Bash script not copying files using udev

Question

I have a script set up to run when a certain usb drive is inserted. It is supposed to copy a folder from the usb drive into a backup folder, and write over this folder if it already exists. However, when I plug in my usb drive, it creates the folder but it doesn't copy any files.

My .rules looks like this:

ACTION=="add", ATTR{idVendor}==Vendor, ATTR{idProduct}==Product, RUN+="/bin/sh /path/to/file/mkBackup"

My file mkBackup looks like this:

#!/bin/bash
a="/path/to/file/Backup"
if [ -e $a ]; then
        rm -rf $a
fi
mkdir $a
cp -r /path/to/usb/folder $a

When it is run by udev, the Backup folder is created but it is empty, but when I just run ./mkBackup, the folder is created and everything is copied. This is my first time using udev to do something like this, so please forgive me if this seems like a simple question. Thank you in advance.

Answer 1

Given that the Backup folder is created, you know the script successfully executes when you plugin the device. I suspect that the script is executing before the device actually finishes mounting. Thus the files are not yet available for cp.

What happens if you add sleep 5 after mkdir $a ? Or what if you try to copy existing files instead?

This link is also pretty helpful as a reference.

I also use:

udevadm monitor --udev

To watch udev events in real time.

udevadm test /path/to/dev

To simulate a udev event run for the device. Note that this does not run any program specified by a RUN key.

EDIT: As Gilles helpfully points out, my idea won't work. And I'm not aware of a solution. Here's why:

Udev will make the USB device available to the OS. Then, the OS will realize that the OS contains a mountable file system and will mount it. At that point the files on the USB device are actually available.

Any script run by a udev rule will execute before the file system is actually mounted. If I think of an alternative I'll update the post.