co2_munin.sh

#!/usr/bin/env bash
set -e -o pipefail

BUS="1"

hex_to_byte() {
  echo "$((16#$1))"
}

extract_byte() {
  echo "$(hex_to_byte "$(cut -d' ' -f$1 | egrep -io '[0-9a-f]+$')")"
}

read_reg() {
  local reg="$1"
  local xfer="$(i2ctransfer -f -y $BUS "r2@0x${reg}")"
  local cmd=$((16#$reg * 2 + 1))
  local data="$(extract_byte 1 <<< "$xfer")"
  local csum="$(extract_byte 2 <<< "$xfer")"
  local csum_expect=$(((cmd + data) % 256))
  [ "$csum" -eq "$csum_expect" ] || {
    return 1
  }
  echo "$data"
}

configure() {
cat <<EOF
graph_title CO2 concentration
graph_args --base 1000 --lower-limit 0
graph_vlabel ppm
graph_info Current CO2 concentration at my desk
graph_category environment
ppm.label CO2 concentration
ppm.draw LINE1
ppm.colour 000000
EOF
}

fetch() {
  low="$(read_reg 70)"
  high="$(read_reg 78)"

  echo ppm.value $((high * 256 + low))
}

case $1 in
  config) configure;;
  *) fetch;;
esac