A current $I$ ampere flows along an infinitely long straight thin walled tube, then the magnetic induction at any point inside the tube is:

According to Ampere's Circuital Law, $\oint \vec{B}\cdot d\vec{l}={\mu }_0{I}_{enclosed}$.For any circular path inside the thin-walled tube, the enclosed current $I_{enclosed}$ is zero because the current flows only on the surface of the tube.Therefore, $B\cdot 2\pi r=0⟹B=0$.The magnetic field inside the tube is zero.