add comments and more error checking, make more parameters as variables

readSensor.sh

@@ -1,37 +1,79 @@
 #!/bin/bash
 
+#
+# Copyright (c) 2018 leopold@shdx.org
+# Author: Der_Leopold
+#
+# This script tries to read temperature and humidity from a DHT11 or DHT22 sensor
+# and writes the results and a timestamp to a file. It is intended to be executed
+# periodically, for example via crontab.
+#
+
+# Type of sensor; '11' for DHT11 or '22' for DHT22
+SENSOR=22
+
+# Change this to the number of the GPIO pin the sensor is connected to
 GPIOPIN=4
+
+# Retry up to n times if the reading fails
+RETRY=3
+
+# Number of seconds to wait between retries
+DELAY=2
+
+# Specify the temperature range where you expect the sensor will be operating
+# This is simply used as a safeguard to detect implausible sensor readings
 MINTEMP=-30
 MAXTEMP=40
-CMD="${PWD}/external/Adafruit_Python_DHT/examples/AdafruitDHT.py"
+
+# CMD should point to the compiled Adafruit binary that does the acutal reading from the sensor
+# See https://github.com/adafruit/Adafruit_Python_DHT for details
+# The 'sudo' is not needed if the user executing the script is member of the group 'gpio'
+CMD="sudo ${PWD}/external/Adafruit_Python_DHT/examples/AdafruitDHT.py"
+
+# Location where the readings should be stored
 OUTFILE="${PWD}/output"
 
-touch "$OUTFILE"
+
+#
+# no configuration/changes should be necessary below this line
+#
+
+touch "$OUTFILE" 2> /dev/null
+
+if [ ! -w $OUTFILE ]; then
+    echo "Error: output file '$OUTFILE' not writeable"
+    exit 1
+fi
+
 CNT=0
 
 function read_sensor () {
-    OUTPUT=`$CMD 22 $GPIOPIN`
+    OUTPUT=`$CMD $SENSOR $GPIOPIN`
     T=`echo "$OUTPUT" | awk -F[=*%] '{print $2}'`
     H=`echo "$OUTPUT" | awk -F[=*%] '{print $4}'`
     CNT=$(($CNT+1))
-    echo "$CNT"
 }
+
 function check_output () {
     if [[ $OUTPUT =~ Fail ]] || [[ ! $OUTPUT =~ Temp ]] || [[ ! $OUTPUT =~ Humidity ]] || [ ${T%.*} -lt $MINTEMP ] || [ ${T%.*} -gt $MAXTEMP ]; then
-        if [ $CNT -le 3 ]; then
-            sleep 5
+        if [ $CNT -le $RETRY ]; then
+            sleep $DELAY
             read_sensor
 	    check_output
         else
             echo "Error" > "$OUTFILE"
-            exit 0
+            exit 1
         fi
     fi
 }
 
-read_sensor
-check_output
-
+function write_results () {
     echo `date +%s` > "$OUTFILE"
     echo "${T}" >> "$OUTFILE"
     echo "${H}" >> "$OUTFILE"
+}
+
+read_sensor
+check_output
+write_results