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iio_reg
is part of the Libiio package, a library that has been developed to ease the development of software interfacing Linux Industrial I/O (IIO) devices. There are four main userspace helper/utilities:
analog@imhotep:~$ iio_reg -h Usage: iio_reg <device> <register> [<value>]
# iio_reg ad9361-phy 0x3 0x5c