If your purpose is to have a document listing the whatis output for every system command on your machine, do:

find {,/usr}/{,s}bin -printf '%f0' | xargs -0 whatis > command-encyclopedia.txt

• To also capture the entries for which there is no whatis output, append 2>&1 to that command


• To put the undocumented entries in their own document, append 2> dark-commands.txt instead

Actually, to be a proper encyclopedia, you'll want to have it sorted as well!

find {,/usr}/{,s}bin -printf '%f0' | xargs -0 whatis | sort > command-encyclopedia.txt

Better yet, make it interactively searchable (thx for the suggestion @Doogfar!):

find {,/usr}/{,s}bin -printf '%f0' | xargs -0 whatis | sort | less

Now use / to search forward and ? to search backward, or Ctrl+F/B to page forward and backward in your encyclopedia.

Tip: if you want to save yourself the typing every time you need your command encyclopedia, put it in a shell function as described in this answer.