SSD or HDD

With lsblk (part of the util-linux package):

lsblk -d -o name,rota

NAME ROTA
sda     0
sdb     0
sdc     1

where ROTA means rotational device (1 if true, 0 if false)
 

 

----or----

 

Linux automatically detects SSD, and since kernel version 2.6.29, you may verify sda with:

cat /sys/block/sda/queue/rotational

You should get 1 for hard disks and 0 for a SSD.

It will probably not work if your disk is a logical device emulated by hardware (like a RAID controller).