If you are a SAP user then you always use the SAP GUI or Webbase logins but you also can login to your SAP system by Command.
Here is the procedure for the same:
START >> Run >>
Enter below command:
sapgui (Server) (Instance Number)
Exa: sapgui ECCCI 00
Or You can use:
sapshcut.exe -system=SID -client=100 -user=YOURUNAME -pw=PASSWORD