on intel / amd cpus/boards:

use lm sensors:

su - root; # become root
apt-get install lm-sensors; # install
sensors-detect; # detect available temperature sensors, answer with YES usually

# monitor cpu temp every second
while true; do sensors|grep Core; sleep 1; clear; done;
# example output


# usually cpu will throttle clock speed and power consumption
# when critical temp is reached (as it is here at around 70)
# i5 is clocking around 2GHz
apt install lscpu; # install lscpu
while true; do lscpu; sleep 1; clear; done; # observe cpu clock speed
service kmod start; # might help too

graphical output/widget/gadget with psensor:

apt-get install psensor;
# hit Ctrl+D to "logout" and become non-root user
psensor; # start psensor

on arm cpus/boards:

script /scripts/cpu_temp.sh will monitor CPU temp and update every second.

this will show only the temp of ONE CORE?

#!/bin/bash

while true ; do

cpu_temp=$(< /sys/class/thermal/thermal_zone0/temp)
cpu_temp=$(($cpu_temp/1000))
echo $cpu_temp C

sleep 1;
clear;

done # executes COMMAND every second

chmod +x /scripts/cpu_temp.sh; # mark runnable