MARS data can be downloaded in many ways:
MARS is only available to certain users who have access to the Operational Archive.
Other users should use the Copernicus Climate Data Store.
For retrieving mars data via the command line, users who have access to internal ECMWF systems can construct their request using the MARS Catalogue and simply copy into a text file and run the text file with the mars command. Details of running the MARS command can be found here : MARS command and request syntax
For instructions on using the MARS Catalogue please see MARS Web and Web-MARS
The MARS WebAPI allows users, who have access to the ECMWF Archive to download data directly from MARS using a script on their local workstation.
For Details on the WebAPI please see : ECMWF Web API