How to know if a disk is an SSD or an HDD

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).

See this answer for more information…

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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)