Q: What is smenu?

A: smenu is a selection tool which acts as a filter that takes words
   from the standard input or a file and presents them on the screen in
   various ways in a scrolling window.

   A cursor that you can easily move allows you to select one or more
   of them.

   The selected words are printed on the standard output.
--------
Q: Why smenu tells me: "The length of a word has reached the limit of
   512 characters." but there is no such word in my entry?

A: There is a good chance you have an unbalanced quote, single or double,
   somewhere. smenu uses quotation marks to be able to have spaces in
   'words', and these quotation marks that do not serve as delimiters
   must be protected.

   You can use something like: sed -e "s/'/\\\'/g" -e 's/"/\\"/g' to
   pre-process the input in such a case.