info sed =>

`-n'

`--quiet'

`--silent'

? ? ?By default, `sed' prints out the pattern space at the end of each

? ? ?cycle through the script (*note How `sed' works: Execution Cycle.).

? ? ?These options disable this automatic printing, and `sed' only

? ? ?produces output when explicitly told to via the `p' command.

`-i[SUFFIX]'

`--in-place[=SUFFIX]'

? ? ?This option specifies that files are to be edited in-place. ?GNU

? ? ?`sed' does this by creating a temporary file and sending output to

? ? ?this file rather than to the standard output.(1).

? ? ?This option implies `-s'.

? ? ?When the end of the file is reached, the temporary file is renamed

? ? ?to the output file's original name. ?The extension, if supplied,

? ? ?is used to modify the name of the old file before renaming the

? ? ?temporary file, thereby making a backup copy(2)).

? ? ?This rule is followed: if the extension doesn't contain a `*',

? ? ?then it is appended to the end of the current filename as a

? ? ?suffix; if the extension does contain one or more `*' characters,

? ? ?then _each_ asterisk is replaced with the current filename. ?This

? ? ?allows you to add a prefix to the backup file, instead of (or in

? ? ?addition to) a suffix, or even to place backup copies of the

? ? ?original files into another directory (provided the directory

? ? ?already exists).

? ? ?If no extension is supplied, the original file is overwritten

? ? ?without making a backup.

我的理解：sed命令有一個緩沖區，-n選項把緩沖區的內容輸出到屏幕，之后緩沖區就被清空了，再執行-i就只剩下空的內容，寫入到文件里文件內容就沒了

加上 -i 參數后 表示 將修改的內容同步到文件中，這時本身就不會顯示啊

手動指定-n 也是沒有效果的