设置变量：export
显示变量：echo
取消变量: unset
只有显示变量的时候，才需要在变量前加 $
[liang@testdb ~]$
[liang@testdb ~]$ export MYNAME=liang
[liang@testdb ~]$ echo $MYNAME
liang
[liang@testdb ~]$ unset MYNAME
[liang@testdb ~]$ echo $MYNAME
[liang@testdb ~]$
[liang@testdb ~]$