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There is a newer "sysfs thermal zone" API (see also LWN article and Linux kernel doc) showing temperatures under e.g.
/sys/class/thermal/thermal_zone0/temp
Readings are in thousandths of degrees Celcius (although in older kernels, it may have just been degrees C).
Depending on your Linux distro, you may find a file under /proc that contains this information. For example, this page suggests /proc/acpi/thermal_zone/THM/temperature.