If you restart the session and enter

  > echo test1 | md5

it will output another random 'hash' each time. But within one session it knows that md5 should produce the same output. Running

  > echo test1 | md5

multiple times, or even

  > VAR1="test1"; echo $VAR1 | md5

produces the same output for me within the same session, while

  > echo test2 | md5

results in a different, but also session-consistent hash.