Command Line Arguments in F#

Command-line arguments are a common way to parameterize execution of programs. For example, dotnet run -- arg1 arg2 uses arg1 and arg2 as arguments to the program.

open System

[<EntryPoint>]
let main argv =
    // Environment.GetCommandLineArgs() provides access to raw command-line
    // arguments. Note that the first value in this array is the path to the
    // program, and argv holds the arguments to the program.
    let argsWithProg = Environment.GetCommandLineArgs()
    let argsWithoutProg = argv

    // You can get individual args with normal indexing.
    let arg = if argv.Length > 2 then argv.[2] else ""

    printfn "%A" argsWithProg
    printfn "%A" argsWithoutProg
    printfn "%s" arg

    0 // return an integer exit code

To experiment with command-line arguments, it’s best to build the program first:

$ dotnet build
$ dotnet run -- a b c d
[|"/path/to/program"; "a"; "b"; "c"; "d"|]
[|"a"; "b"; "c"; "d"|]
c

In F#, the argv parameter in the main function already excludes the program name, which is different from some other languages. If you need the program name, you can use Environment.GetCommandLineArgs().

Next, we’ll look at more advanced command-line processing with flags.

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Command Line Arguments in F#