Command line interface

Command line interface

Creating your own CLI in Raku

Command line interface - an overview

The default command line interface of Raku scripts consists of three parts:

Parsing the command line parameters into a capture

This looks at the values in @*ARGS, interprets these according to some policy, and creates a capture object out of that. An alternative way of parsing may be provided by the developer or installed using a module.

Calling a provided MAIN subroutine using that capture

Standard multi dispatch is used to call the MAIN subroutine with the generated Capture object. This means that your MAIN subroutine may be a multi sub, each candidate of which is responsible for some part of processing the given command line arguments.

Creating / showing usage information if calling MAIN failed

If multi dispatch failed, then the user of the script should be informed as well as possible as to why it failed. By default, this is done by inspecting the signature of each MAIN candidate sub, and any associated Pod information. The result is then shown to the user on STDERR (or on STDOUT if --help was specified). An alternative way of generating the usage information may be provided by the developer or installed using a module.

sub MAIN

The sub with the special name MAIN will be executed after all relevant entry phasers (BEGIN, CHECK, INIT, PRE, ENTER) have been run and the mainline of the script has been executed. No error will occur if there is no MAIN sub: your script will then just have to do the work, such as argument parsing, in the mainline of the script.

Any normal exit from the MAIN sub will result in an exit code of 0, indicating success. Any return value of the MAIN sub will be ignored. If an exception is thrown that is not handled inside the MAIN sub, then the exit code will be 1. If the dispatch to MAIN failed, a usage message will be displayed on STDERR and the exit code will be 2.

The command line parameters are present in the @*ARGS dynamic variable and may be altered in the mainline of the script before the MAIN unit is called.

The signature of (the candidates of the multi) sub MAIN determines which candidate will actually be called using the standard multi dispatch semantics.

A simple example:

# inside file 'hello.raku' 
sub MAIN($name{
    say "Hello $name, how are you?"

If you call that script without any parameters, you get the following usage message:

$ raku hello.raku
  hello.raku <name>

However, if you give a default value for the parameter, running the script either with or without specifying a name will always work:

# inside file 'hello.raku' 
sub MAIN($name = 'bashful'{
    say "Hello $name, how are you?"
$ raku hello.raku
Hello bashfulhow are you?
$ raku hello.raku Liz
Hello Lizhow are you?

Another way to do this is to make sub MAIN a multi sub:

# inside file 'hello.raku' 
multi sub MAIN()      { say "Hello bashful, how are you?" }
multi sub MAIN($name{ say "Hello $name, how are you?"   }

Which would give the same output as the examples above. Whether you should use either method to achieve the desired goal is entirely up to you.

If you want to pass an indeterminate number of parameters to be dealt within sub MAIN, you can use slurpy parameters:

# inside file 'hello-all.raku' 
sub MAIN(*@all{ -> $name { say "Hello, " ~ $name } }
$ raku hello-all.raku peter paul mary

A more complicated example using a single positional and multiple named parameters, and also showing that where clauses can also be applied to MAIN arguments:

# inside "frobnicate.raku" 
sub MAIN(
  Str   $file where *.IO.f = 'file.dat',
  Int  :$length = 24,
  Bool :$verbose
    say $length if $length.defined;
    say $file   if $file.defined;
    say 'Verbosity ', ($verbose ?? 'on' !! 'off');

With file.dat present, this will work this way:

$ raku frobnicate.raku
Verbosity off

Or this way with --verbose:

$ raku frobnicate.raku --verbose
Verbosity on

If the file file.dat is not present, or you've specified another filename that doesn't exist, you would get the standard usage message created from introspection of the MAIN sub:

$ raku frobnicate.raku doesnotexist.dat
  frobnicate.raku [--length=<Int>] [--verbose] [<file>]

Although you don't have to do anything in your code to do this, it may still be regarded as a bit terse. But there's an easy way to make that usage message better by providing hints using pod features:

# inside "frobnicate.raku" 
sub MAIN(
  Str   $file where *.IO.f = 'file.dat',  #= an existing file to frobnicate 
  Int  :$length = 24,                     #= length needed for frobnication 
  Bool :$verbose,                         #= required verbosity 
    say $length if $length.defined;
    say $file   if $file.defined;
    say 'Verbosity ', ($verbose ?? 'on' !! 'off');

Which would improve the usage message like this:

$ raku frobnicate.raku doesnotexist.dat
  frobnicate.raku [--length=<Int>] [--verbose] [<file>]
    [<file>]          an existing file to frobnicate
    --length=<Int>    length needed for frobnication
    --verbose         required verbosity

From release 2021.03, values to single named arguments can be separated by spaces too. Consider a demo program with the following source:

subset name of Any where Str|True;
subset port of Str;
multi MAIN(
    name :$profile,    #= Write profile information to a file 
    port :$debug-port#= Listen for debugger connections on the specified port 
    Bool :v($verbose), #= Display verbose output 
multi MAIN("--process-files"*@images{}

