shell difference

gistfile1.txt

$ cat t
#! /bin/sh
set -e
s() {
  "$@"
}
x=y s env
$ bash ./t|grep ^x
x=y
$ dash ./t|grep ^x
$ dpkg -l bash dash
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version      Architecture Description
+++-==============-============-============-=================================
ii  bash           4.4-5        amd64        GNU Bourne Again SHell
ii  dash           0.5.8-2.4    amd64        POSIX-compliant shell