#########################################################################
# File Name: 9-6.sh
# Author: zhang yunabo
# mail: ma6174@163.com
# Created Time: Tue 18 Sep 2018 05:48:32 PM EDT
#########################################################################
#!/bin/bash
username0=
echo "username has declared,but is et to null"
echo "username0 = ${username0-`whoami`}"
echo
echo username1 has not been declared
echo "username1 = ${username1- `whoami`}"
username2=
echo "username2 has declared , but is set to null"
echo "${username2:-`whoami`}"
variable=
echo "${variable-0}"
echo "${variable:-0}"
unset variable
echo "${variable-2}"
echo "${variable:-3}"
exit 0
