co-test

lab3-programming-reterminal

3 min read

Source: L3 — Programming the ReTerminal

Overview

Lab 3 covers three progressively layered tasks:

  1. Bash script to control the reTerminal backlight
  2. Python script using subprocess to call the bash script
  3. Python with seeed-python-rpi to sequence the built-in LEDs

Part 1: Bash Backlight Script

Script: lab-3/bash/backlight (no extension, executable)

#!/bin/bash
 
# Validate: exactly 1 argument
if [[ $#--ne-1-| -ne 1 ]]; then
    echo "Usage: $0 <0-255>" >&2
    exit 1
fi
 
val=$1
 
# Validate: is it an integer
if ! [[ "$val" =~ ^[0-9]+$ ]]; then
    echo "Error: must be integer" >&2
    exit 1
fi
 
# Validate: in range
if [[ $val -lt 0 || $val -gt 255 ]]; then
    echo "Error: must be 0-255" >&2
    exit 1
fi
 
# Write to device file using sudo+tee (required for root-owned /sys/ files)
echo "$val" | sudo tee /sys/class/leds/<backlight-device>/brightness > /dev/null

Make executable: chmod +x lab-3/bash/backlight

Run: ./lab-3/bash/backlight 255

Part 2: Python Wrapping Bash (subprocess)

Script: lab-3/python/backlight.py

#!/usr/bin/env python3
import argparse
import subprocess
 
parser = argparse.ArgumentParser()
parser.add_argument("mode", type=str, choices=["ON", "OFF", "DIM"])
args = parser.parse_args()
 
mapping = {"ON": "255", "OFF": "0", "DIM": "1"}
value = mapping[args.mode]
 
# Call bash script using relative path from this file's directory
subprocess.run(["../bash/backlight", value])

Make executable: chmod +x backlight.py

Key points:

  • Use relative path (../bash/backlight) to the bash script
  • Re-use the working bash script; do NOT rewrite its logic
  • argparse converts string ON/OFF/DIM to numeric 255/0/1

Part 3: seeed-python-rpi LED Sequence

Script: lab-3/python/leds.py

Setup:

# Create and activate venv
python -m venv .venv
source .venv/bin/activate
pip install git+https://github.com/Seeed-Studio/Seeed_Python_RPi/
pip install RPi-GPIO
import seeed_python_rpi.core as rt
import time
 
while True:
    rt.sta_led = True   # STA: Red on (default color is red)
    time.sleep(1)
    rt.sta_led = False
 
    rt.sta_led_green = True  # STA: Green
    time.sleep(1)
    rt.sta_led_green = False
 
    rt.usr_led = True   # USR: Green
    time.sleep(1)
    rt.usr_led = False

sudo + venv problem

The seeed library needs root to access LED files. But sudo python uses root’s Python, not the venv.

# CORRECT: use venv's Python with sudo
sudo $(which python) leds.py         # command substitution
sudo ./.venv/bin/python leds.py      # direct path to venv python

Do NOT commit .venv folder (add to .gitignore).

subprocess Module Reference

import subprocess
 
# Basic usage
subprocess.run(['command', 'arg1', 'arg2'])
 
# Capture output
result = subprocess.run(['ls', '-la'], capture_output=True, text=True)
print(result.stdout)
print(result.returncode)
 
# Raise on error
subprocess.check_call(['./script.sh'])
 
# Shell string (less safe, avoid if possible)
subprocess.run('echo hello', shell=True)

See Also

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