Errors from whatis command. Unable to rebuild database with makewhatis?
The following can be used as a workaround for the macOS 10.15.1 version of the apropos command, wherein it spews out complaints of the form makewhatis: /usr/lib/lib … .dylib: Not a directory.

First create the workaround script:

$ mkdir -p ~/workarounds
$ sed -e 66s@/usr/lib@@ /usr/bin/apropos &gt; ~/workarounds/apropos.macos_10.15.1 
$ diff /usr/bin/apropos ~/workarounds/apropos.macos_10.15.1 
66c66
&lt;     for d in /var/cache/man $manpath /usr/lib
---
&gt;     for d in /var/cache/man $manpath 
$ chmod +x ~/workarounds/apropos.macos_10.15.1


Next add an alias to your shell to tell it to use the workaround script, until a newer version of the canonical script becomes available.

For Zsh you can use the following command:

$ /bin/cat &lt;&lt;END &gt;&gt; ~/.zshrc
# Workaround for broken apropos command.
alias apropos=~/workarounds/apropos.macos_10.15.1
END


For other shells such as ksh or bash, use ~/.profile or ~/.bash_profile, as appropriate.

What does the workaround do?

Apropos requests (and man -k requests) are handled by the /usr/bin/apropos script. That script searches for “whatis” database files in all the directories of the man path (see man —path), plus /var/cache/man and /usr/lib. The checks for /var/cache/man/whatis and /usr/lib/whatis appear to be there for historical reasons, however those files are not actively generated in Mojave or Catalina. A lot of different people have contributed to the various flavours of Unix over the years, and many of them had different good ideas about where to put different types of files. At some point in time, somebody decided that /usr/lib would be a good place to put a whatis file, and at some other point somebody else figured that /var/cache/man would be a good place. Others thought that the appropriate place would be the respective man page directories. Different solutions that seemed appropriate at the time. The apropos script has traditionally checked those locations in case a whatis file was present.

With the move to making the system directories on Catalina read-only (a good move), whatis database files can not be written to directories such as /usr/share/man. There are different ways that Apple could handle that, but for this release somebody decided to alter the apropos script by making it generate results on the fly by calling /usr/libexec/makewhatis.local for any man page directory that does not contain a whatis file.

That new apropos code works fine for actual man page directories, and for /var/cache/man (since it doesn’t exist), but it fails for /usr/lib. The workaround detailed above just eliminates /usr/lib from the list of directories searched.

As a final step, set yourself a reminder for a month or two from now to check whether Apple has fixed the apropos script. If so remove your workarounds, by removing the alias and the workaround script.