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Is there any way to detect when a USB Wi-Fi adapter is physically disconnected from the Pi, and run a command when that happens? I'm using the Raspberry Pi 1, model B+, and the adapter is connected to the bottom right USB port. It's the Ralink RT5370 adapter, if it helps.

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    If you mean physically disconnected, as in pulled out, yes -- you want to look into udev rules. For an example demonstrating how to shut the pi down cleanly when a particular device is removed see here: http://raspberrypi.stackexchange.com/a/4722/5538 (you could do anything that way, not just run shutdown). If you mean "disconnected from the network" also yes, but the methodology would be unrelated. – goldilocks May 22 '16 at 18:11

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Assuming you don't have any network equipment other that the WIFI dongle that you could unplug, you could simply write

SUBSYSTEM=="net", ACTION=="remove", RUN+="/path/to/command"

into a file named /etc/udev/rules.d/my_custom.rules. Note that you need to specify the full path to your command, since it will be executed directly, without using a shell.

Dmitry Grigoryev
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