#to see the total amount of physical RAM installed:
sudo lshw -c memory
$ free -m
free -h
# total used free shared buff/cache available
#Mem: --> 7.7G <-- 6.2G 157M 503M 1.3G 709M
#Swap: 2.0G 118M 1.9G
# to see ram usage, cpu core usage, and more:
htop
sudo dmidecode | grep -A 15 Memory