因此，一个变量只有在没有在任何makefile或命令行中指定时，才会使用环境变量的设置注意：父进程makefile变量有时候会传递下来，但不总会这样。
Thus, environment variables are used only if a variable is not set in any makefile or on the command line. Note: parent makefile variables are sometimes, but not always, passed down.
shell脚本中执行定时的基本工具是sleep命令，该命令将使运行中的shell在指定的一段时间内暂停执行。
The basic tool for timing things in a shell script is the sleep command, which causes the running shell to stop execution for a specified time.
PREVIEW命令来看看RMAN能够用来恢复到指定时间的备份，但是这时还没有进行恢复。
PREVIEW command to report, but not restore, the backups that RMAN could use to restore to the specified time.
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