How to automatically strip trailing spaces on save in Vi and Vim?

This works (in the .vimrc file) for all files:

autocmd BufWritePre * :%s/\s\+$//e

This works (in the .vimrc file) for just ruby(.rb) files:

autocmd BufWritePre *.rb :%s/\s\+$//e

To keep cursor position use something like:

function! <SID>StripTrailingWhitespaces()
    let l = line(".")
    let c = col(".")
    %s/\s\+$//e
    call cursor(l, c)
endfun

else cursor would end up at beginning of line of last replace after save.

Example: You have a space at end of line 122, you are on line 982 and enter :w. Not restoring position, would result in cursor ending up at beginning of line 122 thus killing work flow.

Set up call to function using autocmd, some examples:

" Using file extension
autocmd BufWritePre *.h,*.c,*.java :call <SID>StripTrailingWhitespaces()

" Often files are not necessarily identified by extension, if so use e.g.:
autocmd BufWritePre * if &ft =~ 'sh\|perl\|python' | :call <SID>StripTrailingWhitespaces() | endif

" Or if you want it to be called when file-type i set
autocmd FileType sh,perl,python  :call <SID>StripTrailingWhitespaces()

" etc.

One can also use, but not needed in this case, getpos() by:

let save_cursor = getpos(".")
" Some replace command
call setpos('.', save_cursor)

" To list values to variables use:
let [bufnum, lnum, col, off] = getpos(".")

My DeleteTrailingWhitespace plugin does this and, in contrast to the various simple :autocmds floating around, also handles special cases, can query the user, or abort writes with trailing whitespace.

The plugin page contains links to alternatives; there's also a large discussion on the Vim Tips Wiki.