This is interesting thank you making the effort.
Does this mean the app could work with piped input/output?

yes | cli-args-clap3-stdin-stdout | xargs echo

Hmm, it would appear these two issues are relevant:

• https://github.com/rust-cli/meta/issues/95
• Common "footguns" when writing CLI applications team#29

Hope that helps some one?

Thanks for wanting to improve the book!

Some quick thoughts

• It seems like we should be consistent in clap3 vs structopt. Personally, I'd wait for clap3 to be released and then completely switch over
• This seems like this belongs more in the in-depth section than the tutorial.