How does the vim "write with sudo" trick work?

In :w !sudo tee %...

% means "the current file"

As eugene y pointed out, % does indeed mean "the current file name", which is passed to tee so that it knows which file to overwrite.

(In substitution commands, it's slightly different; as :help :% shows, it's equal to 1,$ (the entire file) (thanks to @Orafu for pointing out that this does not evaluate to the filename). For example, :%s/foo/bar means "in the current file, replace occurrences of foo with bar." If you highlight some text before typing :s, you'll see that the highlighted lines take the place of % as your substitution range.)

:w isn't updating your file

One confusing part of this trick is that you might think :w is modifying your file, but it isn't. If you opened and modified file1.txt, then ran :w file2.txt, it would be a "save as"; file1.txt wouldn't be modified, but the current buffer contents would be sent to file2.txt.

Instead of file2.txt, you can substitute a shell command to receive the buffer contents. For instance, :w !cat will just display the contents.

If Vim wasn't run with sudo access, its :w can't modify a protected file, but if it passes the buffer contents to the shell, a command in the shell can be run with sudo. In this case, we use tee.

Understanding tee

As for tee, picture the tee command as a T-shaped pipe in a normal bash piping situation: it directs output to specified file(s) and also sends it to standard output, which can be captured by the next piped command.

For example, in ps -ax | tee processes.txt | grep 'foo', the list of processes will be written to a text file and passed along to grep.

     +-----------+    tee     +------------+
     |           |  --------  |            |
     | ps -ax    |  --------  | grep 'foo' |
     |           |     ||     |            |
     +-----------+     ||     +------------+
                       ||   
               +---------------+
               |               |
               | processes.txt |
               |               |
               +---------------+

(Diagram created with Asciiflow.)

See the tee man page for more info.

Tee as a hack

In the situation your question describes, using tee is a hack because we're ignoring half of what it does. sudo tee writes to our file and also sends the buffer contents to standard output, but we ignore standard output. We don't need to pass anything to another piped command in this case; we're just using tee as an alternate way of writing a file and so that we can call it with sudo.

Making this trick easy

You can add this to your .vimrc to make this trick easy-to-use: just type :w!!.

" Allow saving of files as sudo when I forgot to start vim using sudo.
cmap w!! w !sudo tee > /dev/null %

The > /dev/null part explicitly throws away the standard output, since, as I said, we don't need to pass anything to another piped command.

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In the executed command line, % stands for the current file name. This is documented in :help cmdline-special:

In Ex commands, at places where a file name can be used, the following
characters have a special meaning.
        %       Is replaced with the current file name.

As you've already found out, :w !cmd pipes the contents of the current buffer to another command. What tee does is copy standard input to one or more files, and also to standard output. Therefore, :w !sudo tee % > /dev/null effectively writes the contents of the current buffer to the current file while being root. Another command that can be used for this is dd:

:w !sudo dd of=% > /dev/null

As a shortcut, you can add this mapping to your .vimrc:

" Force saving files that require root permission 
cnoremap w!! w !sudo tee > /dev/null %

With the above you can type :w!!<Enter> to save the file as root.

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The accepted answer covers it all, so I'll just give another example of a shortcut that I use, for the record.

Add it to your etc/vim/vimrc (or ~/.vimrc):

cnoremap w!! execute 'silent! write !sudo tee % >/dev/null' edit!

Where:

cnoremap: tells vim that the following shortcut is to be associated in the command line.

w!!: the shortcut itself.

execute '...': a command that execute the following string.

silent!: run it silently

write !sudo tee % >/dev/null: the OP question, added a redirection of messages to NULL to make a clean command

edit!: this trick is the cherry of the cake: it calls also the edit command to reload the buffer and then avoid messages such as the buffer has changed. is how to write the pipe symbol to separate two commands here.

Hope it helps. See also for other problems:

SuperUser: force vim to write a file

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This also works well:

:w !sudo sh -c "cat > %"

This is inspired by the comment of @Nathan Long.

NOTICE:

" must be used instead of ' because we want % to be expanded before passing to shell.

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:w - Write a file.

