From type/Proc/Async

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method pid(Proc::Async:D: --> Promise:D)

Equivalent to ready.

Returns a Promise that will be kept once the process has successfully started. Promise will be broken if the program fails to start. Returned promise will hold the process id (PID).

Implementation-specific note: Available starting from Rakudo 2018.04.

From type/Proc

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method pid()

Returns the PID value of the process if available, or Nil.