# hdparm -I /dev/sda | grep Serial
        Serial Number:      WD-WCAL71856931
#

# smartctl -a /dev/sda | grep 'Serial Number'
Serial Number:    WD-WCAL71856931
#

These utilities are from the hdparm and smartmontools packages:

# dpkg -l | grep hdparm
ii  hdparm                              9.32-1                       tune hard disk parameters for high performance
# dpkg -l | grep smart
ii  smartmontools                       5.39.1+svn3124-2             control and monitor storage systems using S.M.A.R.T.
#