Scheme - standard way in R5RS to execute an external command?

Is there a way in Scheme Revision 5 to call out to an external program? For example:

(system "ls")

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If not, is there any "official" way to do this, such as specified in a SRFI or a later revision of the Scheme spec (R6RS, etc)?

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No.

(It's generally a bad idea to try to write code with "Standard Scheme", and system is one of those things that is unlikely to become standard enough to be usable outside of a particular implementation or via some compatibility library.)