deanwampler / command-line-arguments

A simple Scala library for processing command-line arguments


Command Line Arguments

Dean Wampler, Ph.D. @deanwampler

This is a Scala library for handling command-line arguments. It has few dependencies on other libraries, Parboiled, for parsing, and ScalaTest and ScalaCheck, for testing. So its footprint is small.


This library is built for Scala 2.11.12 and 2.12.6, the default (2.10 support was dropped in the 0.5.0 release). Artifacts are published to Sonatype's OSS service. You'll need the following settings.

resolvers ++= Seq(

scalaVersion := "2.12.6"  // or 2.11.8

libraryDependencies ++= Seq(
  "com.concurrentthought.cla" %% "command-line-arguments"          % "0.5.0"
  "com.concurrentthought.cla" %% "command-line-arguments-examples" % "0.5.0"

The examples can be omitted.


The included com.concurrentthought.cla.CLASampleMain shows two different idiomatic ways to set up and use the API.

The simplest approach parses a multi-line string to specify the command-line arguments com.concurrentthought.cla.Args:

import com.concurrentthought.cla._

object CLASampleMain {

  def main(argstrings: Array[String]) = {
    val initialArgs: Args = """
      |run-main CLASampleMain [options]
      |Demonstrates the CLA API.
      |   -i | --in  | --input      string              Path to input file.
      |  [-o | --out | --output     string=/dev/null]   Path to output file.
      |  [-l | --log | --log-level  int=3]              Log level to use.
      |  [-p | --path               path]               Path elements separated by ':' (*nix) or ';' (Windows).
      |  [--things                  seq([-|])]          String elements separated by '-' or '|'.
      |  [-q | --quiet              flag]               Suppress some verbose output.
      |                             others              Other arguments.
      |Note that --input and "others" are required.

    // Process the input arguments. If help requested or an error occurs,
    // a message is written to stdout and the program exits with an error code.
    // Default arguments for `process` aren't shown. See also Args#parse() for
    // more flexible handling.
    val finalArgs: Args = initialArgs.process(argstrings)

    // If here, successfully parsed the args and none where "--help" or "-h".

The Scaladocs comments for the cla package explain the format and its limitations, but hopefully most of the format is reasonable intuitive from the example.

The first and last lines in the string that don't have leading whitespace are interpreted as lines to show as part of the corresponding help message. It's a good idea to use the first line to show an example of how to invoke the program.

Next come the command-line options, one per line. Each must start with whitespace, followed by zero or more flags separated by |. There can be at most one option that has no flags. It is used to provide a help message for how command-line tokens that aren't associated with flags will be interpreted. (Note that the library will still handle these tokens whether or not you specify a line like this.)

To indicate that an option can be omitted by the user (i.e., it's truly optional), the flags and name must be wrapped in [...]. Otherwise, the user must specify the option explicitly on the command line. However, if a default value is specified (discussed next), it makes an option optional anyway. The purpose of the optional feature is to indicate to the user which arguments are required and to automatically report missing arguments as errors.

In this example, all are optional except for the --input and others arguments.

The center "column" specifies the type of the option. All but the flag and ~flag types accept an optional default value, which is indicated with an equals = sign. The following "types" are supported:

String Interpretation Corresponding Helper Method
flag Boolean value Opt.flag
~flag Boolean value Opt.flag
string String value Opt.string
byte Byte value Opt.byte
char Char value Opt.char
int Int value
long Long value Opt.long
float Float value Opt.float
double Double value Opt.double
seq Seq[String] [1] Opt.seqString
path "path-like" Seq[String] [1] Opt.path
other Only allowed for the single, no-flags case Args.remainingOpt

1: Both path and seq split an argument using the delimiter regular expression. For path, this is the platform-specific path separator, given by sys.props.getOrElse("path.separator", ":"). It is designed for class paths, etc. For seq, you must provide the delimiter regular expression using a suffix of the form (delimRE). In the example above, the regex is [-|] (split on either - or |).

Both flag and ~flag represent Boolean flags where no default value can be supplied (e.g., --help). The value corresponding to a flag defaults to false if the user doesn't invoke the flag on the command line, ~flag ("tilde" or "not" flag) defaults to true.

So, when an option expects something other than a String, the token given on the command line (or as a default value) will be parsed into the correct type, with error handling captured in the Args.failures field.

Finally, the rest of the text on a line is the help message for the option.

Before discussing the process method shown, let's see two alternative, programmatic ways to declare Args using the API:

  def main2(argstrings: Array[String]) = {
    val input  = Opt.string(
      name     = "input",
      flags    = Seq("-i", "--in", "--input"),
      help     = "Path to input file.",
      requiredFlag = true)
    val output = Opt.string(
      name     = "output",
      flags    = Seq("-o", "--out", "--output"),
      default  = Some("/dev/null"),
      help     = "Path to output file.")
    val logLevel =
      name     = "log-level",
      flags    = Seq("-l", "--log", "--log-level"),
      default  = Some(3),
      help     = "Log level to use.")
    val path = Opt.path(
      name     = "path",
      flags    = Seq("-p", "--path"))
    val things = Opt.seqString(delimsRE = "[-|]")(
      name     = "things",
      flags    = Seq("--things"),
      help     = "String elements separated by '-' or '|'.")
    val others = Args.makeRemainingOpt(
      name     = "others",
      help     = "Other arguments",
      requiredFlag = true)

    val initialArgs = Args(
      "run-main CLASampleMain [options]",
      "Demonstrates the CLA API.",
      """Note that --input and "others" are required.""",
      Seq(input, output, logLevel, path, things, Args.quietFlag, others))

    val finalArgs: Args = initialArgs.process(argstrings)

Each option is defined using a com.concurrentthought.cla.Opt value. In this case, there are helper methods in the Opt companion object for constructing options where the values are strings or numbers. The string and int helpers are used here for String and Int arguments, respectively).

