fdisk (list and edit disk partitions)
fdisk is Linux's traditional disk partitioning tool. It has been largely supplanted by graphical partition editors such as GParted. However, fdisk is still useful for getting quick partitioning info on the command line. Try running this command:
sudo fdisk -l
This will list all partitions of all disks available on your system along with their size (in blocks), file system type, etc. The prefix →sudo is necessary because fdisk is an administrative tool residing in root's executable path. If you want to do any actual partitioning work on the command line, I recommend using cfdisk, which offers an interactive menu and is more intuitive to use than fdisk.