Trouble generating random hex numbers using /dev/urandom in bash

Question

I'm trying to generate a random hex number with an specified length ($length) using the following command:

head -c $length /dev/urandom | xxd -p -u -c $length | tr -d '[:space:]\\'

I've noticed that head -c $length /dev/urandom actually prints the double of the $length value. So if $length=1 the output will be 2 characters long. How can I fix this?

Also why does the input number on xxd -p -u -c $length seems not to affect on the output? meaning I can use either:

head -c 4 /dev/urandom | xxd -p -u -c 20000 | tr -d '[:space:]\\'

or

head -c 4 /dev/urandom | xxd -p -u -c 4 | tr -d '[:space:]\\'

and it will print 8 characters in both cases.