Continuesly monitor GPIO input change/events in Bash

Question

On a Raspberry Pi you can set the GPIO pins as an output or an input. An output or an input can be either high or low.

For example, to monitor the input state (high or low) of GPIO pin 26 you can check the value of /sys/class/gpio/gpio26/value.

Now I want to monitor the input value of GPIO pin 26 continuesly and wrote this script.

#!/bin/bash
# return GPIO input status

# select pin
GPIO=26

# prepare the pin
if [ ! -d /sys/class/gpio/gpio${GPIO} ]; then
  echo "${GPIO}" > /sys/class/gpio/export
fi
echo "in" > /sys/class/gpio/gpio"${GPIO}"/direction

# continuously monitor current value
while true; do
  if [ 1 == "$(</sys/class/gpio/gpio"${GPIO}"/value)" ]; then
    printf "high \r"
  else
    printf "low  \r"
  fi
done

This script works as expected. My main concern is that it uses 30% CPU of one of the cores continuesly.

How can I improve this method of monitoring my GPIO pins without such a high CPU usage? Can I skip cycles? Can I use some kind of 'device event manager' or 'system call manager' like udev?

I really prefer to use Bash scripting only (I have seen some Python code that does this ;)

Answer 1

You are limited if you want to only use shell scripts and the sysfs interface to the GPIO.

All I can suggest is you add a short sleep in your script so it doesn't use as much CPU.

sleep (seconds)     CPU (%)
0.50                0.4%
0.25                1.0%
0.10                2.0%
0.07                2.5%
0.05                5.0%
0.01                7.0%
0.001              10.0%

Another easy addition would be to remember the previous GPIO state and only print a message on change.

Answer 2

@Ingo I have seen some docs referenced elsewhere but I don't have a link. I was trying to solve the same problem as the OP but it seems that a simple loop continually checking the contents of the file is unfortunately the only way to achieve this in bash.

See here for a discussion of inotify limitations: https://unix.stackexchange.com/questions/90627/notify-of-changes-on-a-file-under-proc

Apparently languages that implement the select() system call can watch proc/sys but there is no bash command that does this: https://stackoverflow.com/questions/37049612/equivalent-of-select-system-call-in-bash

However I implemented the OPs loop and with a sleep of 0.1 I do not see any CPU issues so I am sticking with it.