A quick snippet that reliably but quickly returns a username for the current real user ID, taking into consideration possible multiple login names for a given user ID. Guaranteed not to return a username with different privileges, but might not return the precise username that was used to log in.

    sub whoami {
      for ($ENV{USER}, $ENV{LOGNAME}, getlogin()) {
        if (getpwnam($_) == $<) {  ## is this a matching uid?
          return $_;  ## return that name
        }
      }
      return scalar getpwuid($<); ## fallback, return name for this ui
+d
    }
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