The arguments to each of these helpers (and also for Opt[V].apply() that they invoke) is the option name, used to retrieve the value later, a Seq of flags for command line invocation, an optional default value if the command-line argument isn't used (defaults to None), a help string (defaults to ""), and a boolean flag indicating whether or not the "option" is required (defaults to false, which is sort of the opposite behavior of the string DSL discussed previously).

There are also two helpers for command-line arguments that are strings that contain sequences of elements. We use one of them here, seqString, for a classpath-style argument, where the elements will be split into a Seq[String], using : and ; as delimiters; the first argument is a regular expression for the delimiter. If you want to support a path-like option, e.g., a CLASSPATH, there is another, even more specific helper, Opt.path, that handles the platform-specific value for the path-element separator.

There is also a more general seq[V] helper, where the string is first split, then parsed into V instances. See Opt.seq[V] for more details.

The first two arguments to the Args.apply() method provide help strings. The first shows how to run the application, e.g., run-main CLASampleMain as shown, or perhaps java -cp ..., etc. The string is arbitrary. The second string is an optional description of the program. Finally, a Seq[Opt[V]] specifies the actual options supported. Note that we didn't define a Flag for quiet, as in the first example, instead we used a built-in flag Args.quietFlag.

Here is a slightly more concise way to write the content in main2:

  def main3(argstrings: Array[String]) = {
    import Opt._
    import Args._
    val initialArgs = Args(
      "run-main CLASampleMain [options]",
      "Demonstrates the CLA API.",
      """Note that --input and "others" are required.""",
        string("input",     Seq("-i", "--in", "--input"),      None,              "Path to input file."),
        string("output",    Seq("-o", "--out", "--output"),    Some("/dev/null"), "Path to output file."),
        int(   "log-level", Seq("-l", "--log", "--log-level"), Some(3),           "Log level to use."),
        path(  "path",      Seq("-p", "--path"),               None),
               "things",    Seq("--things"),                   None,              "String elements separated by '-' or '|'."),
               "others",                                                          "Other arguments", true)))

    val finalArgs: Args = initialArgs.process(argstrings)

This is more concise, but perhaps harder to follow.

The Args#process first calls Args#parse on the user-specified arguments, which returns a new Args instance with updated values for each argument. However, if an error occurs or help is requested, process automatically prints a message and exits. This behavior is configurable by overriding default arguments. See also Args#parse() for more flexible handling.

You'll almost always want to include logic like this in your code that uses this library.

If --quiet wasn't specified, then you might print information about the argument values. We demonstrate this in the CLASampleMain program. where the showResults method prints all the options and the current values for them, either the defaults or the user-specified values.

  protected def showResults(parsedArgs: Args): Unit = {
    if (parsedArgs.getOrElse("quiet", false)) {
      println("(... I'm being very quiet...)")
    } else {
      // Print all the default values or those specified by the user.

      // Print all the values including repeats.

      // Repeat the "other" arguments (not associated with flags).
      println("\nYou gave the following \"other\" arguments: " +
        parsedArgs.remaining.mkString(", "))

What's the difference between printValues and printAllValues. They address the case where the user should be able to repeat some options, for example, multiple sources of input, while other examples should only be used once. To simplify handling, the API remembers all occurrences of an option on the command line. The method printAllValues and the corresponding getAll and getAllOrElse methods print or return all occurrences seen, respectively. So, if you want an option to be repeatable, retrieve the results with getAll or getAllOrElse. Otherwise, use get and getOrElse, which return the last occurrence of an option (or the default, if any). This supports the common practice in POSIX systems of allowing subsequent option occurrences to override previous occurrences on a command line.

Finally, we extract some other values and "use" them.

      showLogLevel(parsedArgs.getOrElse("log-level", 0))

  protected def showPathElements(path: Option[Seq[String]]) = path match {
    case None => println("No path elements to show!")
    case Some(seq) => println(s"Setting path elements to $seq")

  protected def showLogLevel(level: Int) =
    println(s"New log level: $level")

The get[V] method returns values of the expected type. It uses asInstanceOf[] internally, but it should never fail because the parsing process already converted the value to the correct type (and then put it in a Map[String,Any] used by get[V]).

Note that an advantage of getOrElse[V] is that its type parameter can be inferred due to the second argument.

Try running the following examples within SBT (run and run-main com.concurrentthought.cla.CLASampleMain do the same thing):

 run-main com.concurrentthought.cla.CLASampleMain -h
 run -h
 run --help
 run -i /in -o /out -l 4 -p a:b --things x-y|z foo bar baz
 run -i /in -o /out -l 4 -p a:b --things x-y|z foo bar baz --quiet
 run --in /in --out=/out -l=4 --path "a:b" --things=x-y|z foo bar baz

The last example mixes argflag value and argflag=value syntax, which of are both supported.

Try a few runs with unknown flags and other errors. Note the error handling that's done, such as when you omit a value expected by a flag, or you provide an invalid value, such as --log-level foo.