scopt

scopt is a little command line options parsing library.

Sonatype

libraryDependencies += "com.github.scopt" %% "scopt" % "X.Y.Z"

See the Maven Central badge above.

Usage

scopt 4.x provides two styles of constructing a command line option parser: functional DSL and object-oriented DSL. Either case, first you need a case class that represents the configuration:

import java.io.File
case class Config(
    foo: Int = -1,
    out: File = new File("."),
    xyz: Boolean = false,
    libName: String = "",
    maxCount: Int = -1,
    verbose: Boolean = false,
    debug: Boolean = false,
    mode: String = "",
    files: Seq[File] = Seq(),
    keepalive: Boolean = false,
    jars: Seq[File] = Seq(),
    kwargs: Map[String, String] = Map())

During the parsing process, a config object is passed around as an argument into action callbacks.

Functional DSL

Here's how you create a scopt.OParser[Config].

import scopt.OParser
val builder = OParser.builder[Config]
val parser1 = {
  import builder._
  OParser.sequence(
    programName("scopt"),
    head("scopt", "4.x"),
    // option -f, --foo
    opt[Int]('f', "foo")
      .action((x, c) => c.copy(foo = x))
      .text("foo is an integer property"),
    // more options here...
  )
}

// OParser.parse returns Option[Config]
OParser.parse(parser1, args, Config()) match {
  case Some(config) =>
    // do something
  case _ =>
    // arguments are bad, error message will have been displayed
}

See Scaladoc API and the rest of this page for the details on various builder methods.

Abstracting over effects

By default, scopt emits output when needed to stderr and stdout. This is expected behavior when using scopt to process arguments for your stand-alone application. However, if your application requires parsing arguments while not producing output directly, you may wish to intercept the side effects. Use OParser.runParser(...) to do so:

// OParser.runParser returns (Option[Config], List[OEffect])
OParser.runParser(parser1, args, Config()) match {
  case (result, effects) =>
    OParser.runEffects(effects, new DefaultOEffectSetup {
      // override def displayToOut(msg: String): Unit = Console.out.println(msg)
      // override def displayToErr(msg: String): Unit = Console.err.println(msg)
      // override def reportError(msg: String): Unit = displayToErr("Error: " + msg)
      // override def reportWarning(msg: String): Unit = displayToErr("Warning: " + msg)
      
      // ignore terminate
      override def terminate(exitState: Either[String, Unit]): Unit = ()
    })

    result match {
      Some(config) =>
        // do something
      case _ =>
        // arguments are bad, error message will have been displayed
    }
}

Full example

import scopt.OParser
val builder = OParser.builder[Config]
val parser1 = {
  import builder._
  OParser.sequence(
    programName("scopt"),
    head("scopt", "4.x"),
    opt[Int]('f', "foo")
      .action((x, c) => c.copy(foo = x))
      .text("foo is an integer property"),
    opt[File]('o', "out")
      .required()
      .valueName("<file>")
      .action((x, c) => c.copy(out = x))
      .text("out is a required file property"),
    opt[(String, Int)]("max")
      .action({ case ((k, v), c) => c.copy(libName = k, maxCount = v) })
      .validate(x =>
        if (x._2 > 0) success
        else failure("Value <max> must be >0"))
      .keyValueName("<libname>", "<max>")
      .text("maximum count for <libname>"),
    opt[Seq[File]]('j', "jars")
      .valueName("<jar1>,<jar2>...")
      .action((x, c) => c.copy(jars = x))
      .text("jars to include"),
    opt[Map[String, String]]("kwargs")
      .valueName("k1=v1,k2=v2...")
      .action((x, c) => c.copy(kwargs = x))
      .text("other arguments"),
    opt[Unit]("verbose")
      .action((_, c) => c.copy(verbose = true))
      .text("verbose is a flag"),
    opt[Unit]("debug")
      .hidden()
      .action((_, c) => c.copy(debug = true))
      .text("this option is hidden in the usage text"),
    help("help").text("prints this usage text"),
    arg[File]("<file>...")
      .unbounded()
      .optional()
      .action((x, c) => c.copy(files = c.files :+ x))
      .text("optional unbounded args"),
    note("some notes." + sys.props("line.separator")),
    cmd("update")
      .action((_, c) => c.copy(mode = "update"))
      .text("update is a command.")
      .children(
        opt[Unit]("not-keepalive")
          .abbr("nk")
          .action((_, c) => c.copy(keepalive = false))
          .text("disable keepalive"),
        opt[Boolean]("xyz")
          .action((x, c) => c.copy(xyz = x))
          .text("xyz is a boolean property"),
        opt[Unit]("debug-update")
          .hidden()
          .action((_, c) => c.copy(debug = true))
          .text("this option is hidden in the usage text"),
        checkConfig(
          c =>
            if (c.keepalive && c.xyz) failure("xyz cannot keep alive")
            else success)
      )
  )
}

// OParser.parse returns Option[Config]
OParser.parse(parser1, args, Config()) match {
  case Some(config) =>
    // do something
  case _ =>
    // arguments are bad, error message will have been displayed
}

The above generates the following usage text:

scopt 4.x
Usage: scopt [update] [options] [<file>...]

