🛠 Vivre côté Linux : SSH, opkg, Python et routes Lua personnalisées

Objectives

Treat the Yún as a true embedded mini-server: connect via SSH, manage packages with opkg (OpenWrt’s package manager), install Python, and create a custom REST route in Lua served by the RPC daemon — the same mechanism as the official /arduino routes. You will write code that runs on the AR9331, without ever touching the 32U4.

Hardware

• 1 online Arduino Yún (root password known)
• PC with an SSH client (ssh root@arduino.local)
• Outbound network access for opkg (Linino/OpenWrt mirrors may be slow or relocated — plan for a local mirror if needed)
• Optional but recommended: microSD card formatted as ext4 to offload internal storage

Full Session

# 1) Connexion (mot de passe root par défaut : yun)
ssh root@arduino.local

# 2) Inventaire système
cat /proc/cpuinfo | head -5      # AR9331 MIPS 24Kc
df -h                            # ~32 Mo de flash, dont ~10 Mo utilisables !
free                             # 64 Mo de RAM DDR2

# 3) Gestion de paquets (OpenWrt)
opkg update
opkg list | grep python          # python est gros ; python-light existe
opkg install python-light py-pycurl

# 4) Script Python côté Linux — il peut parler au Bridge via HTTP local
cat > /root/temperature.py << 'EOF'
import urllib.request, time
while True:
    v = urllib.request.urlopen(
        "http://localhost/data/get/pot").read.decode.strip
    print("pot =", v)
    time.sleep(2)
EOF
python /root/temperature.py &

# 5) Route REST personnalisée : /cgi.d... en fait via luci-rpc en Lua
#    Fichier /usr/lib/lua/temperature.lua :
#      module("temperature", package.seeall)
#      function get
#        return { valeur = 42, unite = "degC" }
#      end
#    Exposée par /cgi-bin/rpc.lua/temperature/get (selon la version de Linino OS).

On the 32U4 side, nothing changes: the sketch and your Linux scripts coexist on the same Bridge and the same data store /data — this is the full power of the architecture.

Expected Verification

opkg -v responds, python -V displays the installed version, your Python script reads the /data keys published by the sketch (localhost, no network configuration). The Lua route returns JSON in the browser. Check space: df -h before/after each opkg install — this is THE constraint of the Yún.

Common Errors

• Saturating the internal flash: only ~10 MB free; a heavy opkg install fails with “No space left on device”. Install on the microSD (extroot) if you go beyond the minimal setup.
• opkg mirrors not found: since the Yún is discontinued, some Linino repositories no longer respond; edit /etc/opkg/distfeeds.conf to point to an accessible mirror. >
• Killing the Bridge while experimenting: restarting uhttpd or luci-rpc breaks the REST API (and thus your /arduino URLs) until the next reboot — test first on your own route.
• Leaving SSH passwords as “yun” on a shared network: change them; never expose port 22 directly to the Internet.
• Confusing execution spaces: the sketch runs on the 32U4 (no Python, no threads), scripts run on the AR9331 (no direct pin access, only via Bridge) — the boundary is the internal serial port.
• Writing continuously to the internal flash: wear; growing logs should go to the microSD or be sent to remote syslog/journald.
• Busybox: utilities are stripped down (no bash, reduced options); test your scripts with sh.