65. Command-Line Arguments

When main declares an args: Array<String> parameter, the runtime fills it with the words you pass on the command line (after -- when launching with kotlin run). Everything arrives as a String, so you convert and validate yourself.

fun main(args: Array<String>) {
    if (args.isEmpty()) {
        println("Usage: greet <name> [age] [--loud]")
        return
    }

    val loud = "--loud" in args                       // simple flag check
    val positional = args.filterNot { it.startsWith("--") }

    val name = positional[0]
    val age = positional.getOrNull(1)?.toIntOrNull()  // optional, safely parsed

    var greeting = "Hello, $name!"
    if (age != null) greeting += " You are $age."
    if (loud) greeting = greeting.uppercase()

    println(greeting)
    println("received ${args.size} argument(s)")
}

Separating flags from positional arguments keeps simple CLIs readable. For anything richer (subcommands, typed options) reach for a library such as kotlinx-cli or clikt.

Running it:

$ kotlin run -- Alice 42
Hello, Alice! You are 42.
received 2 argument(s)

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