This program generates the following usage message:

  demo [--profile[=name]] [--debug-port=<port>] [-v] <file>
  demo --process-files [<images> ...]
    --profile[=name]       Write profile information to a file
    --debug-port=<port>    Listen for debugger connections on the specified port
    -v                     Display verbose output

The following are valid ways to call demo:

demo --profile ~/foo
demo --profile=/tmp/bar ~/foo
demo --debug-port 4242 ~/foo
demo --debug-port=4242 ~/foo
demo -v ~/foo
demo --process-files *.jpg

These, however, are not valid

demo --profile /tmp/bar ~/foo
demo --debug-port ~/foo

The first is invalid because /tmp/bar and ~/foo are both parsed as positional arguments, which means demo was called with too many positional arguments. The second is invalid because ~/foo is parsed as an argument to --debug-port, and thus demo lacks the required positional argument.

Here's how it works; with Raku distinguishing between three types of options:

  • Boolean options (like -v), which never take an argument; they are ether present or absent.

  • Options with a mandatory argument (like --debug-port), which always take an argument. If you give them an argument with =, they will use that; if not, they'll take the following argument.

  • Options with an optional argument (like --profile), which are valid both with and without an argument. You can only give these arguments an option with the = syntax; if there is a space after the option, that means it was called without an argument.

And here's the signature that produces each type of argument:

  • Boolean options: A Bool type constraint. = Options with a mandatory argument: A type that does not .ACCEPT a Bool.

  • Options with an optional argument: A type that .ACCEPTS a True (because passing an option without an argument is equivalent to passing True)

As any other subroutine, MAIN can define aliases for its named parameters.

sub MAIN(
  Str   $file where *.IO.f = 'file.dat',  #= an existing file to frobnicate 
  Int  :size(:$length= 24,              #= length/size needed for frobnication 
  Bool :$verbose,                         #= required verbosity 
    say $length if $length.defined;
    say $file   if $file.defined;
    say 'Verbosity ', ($verbose ?? 'on' !! 'off');

In which case, these aliases will also be listed as alternatives with --help:

  frobnicate.raku [--size|--length=<Int>] [--verbose] [<file>]
    [<file>]                 an existing file to frobnicate
    --size|--length=<Int>    length needed for frobnication
    --verbose                required verbosity

Enumerations can be used in signatures with arguments converted automatically to its corresponding enum symbol:

enum Flag  (
    FLAG_FOO => 0b001,
    FLAG_BAR => 0b010,
    FLAG_BAZ => 0b100,
sub MAIN(Flag $flag = FLAG_FOO{
    say "Flagging $flag";

This will work correctly with

raku MAIN-enum.raku FLAG_BAR

but will die if called with something that is not a Flag.


It's possible to alter how arguments are processed before they're passed to sub MAIN {} by setting options in the %*SUB-MAIN-OPTS hash. Due to the nature of dynamic variables, it is required to set up the %*SUB-MAIN-OPTS hash and fill it with the appropriate settings. For instance:

  :named-anywhere,    # allow named variables at any location 
  # other possible future options / custom options 
sub MAIN ($a$b:$c:$d{
    say "Accepted!"

Available options are:


By default, named arguments passed to the program (i.e., MAIN) cannot appear after any positional argument. However, if %*SUB-MAIN-OPTS<named-anywhere> is set to a true value, named arguments can be specified anywhere, even after positional parameter. For example, the above program can be called with:

$ raku example.raku 1 --c=2 3 --d=4


When %*SUB-MAIN-OPTS<bundling> is set to a true value, single letter named arguments can be bundled together with a single dash. The following two commands are then equivalent:

$ raku example.raku -a -b -c
$ raku example.raku -abc

Bundled arguments can be understood as flags, that can neither be negated, nor assigned a value though:

$ raku example.raku -/a       # OK 
$ raku example.raku -a=asdf   # OK 
$ raku example.raku -abc=asdf # Error 
$ raku example.raku -/abc     # Error 

This option is only available starting in the 2020.10 release.

is hidden-from-USAGE

Sometimes you want to exclude a MAIN candidate from being shown in any automatically generated usage message. This can be achieved by adding a hidden-from-USAGE trait to the specification of the MAIN candidate you do not want to show. Expanding on an earlier example:

# inside file 'hello.raku' 
multi sub MAIN() is hidden-from-USAGE {
    say "Hello bashful, how are you?"
multi sub MAIN($name{  #= the name by which you would like to be called 
    say "Hello $name, how are you?"

So, if you would call this script with just a named variable, you would get the following usage:

$ raku hello.raku --verbose
  hello.raku <name> -- the name by which you would like to be called

Without the hidden-from-USAGE trait on the first candidate, it would have looked like this:

$ raku hello.raku --verbose
  hello.raku <name> -- the name by which you would like to be called

Which, although technically correct, doesn't read as well.

