How to detect disconnection of usb0 device (USB gadget aka g_ether)?

Question

Two Linux machines (embedded board and PC) are connected to each other via USB cable, and communication is established via USB gadget/Ethernet. Both sides have IP address, ping works, etc.

I need to find a way to detect USB cable disconnect on the board side, ideally with a single Bash command.

ifconfig usb0 still shows IP address and that device is "UP" and "RUNNING" even after unplugging the cable.

Answer 1

One way to do this is to check the file:

/sys/class/net/usb0/operstate

If the file content is "up" then the link exists. If the content is "down", then there is no link.

Answer 2

In my particular case it turned out to be HW+SW issue: driver (atmel_usba_udc) is using USB power (VBUS) to detect connection, since it's acting as a slave and power is provided by host. However in PCB power is provided unconditionally by the board itself for all USB ports at once.

Normally any standard method should work: ifconfig or sysfs.