2019独角兽企业重金招聘Python工程师标准>>> 
$ str= null value
$ unset str 不存在
$ str= # 设为 null
$ unset str # 取消
#!/bin/bash
A=B
echo "PID for 1.sh before exec/source/fork:$$"
export A
echo "1.sh: \$A is $A"
case $1 in
exec)
echo "using exec..."
exec ./2.sh ;;
source)
echo "using source..."
. ./2.sh ;;
*)
echo "using fork by default..."
./2.sh ;;
esac