await.result(system.terminate(), 5.seconds) }
Save the above program in Demo.scala. Use the following commands to compile and execute this program.
Command
> scalac Demo.scala > scala Demo
Output
Processing: Sample data
Future and ExecutionContext
You can control the number of concurrent tasks with ExecutionContext.
Example
import scala.concurrent.{ExecutionContext, Future} import java.util.concurrent.Executors import scala.util.{Success, Failure} object Demo extends App { // Execution context with a fixed thread pool implicit val ec: ExecutionContext = ExecutionContext.fromExecutor(Executors.newFixedThreadPool(5)) // Tasks for concurrent processing val tasks = (1 to 20).map { i => Future { // Simulate some computation Thread.sleep(200) i * i } } // Handle the results of the tasks val aggregatedResult = Future.sequence(tasks) aggregatedResult.onComplete { case Success(result) => println(s"Results: $result") case Failure(e) => println(s"Error occurred: $e") } // Keep the application running to allow async processing to complete Thread.sleep(5000) }
Save the above program in Demo.scala. Use the following commands to compile and execute this program.
Command
> scalac Demo.scala > scala Demo
Output
Results: Vector(1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400)
Using ProcessLogger for Enhanced Output Handling
You can use ProcessLogger to capture and handle process output.
Example
import scala.sys.process._ object ProcessLoggerExample extends App { val buffer = new StringBuffer() val logger = ProcessLogger(buffer.append(_)) // Execute a command and log its output "cmd /c dir".! (logger) println(s"Process output: ${buffer.toString}") }
Save the above program in Demo.scala. Use the following commands to compile and execute this program.
Command
> scalac Demo.scala > scala Demo
It will execute successfully.
Combining Processes
You can combine multiple processes to create complex workflows.
Example
import scala.sys.process._ object Demo extends App { val buffer = new StringBuffer() val logger = ProcessLogger(buffer.append(_)) // Combine the commands using cmd /c and findstr for Windows val combined = Seq("cmd", "/c", "echo Hello && echo Hello | findstr Hello && echo Success || echo Failure") // Execute the combined process combined ! logger println(s"Process output: ${buffer.toString}") }
Save the above program in Demo.scala. Use the following commands to compile and execute this program.
Command
> scalac Demo.scala > scala Demo
Output
Process output: Hello Hello Success
Notes
- You can manage external processes using the sys.process package. So you can execute, chaining, and manage commands similar to Unix shells.
- It is built on Java Process and ProcessBuilder classes.
- There are tools like Scala concurrency (Futures and Akka actor system) for reliable error handling, robust logging, and scalable concurrent programming.
- You can execute a process using various methods, like ! for blocking until exit, !! for capturing output as a string, and run for concurrent execution.
- You can compose processes similarly to Unix shell commands using operators like #| for piping, ### for sequential execution, and #&& for conditional execution.
- There are abstractions for handling I/O, like ProcessIO for low-level control, ProcessLogger for logging, and BasicIO for helper methods.
- You can use concurrency tools like Future and ExecutionContext for managing concurrent tasks and Akka actors for lightweight concurrent processing models.