修改全局环境变量（）：
vim /etc/vim/vimrc
添加如下配置（推荐）
set tabstop=4
set shiftwidth=4
set softtabstop=4
set expandtab
释义：
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
cited from https://stackoverflow.com/questions/1878974/redefine-tab-as-4-spaces
