If you write a lot of bash scripts you'll find you end up repeating the same functions. That's a signal to pull them out into a separate library to save you having to keep reinventing the wheel. That's how bashfun "bash function library" began. So far there isn't much in here. The only item of any note is bst: a simple, bash unit-testing framework.
First you'll need to install layman. See Gentoo Overlays User Guide.
Note that in future when you "emerge --sync" you should also "layman -S" to update your layman overlays.
Next, add the underlay overlay:
$ layman -a underlay
You need to edit package.keywords so that portage can install bashfun. For example, for x86:
$ echo "app-misc/bashfun ~x86" >> /etc/portage/package.keywords
Now you can emerge bashfun:
$ emerge -av bashfun
All files are installed to /opt/bashfun.