##Recover boot to a shell
一：正文
In normal operation,systems boot themselves independently and are then accessed remotely by administrators and users.However,administrators need a recovery tool they can use if a disk crashes or a configuration problem prevents the system from completing the normal boot process.Instead of shooting for full system operation,UNIX systems can boot just enough to run a shell on the system console.This option is traditionally known as booting to single-user mode,recovery mode,or maintenance mode,all terms that we use interchangeably in this

chapter.As its name implies,single-user mode does not allow network operation;you need physical access to the system console to use it.
On most systems,you request a boot to single-mode by passing an argument to the kernel at boot time.If the system is already up and running,you can bring it down to single-user mode with the shutdown or telinit command.
二：單詞
*：operation /ɒpə’reɪʃ(ə)n/ 操作
*：independent /,ɪndɪ’pendənt/ 獨立的
*：access /'ækses/ 訪問
*：remotely /rɪ’məʊtlɪ/ 遠端的
*：recovery /rɪ’kʌv(ə)rɪ/ 恢復
*：crashes 崩潰
*：completing 完成
*：interchangeably 可交換的
*：implies 蘊含
三：全文
在正常的執行情況下，系統可以自行引導，引導起來以後，系統管理員和使用者就能夠遠端訪問它。不過，如果因為硬碟損壞或者配置有問題，造成系統不能完成正常的引導過程，系統管理員就需要工具來恢復。UNIX系統也可以不追求完整啟動，而只是部分引導，只要在控制檯執行shell即可。這種方式傳統上稱為引導進入單使用者模式，恢復模式或者維護模式，我們不加區分。顧明思議，單使用者模式不能進行網路操作；使用者要到控制檯前使用它。
在大多數系統上，我們通過引導時候傳給核心一個引數，引導進入單使用者模式。