Redefine tab as 4 spaces

It depends on what you mean. Do you want actual tab characters in your file to appear 4 spaces wide, or by "tab" do you actually mean an indent, generated by pressing the tab key, which would result in the file literally containing (up to) 4 space characters for each "tab" you type?

Depending on your answer, one of the following sets of settings should work for you:

For tab characters that appear 4-spaces-wide: set tabstop=4 If you're using actual tab character in your source code you probably also want these settings (these are actually the defaults, but you may want to set them defensively): set softtabstop=0 noexpandtab Finally, if you want an indent to correspond to a single tab, you should also use: set shiftwidth=4

For indents that consist of 4 space characters but are entered with the tab key: set tabstop=8 softtabstop=0 expandtab shiftwidth=4 smarttab

To make the above settings permanent add these lines to your vimrc.

In case you need to make adjustments, or would simply like to understand what these options all mean, here's a breakdown of what each option means:

tabstop The width of a hard tabstop measured in "spaces" -- effectively the (maximum) width of an actual tab character. shiftwidth The size of an "indent". It's also measured in spaces, so if your code base indents with tab characters then you want shiftwidth to equal the number of tab characters times tabstop. This is also used by things like the =, > and < commands. softtabstop Setting this to a non-zero value other than tabstop will make the tab key (in insert mode) insert a combination of spaces (and possibly tabs) to simulate tab stops at this width. expandtab Enabling this will make the tab key (in insert mode) insert spaces instead of tab characters. This also affects the behavior of the retab command. smarttab Enabling this will make the tab key (in insert mode) insert spaces or tabs to go to the next indent of the next tabstop when the cursor is at the beginning of a line (i.e. the only preceding characters are whitespace).

For more details on any of these see :help 'optionname' in vim (e.g. :help 'tabstop')

To define this on a permanent basis for the current user, create (or edit) the .vimrc file:

$ vim ~/.vimrc

Then, paste the configuration below into the file. Once vim is restarted, the tab settings will apply.

set tabstop=4       " The width of a TAB is set to 4.
                    " Still it is a \t. It is just that
                    " Vim will interpret it to be having
                    " a width of 4.

set shiftwidth=4    " Indents will have a width of 4

set softtabstop=4   " Sets the number of columns for a TAB

set expandtab       " Expand TABs to spaces

or shorthand for vim modeline:

vim :set ts=4 sw=4 sts=4 et :

There are few settings which define whether to use spaces or tabs.

So here are handy functions which can be defined in your ~/.vimrc file:

function! UseTabs()
  set tabstop=4     " Size of a hard tabstop (ts).
  set shiftwidth=4  " Size of an indentation (sw).
  set noexpandtab   " Always uses tabs instead of space characters (noet).
  set autoindent    " Copy indent from current line when starting a new line (ai).
endfunction

function! UseSpaces()
  set tabstop=2     " Size of a hard tabstop (ts).
  set shiftwidth=2  " Size of an indentation (sw).
  set expandtab     " Always uses spaces instead of tab characters (et).
  set softtabstop=0 " Number of spaces a <Tab> counts for. When 0, featuer is off (sts).
  set autoindent    " Copy indent from current line when starting a new line.
  set smarttab      " Inserts blanks on a <Tab> key (as per sw, ts and sts).
endfunction

Usage:

:call UseTabs()
:call UseSpaces()

To use it per file extensions, the following syntax can be used (added to .vimrc):

au! BufWrite,FileWritePre *.module,*.install call UseSpaces()

See also: Converting tabs to spaces.

Here is another snippet from Wikia which can be used to toggle between tabs and spaces:

" virtual tabstops using spaces
set shiftwidth=4
set softtabstop=4
set expandtab
" allow toggling between local and default mode
function TabToggle()
  if &expandtab
    set shiftwidth=8
    set softtabstop=0
    set noexpandtab
  else
    set shiftwidth=4
    set softtabstop=4
    set expandtab
  endif
endfunction
nmap <F9> mz:execute TabToggle()<CR>'z

It enables using 4 spaces for every tab and a mapping to F9 to toggle the settings.

I copied and pasted this into my .vimrc file:

" size of a hard tabstop
set tabstop=4

" always uses spaces instead of tab characters
set expandtab

" size of an "indent"
set shiftwidth=4

The first 2 settings mean that when I press Tab I get 4 spaces. The third setting means that when I do V> (i.e. visual and indent) I also get 4 spaces.

Not as comprehensive as the accepted answer but it might help people who just want something to copy and paste.

Put your desired settings in the ~/.vimrc file -- See below for some guidelines and best practices.

There are four main ways to use tabs in Vim:

Always keep 'tabstop' at 8, set 'softtabstop' and 'shiftwidth' to 4 (or 3 or whatever you prefer) and use 'noexpandtab'. Then Vim will use a mix of tabs and spaces, but typing and will behave like a tab appears every 4 (or 3) characters. Note: Setting 'tabstop' to any other value than 8 can make your file appear wrong in many places (e.g., when printing it). Set 'tabstop' and 'shiftwidth' to whatever you prefer and use 'expandtab'. This way you will always insert spaces. The formatting will never be messed up when 'tabstop' is changed. Set 'tabstop' and 'shiftwidth' to whatever you prefer and use a |modeline| to set these values when editing the file again. Only works when using Vim to edit the file. Always set 'tabstop' and 'shiftwidth' to the same value, and 'noexpandtab'. This should then work (for initial indents only) for any tabstop setting that people use. It might be nice to have tabs after the first non-blank inserted as spaces if you do this though. Otherwise aligned comments will be wrong when 'tabstop' ischanged.

Source:

vimdoc.sourceforge.net/htmldoc/options.html#'tabstop'

:help tabstop

One more thing, use
:retab
to convert existing tab to spaces http://vim.wikia.com/wiki/Converting_tabs_to_spaces

Add line
set ts=4
in
~/.vimrc file for per user
or
/etc/vimrc file for system wide

:set sw=4

See Mastering the VI editor

My basic ~/.vimrc with comment:

set number " show line number                                                                                           
set tabstop=2 " set display width of tab; 1 tab = x space with                                                           
set expandtab " transform tab to x space (x is tabstop)                                                               
set autoindent " auto indent; new line with number of space at the beginning same as previous                                                                      
set shiftwidth=2 " number of space append to lines when type >> 

Permanent for all users (when you alone on server):

# echo "set tabstop=4" >> /etc/vim/vimrc

Appends the setting in the config file. Normally on new server apt-get purge nano mc and all other to save your time. Otherwise, you will redefine editor in git, crontab etc.

Make sure vartabstop is unset

set vartabstop=

Set tabstop to 4

set tabstop=4