How to Use md5sum to Verify Data Integrity

First of all, what’s a hash? A hash is the output of a one-way, reproducible function for creating a small fingerprint from a chunk of data. For example, when the popular hash function MD5 is given the word “cat”, it produces the following output every time:
54b8617eca0e54c7d3c8e6732c6b687a

If “cat” is changed to “cats” the resulting hash is completely different:
48ccbaffbb0675f2a0d9e6ef8875a03c

No matter how much data the hash function is given, the fingerprint stays the same size. No matter how small the change to the data, the fingerprint will be totally different.

What’s md5sum?