TL;DR: $PATH will call the value stored in the environmental variable PATH as a command, but echo $PATH will print out the value stored in the variable PATH.
How I found out:
Recently I have been trying to integrate WSL with RStudio.
One thing intrigues me is that the bash in Rmd has a different PATH with WSL. For example, one cannot run the software installed with conda directly from Rmd - the conda bin directory needs to be added into PATH (using export .