C:\> wmic useraccount get name,sid   - sample output -  
Name SID
admin S-1-5-21-615456588-3658538152-758053764-1009
myUser S-1-5-21-615456588-3658538152-758053764-1008 
 C:\> whoami /user   - or -  
C:\> wmic useraccount where name='%username%' get sid 
 C:\> wmic useraccount where name='<  username 
>' get sid   - example -  
C:\> wmic useraccount where name='myUser' get sid   - sample output -  
SID
S-1-5-21-615456588-3658538152-758053764-1008 
 C:\> wmic useraccount where sid='<  sid 
>' get name   - example -  
C:\> wmic useraccount where sid='S-1-5-21-615456588-3658538152-758053764-1008' get name   - sample output -  
Name
myUser