  Non-Hermitian skin effect is a basic phenomenon in non-Hermitian system,
which means that an extensive number of eigenstates can be localized at the
boundary. In this Letter, we systematically investigate the constraints from
all internal symmetries on the non-Hermitian skin effect in arbitrary
dimensions. By adopting the powerful Amoeba formulation, we build a generic
correspondence between the various internal symmetries and the behavior of the
non-Hermitian skin effect. Notably, we find that, for non-Hermitian systems
with the time-reversal$^\dagger$ symmetry, the eigenstates can simultaneously
localize at opposite boundaries, which is beyond the Amoeba formulation, and we
dub the phenomenon bidirectional skin effect. Our work provides an overall
perspective from the internal symmetry to the non-Hermitian skin effect.
