How do I find the temperatures without 3rd party software?

It sounds like what you want is /sys/class/hwmon and /sys/class/thermal.

Both of these provide simple shell based access to the data you want (the hwmon directory will also include other sensor types). Each has one directory for each sensor interface in your system (which may have more than one sensor).

Three other things to note though:

  • Unless you have a really bad system, running the sensors command even once a second shouldn't have any percievable effect. In fact, regularly reading the files in the above mentioned directories may have more of an effect.
  • Graphs are your friend here. I would suggest getting something like netdata or collectd running so you have realtime (or near realtime) data to look at.
  • Check your voltages too. What your describing could also be due to having a power supply that can't keep up with your system's power requirements. This will be indicated by voltages dropping significantly when the system is under high load.

This answer is about one way to monitor processor temperature manually for some Intel processors via accessing the Machine Specific Registers (MSR) directly.

The first thing to note is, in this case, what is read out of the MSR is relative to the Tcc, the limit temperature, so an additional calculation is required to determine the actual temperature.

Refer to the Intel® 64 and IA-32 Architectures Software Developer’s Manual, or in your case the AMD equivalent.

In my case, I want bits 22-16 of the MSR at 0x1B1, aka IA32_PACKAGE_THERM_STATUS. The Tcc for my older i7-2600K is 98 degrees.

Here is a simple script to monitor the temperature (and CPU frequency) manually:

#! /bin/dash
#
# temp_mon3 Smythies 2016.10.05
#       a simplified version of temp_mon2,
#       for monitoring temp.
#       Note: it is on purpose that -a is not used.
#       Also CPU0 frequency (1 is good enough, when all
#       are loaded).
#
# temp_mon2 Smythies 2016.09.29
#       Monitor Package temperatures.
#       Use clock modulation to control temps.
#       i.e. simulate the second to last level
#       of defense.
#       Use simple primatives.
#       run as sudo
#       hardcoded for my tcc of 98 degrees.
#
echo ... begin package temperature monitoring ...

#
# In case I forgot (which I often do)

modprobe msr

#
# first let the drastic effect of the sudo command decay
# Done later in temp_mon3.

#
# some stuff

COMMAND="rdmsr --bitfield 22:16 -u 0x1B1"
COMMAND2="cat /sys/devices/system/cpu/cpufreq/policy0/scaling_cur_freq"

#
# then get on with it

while [ 1 ];do
  sleep 4
  TEMP_RAW=$(eval $COMMAND)
  CPU0_FREQ=$(eval $COMMAND2)
  TEMP_ACT=$((98-TEMP_RAW))
  echo "$TEMP_ACT   $CPU0_FREQ"
done

And here is some sample output, where I add some CPU load after awhile (temp goes from 31 to 73 degrees):

$ sudo ./temp_mon3
[sudo] password for doug:
... begin package temperature monitoring ...
31   1605275
31   1605154
32   1605164
30   1605148
31   1605176
51   3511279
54   3511278
55   3511279
57   3511283
58   3511279
60   3511278
61   3511284
63   3511279
64   3511280
64   3511280
66   3511280
67   3511278
68   3511280
68   3511281
69   3511278
71   3511280
70   3511281
71   3511281
71   3511280
72   3511276
72   3511277
73   3511283
73   3511279
^C

The lm-sensors project (and therefore the sensors command) makes use of the libsensor library; the libraries and package are:

  • Ubuntu 18.04: libsensors4, version 3.4.0-4

     sudo apt install libsensors4

  • Ubuntu 20.04: libsensors5, version 3.6.0.

     sudo apt install libsensors5

I also recommend to install the developer package (includes man pages), which is the same for LTS 18.04 and LTS 20.04:

sudo apt install libsensors4-dev

Some background information can no be found on:

man libsensors
man sensors.conf

BTW this is the same library which reads the temperature as seen in the sysdirectory.

To sum it up, sensors is a good choice. Watching the temperature from command-line can be easily done by

watch -n 1 sensors

If you intend to write a program, take a look at the libsensors manual man libsensorsor make use of your /usr/share/doc/ documentation. In a C-program you have to include #include <sensors/sensors.h> headers. It will use the sensors.conffiles /etc/sensors3.conf and/or /etc/sensors.conf. Further (users) configuration can be found in /etc/sensors.d/, if you make use of this option.

If you think that you are missing some sensors take a look at the /sys/class/thermal or the linked /sys/devices/virtual/thermal directory.

To get the temperatures of the all thermal zones use the command

$ cat /sys/devices/virtual/thermal/thermal_zone?/temp

77000
66000
67000

The temperature is measured in miliCelcius (mC), in the case above, the temperature measured in celsius are: 77.0, 66.0, 67.0 °C.

To watch the temperatures continuously use

watch -n 1 cat /sys/devices/virtual/thermal/thermal_zone?/temp

In this directory you can also find information about your cooling (fan) devices, and how the PID regulators are programmed.

Further some over-temperature protection is coded firmware/hardware based (this is a good idea), and its set-up data is placed in your bios.

If you prefer to display the temperature in °C instead of °mC use the bash computation $((value / 1000 )) or awk to make this conversion:

cat /sys/devices/virtual/thermal/thermal_zone?/temp | awk '{printf " %5.2f °C\n" , $1/1000}'

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