If I'm reading your entry correctly, was it "tabcmd" that you used previously? For 2018.2.x we replaced "tabadmin" with "tsm" but "tabcmd" is still around.
Tabcmd only works in version prior 2018.2. Start from version 2018.2, they change the command to TSM, i tried to use tsm export command to do the export however, it said the export is an unknown command.
TSM is used to manage installation and configuration of Tableau Server. it is not the command line for use in exporting content.
Tabcmd will still be the method for exporting content. It should still have the same behavior as before.
You are right, forgot to install the tabcmd. It's work now. Thanks!