Entware is a modern alternative to Optware.
For those unfamiliar with Optware: it's a software repository that offers various software programs that can be installed on your router. They allow you to add new functionality to your router (provided you have the know-how to properly configure them).
Entware system library is specially modified (patched) so that standard linux files that are normally located in /etc
directory are now located in /opt/etc/
directory. To simplify things let's consider /etc/passwd
file. On Asuswrt-merlin it normally looks something like:
admin:x:0:0:admin:/root:/bin/sh
nas:x:100:100:nas:/dev/null:/dev/null