$ id -a
uid=0(root) gid=1(other) groups=1(other),0(root),2(bin),3(sys),4(adm),5(uucp),6(mail),7(tty),8(lp),9(nuucp),12(daemon)

$ id -un
root

$ id -gn
other

-u   Output only the effective user  ID,  using  the  format
     "%u\n".

-g   Output only the effective group ID,  using  the  format
     "%u\n".

-n   Output the name in  the  format  "%s"  instead  of  the
     numeric ID using the format "%u".