M-x auto-insert will put some predefined text at the beginning of
the buffer. The main application for this function, as its name suggests,
is to have it be called automatically every time an empty, and only an
empty file is visited. This is accomplished by putting
(auto-insert-mode t) into your init file
(see Init File in The GNU Emacs Manual).
What gets inserted, if anything, is determined by the variable
auto-insert-alist. The CAR of each element of this list
is either a mode name, making the element applicable when a buffer is
in that mode, or a string, which is a regexp matched against a
buffer’s file name (the latter allows to distinguish between different
kinds of files that have the same mode in Emacs). The CAR of an
element may also be a cons cell, consisting of mode name or regexp, as
above, and an additional descriptive string.
When a matching element is found, the CDR says what to do. It may
be a string, which is a file name, whose contents are to be inserted, if
that file is found in the directory auto-insert-directory or under a
absolute file name. Or it can be a skeleton (see Skeleton Language) to
be inserted.
It can also be a function, which allows doing various things. The function can simply insert some text, indeed, it can be skeleton command (see Using Skeletons). It can be a lambda function which will for example conditionally call another function. Or it can even reset the mode for the buffer. If you want to perform several such actions in order, you use a vector, i.e., several of the above elements between square brackets (‘[…]’).
By default C and C++ headers insert a definition of a symbol derived from the filename to prevent multiple inclusions. C and C++ sources insert an include of the header. Makefiles insert the file makefile.inc if it exists.
TeX and bibTeX mode files insert the file tex-insert.tex if it exists, while
LaTeX mode files insert a typical \documentclass frame. HTML
files insert a skeleton with the usual frame.
Ada mode files call the Ada header skeleton command. Emacs Lisp
source files insert the usual header, with a copyright of your
environment variable $ORGANIZATION or else the name of the
current user, and prompt for valid keywords describing the contents.
Files in a bin directory for which Emacs could determine no
specialized mode (see Choosing Modes in The GNU Emacs
Manual) are set to Shell script mode.
In Lisp (see Init File in The GNU Emacs Manual) you can use the function
define-auto-insert to add to or modify
auto-insert-alist. See its documentation with C-h f
define-auto-insert.
The variable auto-insert says what to do when auto-insert is
called non-interactively, e.g., when a newly found file is empty (see above):
nilDo nothing.
tInsert something if possible, i.e., there is a matching entry in
auto-insert-alist.
Insert something if possible, but mark as unmodified.
The variable auto-insert-query controls whether to ask about
inserting something. When this is nil, inserting is only done with
M-x auto-insert. When this is function, you are queried
whenever auto-insert is called as a function, such as when Emacs
visits an empty file and you have set the above-mentioned hook. Otherwise
you are always queried.
When querying, the variable auto-insert-prompt’s value is used as a
prompt for a y-or-n-type question. If this includes a ‘%s’ construct,
that is replaced by what caused the insertion rule to be chosen. This is
either a descriptive text, the mode-name of the buffer or the regular
expression that matched the filename.