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bashshellraspberry-piraspbiangpio

gpio script returned permission denied when run by user on gpio group raspberry pi

On raspberry pi 1, raspbian stretch lite release 9.1 - kernel version 4.9.52+

I made a shell program to light up a led via raspberry pi's gpio (filename is led.sh). There's a part where I need to interact with some files:

function makeOn {
  # status has value 1 if led is on and 0 if led is off
  if [ $(status) -eq 1 ]; then
    echo "led is already on"
  else
    #say we are using pin 18 and set it to output mode
    echo "18" > /sys/class/gpio/export
    echo "out" > /sys/class/gpio/gpio18/direction # this is line 38

    # write output
    echo "1" > /sys/class/gpio/gpio18/value # this is line 41
    echo "led is on"
  fi
}

I am running this as a user in the gpio group. These are the files I am trying to mess with in my program:

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This is what happens when I run led.sh:

enter image description here

The weird thing is, I ran those commands on the shell one by one and no problem. But when I run a script that calls them, I get a permission denied even though I am in the gpio group. First I thought the shell might run commands as a special user and checked by adding an echo $EUID to the led.sh in several spots and they all returned my user id.

Why does this program have permission denied to those files and how do I give it permission/fix this?

Solution

  • Both value and direction files are created when the pin is exported in echo "18" > /sys/class/gpio/export.

    A small delay is required (ex: sleep 0.1) after it so that the system has time "to properly create and set the file's permission".

    A working example is:

    function makeOn {
  # status has value 1 if led is on and 0 if led is off
  if [ $(status) -eq 1 ]; then
    echo "led is already on"
  else
    # say we are using pin 18 and set it to output mode
    echo "18" > /sys/class/gpio/export
    # added to allow time for the file to be created before trying to use it
    sleep 0.1
    echo "out" > /sys/class/gpio/gpio18/direction

    # write output
    echo "1" > /sys/class/gpio/gpio18/value
    echo "led is on"
  fi
}