!sudo - Call shell sudo command.

tee - The output of write (vim :w) command redirected using tee. The % is nothing but current file name i.e. /etc/apache2/conf.d/mediawiki.conf. In other words tee command is run as root and it takes standard input and write it to a file represented by %. However, this will prompt to reload file again (hit L to load changes in vim itself):

tutorial link

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I'd like to suggest another approach to the "Oups I forgot to write sudo while opening my file" issue:

Instead of receiving a permission denied, and having to type :w!!, I find it more elegant to have a conditional vim command that does sudo vim if file owner is root.

This is as easy to implement (there might even be more elegant implementations, I'm clearly not a bash-guru):

function vim(){
  OWNER=$(stat -c '%U' $1)
  if [[ "$OWNER" == "root" ]]; then
    sudo /usr/bin/vim $*;
  else
    /usr/bin/vim $*;
  fi
}

And it works really well.

This is a more bash-centered approach than a vim-one so not everybody might like it.

Of course:

there are use cases where it will fail (when file owner is not root but requires sudo, but the function can be edited anyway)

it doesn't make sense when using vim for reading-only a file (as far as I'm concerned, I use tail or cat for small files)

But I find this brings a much better dev user experience, which is something that IMHO tends to be forgotten when using bash. :-)

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FOR NEOVIM

Due to problems with interactive calls (https://github.com/neovim/neovim/issues/1716), I am using this for neovim, based on Dr Beco's answer:

cnoremap w!! execute 'silent! write !SUDO_ASKPASS=`which ssh-askpass` sudo tee % >/dev/null' <bar> edit!

This will open a dialog using ssh-askpass asking for the sudo password.

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A summary (and very minor improvement) on the most common answers that I found for this as at 2020.

tl;dr

Call with :w!! or :W!!. After it expands, press enter.

If you are too slow in typing the !! after the w/W, it will not expand and might report: E492: Not an editor command: W!!

NOTE Use which tee output to replace /usr/bin/tee if it differs in your case.

Put these in your ~/.vimrc file:

    " Silent version of the super user edit, sudo tee trick.
    cnoremap W!! execute 'silent! write !sudo /usr/bin/tee "%" >/dev/null' <bar> edit!
    " Talkative version of the super user edit, sudo tee trick.
    cmap w!! w !sudo /usr/bin/tee >/dev/null "%"

More Info:

First, the linked answer below was about the only other that seemed to mitigate most known problems and differ in any significant way from the others. Worth reading: https://stackoverflow.com/a/12870763/2927555

My answer above was pulled together from multiple suggestions on the conventional sudo tee theme and thus very slightly improves on the most common answers I found. My version above:

Works with whitespace in file names

Mitigates path modification attacks by specifying the full path to tee.

Gives you two mappings, W!! for silent execution, and w!! for not silent, i.e Talkative :-)

The difference in using the non-silent version is that you get to choose between [O]k and [L]oad. If you don't care, use the silent version. [O]k - Preserves your undo history, but will cause you to get warned when you try to quit. You have to use :q! to quit. [L]oad - Erases your undo history and resets the "modified flag" allowing you to exit without being warned to save changes.

[O]k - Preserves your undo history, but will cause you to get warned when you try to quit. You have to use :q! to quit.

[L]oad - Erases your undo history and resets the "modified flag" allowing you to exit without being warned to save changes.

Information for the above was drawn from a bunch of other answers and comments on this, but notably:

Dr Beco's answer: https://stackoverflow.com/a/48237738/2927555

idbrii's comment to this: https://stackoverflow.com/a/25010815/2927555

Han Seoul-Oh's comment to this: How does the vim "write with sudo" trick work?

Bruno Bronosky comment to this: https://serverfault.com/a/22576/195239

This answer also explains why the apparently most simple approach is not such a good idea: https://serverfault.com/a/26334/195239

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The only problem with cnoremap w!! is that it replaces w with ! (and hangs until you type the next char) whenever you type w! at the : command prompt. Like when you want to actually force-save with w!. Also, even if it's not the first thing after :.

Therefore I would suggest mapping it to something like <Fn>w. I personally have mapleader = F1, so I'm using <Leader>w.

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