  -f, --foo <value>        foo is an integer property
  -o, --out <file>         out is a required file property
  --max:<libname>=<max>    maximum count for <libname>
  -j, --jars <jar1>,<jar2>...
                           jars to include
  --kwargs k1=v1,k2=v2...  other arguments
  --verbose                verbose is a flag
  --help                   prints this usage text
  <file>...                optional unbounded args
some notes.

Command: update [options]
update is a command.
  -nk, --not-keepalive     disable keepalive
  --xyz <value>            xyz is a boolean property

Options

Command line options are defined using opt[A]('f', "foo") or opt[A]("foo") where A is any type that is an instance of Read typeclass.

• Unit works as a plain flag --foo or -f
• Int, Long, Double, String, BigInt, BigDecimal, java.io.File, java.nio.file.Path, java.net.URI, and java.net.InetAddress accept a value like --foo 80 or --foo:80
• Boolean accepts a value like --foo true or --foo:1
• java.util.Calendar accepts a value like --foo 2000-12-01
• scala.concurrent.duration.Duration accepts a value like --foo 30s
• A pair of types like (String, Int) accept a key-value like --foo:k=1 or -f k=1
• A Seq[File] accepts a string containing comma-separated values such as --jars foo.jar,bar.jar
• A Map[String, String] accepts a string containing comma-separated pairs like --kwargs key1=val1,key2=val2

This could be extended by defining Read instances in the scope. For example,

object WeekDays extends Enumeration {
  type WeekDays = Value
  val Mon, Tue, Wed, Thur, Fri, Sat, Sun = Value
}
implicit val weekDaysRead: scopt.Read[WeekDays.Value] =
  scopt.Read.reads(WeekDays withName _)

By default these options are optional.

Short options

For plain flags (opt[Unit]) short options can be grouped as -fb to mean --foo --bar.

opt accepts only a single character, but using abbr("ab") a string can be used too:

opt[Unit]("no-keepalive").abbr("nk").action( (x, c) => c.copy(keepalive = false) )

Help, Version, and Notes

There are special options with predefined action called help("help") and version("version"), which prints usage text and header text respectively. When help("help") is defined, parser will print out short error message when it fails instead of printing the entire usage text.

note("...") is used add given string to the usage text.

Arguments

Command line arguments are defined using arg[A]("<file>"). It works similar to options, but instead it accepts values without -- or -. By default, arguments accept a single value and are required.

arg[String]("<file>...")

Occurrence

Each opt/arg carries occurrence information minOccurs and maxOccurs. minOccurs specify at least how many times an opt/arg must appear, and maxOccurs specify at most how many times an opt/arg may appear.

Occurrence can be set using the methods on the opt/arg:

opt[String]('o', "out").required()
opt[String]('o', "out").required().withFallback(() => "default value")
opt[String]('o', "out").minOccurs(1) // same as above
arg[String]("<mode>").optional()
arg[String]("<mode>").minOccurs(0) // same as above
arg[String]("<file>...").optional().unbounded()
arg[String]("<file>...").minOccurs(0).maxOccurs(1024) // same as above

Visibility

Each opt/arg can be hidden from the usage text using hidden() method:

opt[Unit]("debug")
  .hidden()
  .action( (_, c) => c.copy(debug = true) )
  .text("this option is hidden in the usage text")

Validation

Each opt/arg can carry multiple validation functions.

opt[Int]('f', "foo")
  .action( (x, c) => c.copy(intValue = x) )
  .validate( x =>
    if (x > 0) success
    else failure("Option --foo must be >0") )
  .validate( x => failure("Just because") )

The first function validates if the values are positive, and the second function always fails.

Check configuration

Consistency among the option values can be checked using checkConfig.

checkConfig( c =>
  if (c.keepalive && c.xyz) failure("xyz cannot keep alive")
  else success )

These are called at the end of parsing.

