`wye` is an input variant of the `tee` utility

Usage is like `tee`: wye [FILE]...

wye always consumes from its stdin, the optional files specified are
read-multiplexed as alternative inputs to stdin.  All reads are
performed using PIPE_BUF sized buffers which are the atomic units for
unix pipes, any read data is immediately written to stdout.

Someone should try get this command added upstream in GNU Coreutils
upstream, and give me credit for the name.  This implementation is a
quick and dirty hack and not particularly robust.