/lib/udev/rules.d/60-persistent-serial.rules is in udev 175-0ubuntu9.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | # do not edit this file, it will be overwritten on update
ACTION=="remove", GOTO="persistent_serial_end"
SUBSYSTEM!="tty", GOTO="persistent_serial_end"
KERNEL!="ttyUSB[0-9]*|ttyACM[0-9]*", GOTO="persistent_serial_end"
SUBSYSTEMS=="usb-serial", ENV{.ID_PORT}="$attr{port_number}"
IMPORT{builtin}="path_id"
ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="", SYMLINK+="serial/by-path/$env{ID_PATH}"
ENV{ID_PATH}=="?*", ENV{.ID_PORT}=="?*", SYMLINK+="serial/by-path/$env{ID_PATH}-port$env{.ID_PORT}"
IMPORT{builtin}="usb_id"
ENV{ID_SERIAL}=="", GOTO="persistent_serial_end"
SUBSYSTEMS=="usb", ENV{ID_USB_INTERFACE_NUM}="$attr{bInterfaceNumber}"
ENV{ID_USB_INTERFACE_NUM}=="", GOTO="persistent_serial_end"
ENV{.ID_PORT}=="", SYMLINK+="serial/by-id/$env{ID_BUS}-$env{ID_SERIAL}-if$env{ID_USB_INTERFACE_NUM}"
ENV{.ID_PORT}=="?*", SYMLINK+="serial/by-id/$env{ID_BUS}-$env{ID_SERIAL}-if$env{ID_USB_INTERFACE_NUM}-port$env{.ID_PORT}"
LABEL="persistent_serial_end"
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