How to get markdown preview mode working on Linux?

[David M. Karr]

I'd like to get some sort of Markdown preview mode working on my Linux 
laptop.

I installed the the three main "markdown" packages ("markdown-mode", 
"markdown-mode+", and "markdown-preview-mode"). While viewing a markdown 
file that I've created (almost empty), I pressed the key bound to 
"markdown-preview".  It brought up a browser page that said exactly this:
-----------------
 /usr/bin/bash: markdown: command not found
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I read the README on the github page for markdown-preview-mode, and it 
doesn't really say much.