Course of Raku / Advanced / Containers / Special and dynamic variables 🆕

Built-in special variables

Raku predefines a set of variables for you. Most of them are dynamic (the * twigil), so — as you saw with dynamic variables — you can read them anywhere and even override them for a scope. Here are the ones you will reach for most often.

Standard streams

Input and output flow through three dynamic variables: $*OUT (standard output), $*ERR (standard error), and $*IN (standard input). say and print write to $*OUT; to send text to standard error instead, call the method on $*ERR:

$*OUT.say('normal output'); # goes to standard output
$*ERR.say('a diagnostic');  # goes to standard error

Because these are dynamic, redirecting $*OUT in a block sends the output of everything called inside that block to the new destination — without changing any of that code.

The program and its process

A few variables describe the running program itself:

say $*PROGRAM-NAME; # the path of the script being run
say $*PID;          # the process id of this program
say $*CWD;          # the current working directory

Their values change from run to run, so no fixed output is shown here.

Command line and environment

The arguments and the environment arrive in two ready-made containers:

• @*ARGS — the list of command-line arguments (the same ones a MAIN subroutine receives)
• %*ENV — the environment variables, as a hash

say @*ARGS.elems; # how many arguments were passed
say %*ENV<HOME>;  # the value of the HOME environment variable

These built-ins save you from wiring up the same information by hand. And because they are dynamic, looked up through the call stack, any $* built-in can be overridden for a scope exactly like a $* variable of your own.

Alongside these dynamic variables, Raku also has the compile-time ? built-ins — $?FILE and $?LINE — which you already met on the twigils page.

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