Commands

Commands may be defined using cmd("update"). Commands could be used to express git branch kind of argument, whose name means something. Using children method, a command may define child opts/args that get inserted in the presence of the command. To distinguish commands from arguments, they must appear in the first position within the level. It is generally recommended to avoid mixing args both in parent level and commands to avoid confusion.

cmd("update")
  .action( (_, c) => c.copy(mode = "update") )
  .text("update is a command.")
  .children(
    opt[Unit]("not-keepalive").abbr("nk").action( (_, c) =>
      c.copy(keepalive = false) ).text("disable keepalive"),
    opt[Boolean]("xyz").action( (x, c) =>
      c.copy(xyz = x) ).text("xyz is a boolean property"),
    checkConfig( c =>
      if (c.keepalive && c.xyz) failure("xyz cannot keep alive")
      else success )
  )

In the above, update test.txt would trigger the update command, but test.txt update won't.

Commands could be nested into another command as follows:

cmd("backend")
  .text("commands to manipulate backends:\n")
  .action( (x, c) => c.copy(flag = true) )
  .children(
    cmd("update").children(
      arg[String]("<a>").action( (x, c) => c.copy(a = x) )
    )
  )

Object-oriented DSL, immutable parsing

Here's the object-oriented DSL that's mostly source-compatible with scopt 3.x.

Create a parser by extending scopt.OptionParser[Config]. See Scaladoc API for the details on various builder methods.

val parser = new scopt.OptionParser[Config]("scopt") {
  head("scopt", "4.x")

  opt[Int]('f', "foo")
    .action((x, c) => c.copy(foo = x))
    .text("foo is an integer property")

  opt[File]('o', "out")
    .required()
    .valueName("<file>")
    .action((x, c) => c.copy(out = x))
    .text("out is a required file property")
}

// parser.parse returns Option[C]
parser.parse(args, Config()) match {
  case Some(config) =>
    // do stuff

  case None =>
    // arguments are bad, error message will have been displayed
}

Object-oriented DSL, mutable parsing

Create a scopt.OptionParser[Unit] and customize it with the options you need, passing in functions to process each option or argument. Use foreach instead of action.

Mutable parsing has changed from scopt 3.x. When upgrading from 3.x, replace parser.parse(args) with parser.parse(args, ()).isDefined.

val parser = new scopt.OptionParser[Unit]("scopt") {
  head("scopt", "4.x")

  opt[Int]('f', "foo")
    .foreach( x => c = c.copy(foo = x) )
    .text("foo is an integer property")

  opt[File]('o', "out")
    .required()
    .valueName("<file>")
    .foreach( x => c = c.copy(out = x) )
    .text("out is a required file property")
}
if (parser.parse(args, ()).isDefined) {
  // do stuff
}
else {
  // arguments are bad, usage message will have been displayed
}

Advanced: showUsageOnError

When help("help") is defined, parser will print out short error message when it fails instead of printing the entire usage text.

This behavior could be changed by overriding showUsageOnError as follows:

import scopt.{ OParserSetup, DefaultOParserSetup }
val setup: OParserSetup = new DefaultOParserSetup {
  override def showUsageOnError = Some(true)
}
val result = OParser.parse(parser1, args, Config(), setup)

Advanced: Rendering mode

scopt 3.5.0 introduced rendering mode, and adopted two-column rendering of the usage text by default. To switch back to the older one-column rendering override the renderingMode method:

import scopt.{ OParserSetup, DefaultOParserSetup }
val setup: OParserSetup = new DefaultOParserSetup {
  override def renderingMode = scopt.RenderingMode.OneColumn
}
val result = OParser.parse(parser1, args, Config(), setup)

Advanced: Termination handling

By default, when the --help or --version are invoked, they call sys.exit(0) after printing the help or version information. If this is not desired (e.g. testing purposes), you can override the terminate(exitState: Either[String, Unit]) method:

import scopt.{ OParser, DefaultOEffectSetup }

OParser.runParser(parser1, args, Config()) match {
  case (result, effects) =>
    OParser.runEffects(effects, new DefaultOEffectSetup {
      // ignore terminate
      override def terminate(exitState: Either[String, Unit]): Unit = ()
    })

    result match {
      Some(config) =>
        // do something
      case _ =>
        // arguments are bad, error message will have been displayed
    }
}

Building

sbt to build scopt.

License

MIT License.

Credits

• January 13, 2008: Aaron Harnly creates aaronharnly/scala-options.
• December 1, 2009: Tim Perrett introduces it as a gist on Parsing command lines argument in a "scalaesque" way.
• January 10, 2010: James Strachan takes the code, adds usage text, sbt build, etc and creates jstrachan/scopt, which is also mentioned in Scala CLI Library?.
• March 4th, 2010: Eugene Yokota joins scopt project, improves usage text, and adds support for key=value option and argument list.
• May 27, 2011: scopt 1.0.0 is released to scala-tools.org.
• March 18, 2012: Eugene adds immutable parser, forks the project to scopt/scopt, and releases scopt 2.0.0.
• June 7, 2013: Eugene rewrites scopt from scratch for polymorphic options, and releases scopt 3.0.0.

Changes

See notes.