How to add line numbers to range of lines in Vim?

How can I add line numbers to a range of lines in a file opened in Vim? Not as in :set nu—this just displays line numbers—but actually have them be prepended to each line in the file?

With

:%s/^/\=line('.')/

EDIT: to sum up the comments.

This command can be tweaked as much as you want.

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Let's say you want to add numbers in front of lines from a visual selection (V + move), and you want the numbering to start at 42.

:'<,'>s/^/\=(line('.')-line("'<")+42)/

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If you want to add a string between the number and the old text from the line, just concatenate (with . in VimL) it to the number-expression:

:'<,'>s/^/\=(line('.')-line("'<")+42).' --> '/

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If you need this to sort as text, you may want to zero pad the results, which can be done using printf for 0001, 0002 ... instead of 1, 2... eg:

:%s/^/\=printf('%04d', line('.'))/

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Anyway, if you want more information, just open vim help: :h :s and follow the links (|subreplace-special|, ..., |submatch()|)

cat -n adds line numbers to its input. You can pipe the current file to cat -n and replace the current buffer with what it prints to stdout. Fortunately this convoluted solution is less than 10 characters in vim:

 :%!cat -n

Or, if you want just a subselection, visually select the area, and type this:

 :!cat -n

That will automatically put the visual selection markers in, and will look like this after you've typed it:

 :'<,'>!cat -n

In order to erase the line numbers, I recommend using control-v, which will allow you to visually select a rectangle, you can then delete that rectangle with x.

On a GNU system: with the external nl binary:

:%!nl

With Unix-like environment, you can use cat or awk to generate a line number easily, because vim has a friendly interface with shell, so everything work in vim as well as it does in shell.
From Vim Tip28:

:%!cat -n

or

:%!awk '{print NR,$0}'

But, if you use vim in MS-DOS, of win9x, win2000, you loss these toolkit. here is a very simple way to archive this only by vim:

fu! LineIt()
  exe ":s/^/".line(".")."/"
endf

Or, a sequence composed with alphabet is as easy as above:

exe "s/^/".nr2char(line("."))."/" 

You can also use a subst:

:g/^/exe ":s/^/".line(".")."^I/"

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You can also only want to print the lines without adding them to the file:

> "Sometimes it could be useful especially be editing large source files to print the line numbers out on paper.
To do so you can use the option :set printoptions=number:y to activate and :set printoptions=number:n to deactivate this feature.
If the line number should be printed always, place the line set printoptions=number:y in the vimrc."

First, you can remove the existing line numbers if you need to:

:%s/^[0-9]*//

Then, you can add line numbers. NR refers to the current line number starting at one, so you can do some math on it to get the numbering you want. The following command gives you four digit line numbers:

:%!awk '{print 1000+NR*10,$0}'

The "VisIncr" plugin is good for inserting columns of incrementing numbers in general (or letters, dates, roman numerals etc.). You can control the number format, padding, and so on. So insert a "1" in front of every line (via :s or :g or visual-block insert), highlight that column in visual-block mode, and run one of the commands from the plugin.

If someone wants to put a tab (or some spaces) after inserting the line numbers using the this excellent answer, here's a way. After going into the escape mode, do:

:%s/^/\=line('.').'    '/

^ means beginning of a line and %s is the directive for substitution. So, we say that put a line number at the beginning of each line and add 4 spaces to it and then put whatever was the contents of the line before the substitution, and do this for all lines in the file.

This will automatically substitute it. Alternatively, if you want the command to ask for confirmation from you, then do:

:%s/^/\=line('.').'    '/igc

P.S: power of vim :)