Darwin/Linux/Unix command line email client that supports iCloud? I need to access my iCloud email from a command line.

Is there a command line client for Mac OS X (and/or Linux and/or Unix) that can connect to Apple's (I assume it is IMAP) servers?
A popular command line e-mail clients is Mutt. Use a package manager such as Brew or Macports to install it on MacOS. Have a look at this page for some pointers.

You can find the IMAP settings for iCloud in this Support article.