#!/bin/bash
s="Text start
This is a line I want to delete,
this is second line that should be deleted.
And this is stop-pattern which should terminate deletion.
end"
sed '/delete,/,/deletion\./d' <<< "$s"
IyEvYmluL2Jhc2gKcz0iVGV4dCBzdGFydApUaGlzIGlzIGEgbGluZSBJIHdhbnQgdG8gZGVsZXRlLAp0aGlzIGlzIHNlY29uZCBsaW5lIHRoYXQgc2hvdWxkIGJlIGRlbGV0ZWQuCkFuZCB0aGlzIGlzIHN0b3AtcGF0dGVybiB3aGljaCBzaG91bGQgdGVybWluYXRlIGRlbGV0aW9uLgplbmQiCnNlZCAnL2RlbGV0ZSwvLC9kZWxldGlvblwuL2QnIDw8PCAiJHMi