Most of the nouns (usually, mascul. gender) ending with ш DO NOT REQUIRE ь sign at the end. Examples: ёрш, пунш, куш (all masculine, 2 склонение).

But if it's feminine, ь is used. Examples: мышь, глушь, пустошь (all feminine, 3 склонение)

In case with verbs шь (знаешь, думаешь, etc) will be the ending. In this case YOU CANNOT OMIT the ь sign, because this is the rule for spelling of verb endings.

At the end of the word шь and ш are pronounced as a hard sound. This is the rule.