journal-cron.sh
The cron + short-script pair is Linux' universal scheduler: every minute, CPU load and temperature are appended to a CSV with built-in rotation. Zero dependencies, zero daemons, survives reboots on its own.
Hardware
An ODROID-XU4 (Samsung Exynos5422, 4 Cortex-A15 + 4 Cortex-A7, 2 GB) running Ubuntu Hardkernel or Armbian, with gigabit Ethernet and an eMMC port. None: readings come from the board itself.
Wiring
No wiring. Check cron (or cronie) is installed and running on the distribution you use.
How it works
cron runs the script on schedule; the script reads /proc and sysfs, appends a CSV line then truncates past 100,000 lines to bound storage.
Running it
On Ubuntu Hardkernel or Armbian: apt install gpiod python3-gpiod; the 40-pin header shows up in gpiodetect (gpiochip0/1 chips on Exynos), check pins with gpioinfo.
Variants
Push the CSV to a server (curl -T), or replace cron with a systemd timer (OnUnitActiveSec=60) for integrated journalctl logs.
journal-cron.sh
#!/bin/bash
# Mesure ponctuelle appelee par cron (ici toutes les minutes).
# Installation :
# sudo cp journal-cron.sh /usr/local/bin/mesure-cart.sh
# sudo chmod +x /usr/local/bin/mesure-cart.sh
# (crontab -e) puis : * * * * * /usr/local/bin/mesure-cart.sh
JOURNAL="/var/log/mesure-cart.csv"
[ -f "$JOURNAL" ] || echo "horodatage,charge,temp_c" > "$JOURNAL"
# 1) Charge moyenne (1 minute) depuis /proc/loadavg
CHARGE=$(awk '{print $1}' /proc/loadavg)
# 2) Temperature SoC (tolere l'absence du capteur)
TEMP="NA"
for z in /sys/class/thermal/thermal_zone*/temp; do
[ -r "$z" ] && TEMP=$(( $(cat "$z") / 1000 )) && break
done
# 3) Ligne CSV horodatee, append atomique
echo "$(date -Is),$CHARGE,$TEMP" >> "$JOURNAL"
# 4) Rotation maison : garder ~100 000 lignes (~70 jours a la minute)
if [ "$(wc -l < "$JOURNAL")" -gt 100000 ]; then
tail -n 50000 "$JOURNAL" > "$JOURNAL.tmp" && mv "$JOURNAL.tmp" "$JOURNAL"
fi