#!/bin/sh
#Try this with $1 set and not set
t=${1-t_default}
echo '$t=' $t
#Try this with $c set (and exported) and not set (or not exported).
t=${c-c_default}
echo '$c=' $c
echo '$t=' $t
#unset t
echo t is now unset: $t is not present
v=${t=65}
echo t is $t
echo v is $v