How to determine if cp -u actually copied the file

I don't know why changing the kernel name would have made a difference (perhaps sd*1 runs after sd* allowing a bit more time for work to get done?), but udev doesn't like long-running actions in events:

Starting daemons or other long running processes is not appropriate for udev; the forked processes, detached or not, will be unconditionally killed after the event handling has finished.

Original nohup suggestion

I originally suggested nohup before fully reading my own links :) -- which suggest that this may not actually work

udev rule:

ACTION=="add", SUBSYSTEM=="block", KERNEL=="sd[a-z]1" SYMLINK+="usbflash", RUN+="/path/to/mywrapper.sh"

mywrapper.sh (Note: if you don't redirect the output nohup may litter your directory with a nohup.out file):

#!/bin/sh
nohup /path/to/myscript.sh >/log/myscript.log 2>&1 &

Then myscript.sh can be what it is.

Newer systemd suggestion

The third link below suggests firing off a systemd service when the device is plugged in. That blog entry does more work than I think you need due to ensuring that the device information is passed to the service, I think you can get away with simply using systemd as your method of daemonization:

ACTION=="add", SUBSYSTEM=="block", KERNEL=="sd[a-z]1" SYMLINK+="usbflash", RUN+="/usr/bin/systemctl start my-usb-backup.service"

With a simple oneshot service my-usb-backup.service:

[Unit]
Description=run myscript
[Service]
Type=oneshot
ExecStart=/path/to/myscript.sh

See also:

• How to run long time process on Udev event?
• What is the easiest way to “detach/daemonize” a Bash script?
• Proper(-ish) way to start long-running systemd service on udev event (device hotplug)
• More than you wanted to know about systemd

If you can use Zsh, this seems to do what you wish:

$ mkdir -p x/y; touch x/y/a x/b; sleep .1; touch x/y/a; sleep .1; touch x/b
$ find -depth -type d -execdir zsh -c 'touch "$1" -r "$1"/*(om[1])' zsh {} \;

Test output with GNU find, ./x/b was the newest, its timestamp is copied to ./x and ..

$ find -printf "%TT %p\n"  | sort -n
15:16:24.0182222830 ./x/y
15:16:24.0182222830 ./x/y/a
15:16:24.1222150510 .
15:16:24.1222150510 ./x
15:16:24.1222150510 ./x/b

Finding the newest file by timestamp is somewhat tricky in general, if there are filenames with whitespace/newlines or such. (See BashFAQ 099.) Zsh does make it rather easy, though, the trick above is from Is it possible to reference the most recently modified file in a command line argument?

-depth tells find to handle subdirectories before their parents, so that the bubbling up actually works.

This doesn't actually fully ignore timestamps of directories since they must be counted when the timestamps bubble up. A directory containing files will get its timestamp from the files, but an empty directory will keep its original timestamp, and that timestamp will bubble up. I'm not sure if it's worth working around that.