ifconfig error showing invalid errors for ifconfig en0 | grep ether
You probably installed inetutils 1.9.3 with a package manager like brew or MacPorts.

Besides GNU versions of various tools (telnet, rsh etc.) it also installs a GNU version of ifconfig.

To distinguish the GNU tools from macOS tools with the same names they are usually linked from the install dir (installed with brew this is: /usr/local/opt/inetutils/gnubin) to a directory in the standard PATH prefixed with a g (e.g. ifconfig > gifconfig or ftp > gftp).

If you add this install dir to your PATH variable in a way that gives preference over the standard paths, the GNU tools will be executed instead of the original ones.



Check your PATH variable: echo $PATH. You will likely see something like:

/usr/local/opt/inetutils/gnubin:/usr/local/bin:/usr/bin:/bin:...


Now edit the file which determines your PATH variable (probably either ~/.bashrc or ~/.bash_profile) and remove the gnubin path. 

Example: nano ./bash_profile

...
export PATH="/usr/local/opt/inetutils/gnubin:/usr/local/mysql/bin:$PATH"
...


Change this to:

...
export PATH="/usr/local/mysql/bin:$PATH"
...


Then close the Terminal window or relaunch Terminal.app. This will "reenable" the standard versions of the commands. The GNU tools are still available by entering a command prefixed with a g (e.g. gifconfig or gftp).