Unit-scoped definition of MAIN

If the entire program body resides within MAIN, you can use the unit declarator as follows (adapting an earlier example):

unit sub MAIN(
  Str   $file where *.IO.f = 'file.dat',
  Int  :$length = 24,
  Bool :$verbose,
);  # <- note semicolon here 
say $length if $length.defined;
say $file   if $file.defined;
say 'Verbosity ', ($verbose ?? 'on' !! 'off');
# rest of script is part of MAIN 

Note that this is only appropriate if you can get by with just a single (only) sub MAIN.


If no multi candidate of MAIN is found for the given command line parameters, the sub USAGE is called. If no such method is found, the compiler will output a default usage message.

#|(is it the answer) 
multi MAIN(Int $i{ say $i == 42 ?? 'answer' !! 'dunno' }
#|(divide two numbers) 
multi MAIN($a$b){ say $a/$b }
sub USAGE() {
    print Q:c:to/EOH/; 
    Usage: {$*PROGRAM-NAME} [number]
    Prints the answer or 'dunno'.

The default usage message is available inside sub USAGE via the read-only $*USAGE variable. It will be generated based on available sub MAIN candidates and their parameters. As shown before, you can specify an additional extended description for each candidate using a #|(...) Pod block to set WHY.

Intercepting CLI argument parsing (2018.10, v6.d and later)

You can replace or augment the default way of argument parsing by supplying a ARGS-TO-CAPTURE subroutine yourself, or by importing one from any of the Getopt modules available in the ecosystem.


The ARGS-TO-CAPTURE subroutine should accept two parameters: a Callable representing the MAIN unit to be executed (so it can be introspected if necessary) and an array with the arguments from the command line. It should return a capture object that will be used to dispatch the MAIN unit. The following is a very contrived example that will create a Capture depending on some keyword that was entered (which can be handy during testing of a command line interface of a script):

sub ARGS-TO-CAPTURE(&main@args --> Capture{
    # if we only specified "frobnicate" as an argument 
    @args == 1 && @args[0eq 'frobnicate'
      # then dispatch as MAIN("foo","bar",verbose => 2) 
      ?? Capture.newlist => <foo bar>hash => { verbose => 2 } )
      # otherwise, use default processing of args 
      !! &*ARGS-TO-CAPTURE(&main@args)

Note that the dynamic variable &*ARGS-TO-CAPTURE is available to perform the default command line arguments to Capture processing so you don't have to reinvent the whole wheel if you don't want to.

Intercepting usage message generation (2018.10, v6.d and later)

You can replace or augment the default way of usage message generation (after a failed dispatch to MAIN) by supplying a GENERATE-USAGE subroutine yourself, or by importing one from any of the Getopt modules available in the ecosystem.


sub RUN-MAIN(&main$mainline:$in-as-argsfiles)

This routine allows complete control over the handling of MAIN. It gets a Callable that is the MAIN that should be executed, the return value of the mainline execution and additional named variables: :in-as-argsfiles which will be True if STDIN should be treated as $*ARGFILES.

If RUN-MAIN is not provided, a default one will be run that looks for subroutines of the old interface, such as MAIN_HELPER and USAGE. If found, it will execute following the "old" semantics.

class Hero {
    has @!inventory;
    has Str $.name;
    submethod BUILD:$name:@inventory ) {
        $!name = $name;
        @!inventory = @inventory
sub new-main($name*@stuff ) {$name), :inventory(@stuff) ).raku.say
RUN-MAIN&new-mainNil );

This will print the name (first argument) of the generated object.


The GENERATE-USAGE subroutine should accept a Callable representing the MAIN subroutine that didn't get executed because the dispatch failed. This can be used for introspection. All the other parameters are the parameters that were set up to be sent to MAIN. It should return the string of the usage information you want to be shown to the user. An example that will just recreate the Capture that was created from processing the arguments:

sub GENERATE-USAGE(&main|capture{
      ?? "You're not allowed to specify a --foo"
      !! &*GENERATE-USAGE(&main|capture)

You can also use multi subroutines to create the same effect:

multi sub GENERATE-USAGE(&main:$foo!{
    "You're not allowed to specify a --foo"
multi sub GENERATE-USAGE(&main|capture{

Note that the dynamic variable &*GENERATE-USAGE is available to perform the default usage message generation so you don't have to reinvent the whole wheel if you don't want to.

Intercepting MAIN calling (before 2018.10, v6.e)

An older interface enabled one to intercept the calling to MAIN completely. This depended on the existence of a MAIN_HELPER subroutine that would be called if a MAIN subroutine was found in the mainline of a program.

This interface was never documented. However, any programs using this undocumented interface will continue to function until v6.e. From v6.d onward, the use of the undocumented API will cause a DEPRECATED message.

Ecosystem modules can provide both the new and the old interface for compatibility with older versions of PerlĀ 6 and Raku: if a newer Raku recognizes the new (documented) interface, it will use that. If there is no new interface subroutine available, but the old MAIN_HELPER interface is, then it will use the old interface.

If a module developer decides to only offer a module for v6.d or higher, then the support for the old interface can be removed from the module.