[Linux 常見問題] Getting root permissions on a file inside of vi?

Source From Here 
Question 
Often while editing config files, I'll open one with vi and then when I go to save it realize that I forget the "sudo". For example: 

# sudo vi filename

Is there any way to give vi sudo privileges to save the file while already opening the file? I seem to recall seeing something about this while looking up some stuff about vi a while ago, but now I can't find it. 

How-To 
Refer to below command inside command-line mode. % is replaced with the current file name, thus you can use: 

:w !sudo tee %

vim will detect that the file has been changed and ask whether you want to it to be reloaded. 

As a shortcut, you can define your own command. Put the following in your ~/.vimrc: 

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1. command W w !sudo tee % >/dev/null  

With the above you can type :W to save the file. Since I wrote this, I have found a nicer way (in my opinion) to do this: 

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1. cmap w!! w !sudo tee >/dev/null %  

This way you can type :w!! and it will be expanded to the full command line, leaving the cursor at the end, so you can replace the % with a file name of your own, if you like.