udev rule to run shell script works halfway through script, but doesn't complete

Question

I have a udev rule that will run a shell script to create a directory and mount a device in read-only mode. The problem is that when the rule runs (when I insert the device), it creates the directory but doesn't mount the device.

Naturally, something must be wrong with the script then. However, I can run the script (I insert the device, delete the directory that is created by the udev rule manually, then run the script manually as well) and it creates the directory and mounts the device just fine.

Relevant section of udev rule (filename = '10-magicusb.rules'):

ENV{MYMOUNT}=="1", RUN+="/etc/udev/scripts/romount.sh"

romount.sh:

#!/bin/bash
mkdir /media/sdb1
/bin/mount /dev/sdb1 -o ro,noatime /media/sdb1

Technically the device name comes from further up in the udev rule (its not hardcoded, trust me). I run a script that creates romount.sh and I pass that script the device with %k. However, that shouldn't be related since the script runs from udev, but stops halfway through execution.

My guess is there is some timing issue going on here. When the rule runs, for some reason it cannot mount the device, but when I do it later manually the system has had time to sort things out I guess. I added a sleep 10 to the top of the bash script to no avail.

Thank you in advance.

EDIT 1

Based on what David G. has posted, I did some testing with the improved script so now romount.sh looks like this:

#!/bin/bash
mkdir /media/sdb1
(
    echo 'before while iteration' > /etc/udev/scripts/iteration.txt
    while [[ ! -b /dev/sdb1 ]]
    do
        sleep 1
        echo 'iteration performed' > /etc/udev/scripts/iteration.txt
    done
    /bin/mount /dev/sdb1 -o ro,noatime /media/sdb1
) &

and the resulting iteration.txt file has: before while iteration only. Which means the condition for the while loop is never triggered. This means the device node is available at time of execution, right?

EDIT 2

I finally got it to work with the second answer here: Udev rule to mount disk does not work

Adding PrivateMounts=No to the systemd-udevd.service worked for me.

Answer 1

I suspect that the issue is that the script is run before the device node is created. Try something like (i.e. without the hardcoding):

#!/bin/bash
mkdir /media/sdb1
(
    while [[ ! -b /dev/sdb1 ]]
    do
        sleep 1
    done
    /bin/mount /dev/sdb1 -o ro,noatime /media/sdb1
) &

The important part is the backgrounded subshell.

Depending on what udev does, you might have issues with needing to do something with stdin, stdout, and stderr. You might also need to ignore SIGHUP or some other signal (though probably not).