# Is digital up and down sampling linear/causal/time-invariant?

So I am trying to determine whether the process of up sampling (zero padding) and downsampling are linear, causal and/or time-invariant.

Based on some resources I found online, I am under the impression that it is linear, but not time-invariant. However I am unsure about causality (since it depends neither on future nor past values?).

Upsampling and downsampling are both causal, since the output at time $m$ depends only on samples you've seen up time $m$.