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(emacs-lisp-intro.info)Buffer Names

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Buffer Names
============

   The two functions, `buffer-name' and `buffer-file-name', show the
difference between a file and a buffer.  When you evaluate the
following expression, `(buffer-name)', the name of the buffer appears
in the echo area.  When you evaluate `(buffer-file-name)', the name of
the file to which the buffer refers appears in the echo area.  Usually,
the name returned by `(buffer-name)' is the same as the name of the
file to which it refers, and the name returned by `(buffer-file-name)'
is the full path-name of the file.

   A file and a buffer are two different entities.  A file is
information recorded permanently in the computer (unless you delete
it).  A buffer, on the other hand, is information inside of Emacs that
will vanish at the end of the editing session (or when you kill the
buffer).  Usually, a buffer contains information that you have copied
from a file; we say the buffer is "visiting" that file.  This copy is
what you work on and modify.  Changes to the buffer do not change the
file, until you save the buffer.  When you save the buffer, the buffer
is copied to the file and is thus saved permanently.

   If you are reading this in Info inside of GNU Emacs, you can evaluate
each of the following expressions by positioning the cursor after it and
typing `C-x C-e'.

     (buffer-name)
     
     (buffer-file-name)

When I do this, `"introduction.texinfo"' is the value returned by
evaluating `(buffer-name)', and
`"/gnu/work/intro/introduction.texinfo"' is the value returned by
evaluating `(buffer-file-name)'.  The former is the name of the buffer
and the latter is the name of the file.  (In the expressions, the
parentheses tell the Lisp interpreter to treat `buffer-name' and
`buffer-file-name' as functions; without the parentheses, the
interpreter would attempt to evaluate the symbols as variables.  Note:
Variables.)

   In spite of the distinction between files and buffers, you will often
find that people refer to a file when they mean a buffer and vice-versa.
Indeed, most people say, "I am editing a file," rather than saying, "I
am editing a buffer which I will soon save to a file."  It is almost
always clear from context what people mean.  When dealing with computer
programs, however, it is important to keep the distinction in mind,
since the computer is not as smart as a person.

   The word `buffer', by the way, comes from the meaning of the word as
a cushion that deadens the force of a collision.  In early computers, a
buffer cushioned the interaction between files and the computer's
central processing unit.  The drums or tapes that held a file and the
central processing unit were pieces of equipment that were very
different from each other, working at their own speeds, in spurts.  The
buffer made it possible for them to work together effectively.
Eventually, the buffer grew from being an intermediary, a temporary
holding place, to being the place where work is done.  This
transformation is rather like that of a small seaport that grew into a
great city: once it was merely the place where cargo was warehoused
temporarily before being loaded onto ships; then it became a business
and cultural center in its own right.

   Not all buffers are associated with files.  For example, when you
start an Emacs session by typing the command `emacs' alone, without
naming any files, Emacs will start with the `*scratch*' buffer on the
screen.  This buffer is not visiting any file.  Similarly, a `*Help*'
buffer is not associated with any file.

   If you switch to the `*scratch*' buffer, type `(buffer-name)',
position the cursor after it, and type `C-x C-e' to evaluate the
expression, the name `"*scratch*"' is returned and will appear in the
echo area.  `"*scratch*"' is the name of the buffer.  However, if you
type `(buffer-file-name)' in the `*scratch*' buffer and evaluate that,
`nil' will appear in the echo area.  `nil' is from the Latin word for
`nothing'; in this case, it means that the `*scratch*' buffer is not
associated with any file.  (In Lisp, `nil' is also used to mean `false'
and is a synonym for the empty list, `()'.)

   Incidentally, if you are in the `*scratch*' buffer and want the
value returned by an expression to appear in the `*scratch*' buffer
itself rather than in the echo area, type `C-u C-x C-e' instead of `C-x
C-e'.  This causes the value returned to appear after the expression.
The buffer will look like this:

     (buffer-name)"*scratch*"

You cannot do this in Info since Info is read-only and it will not allow
you to change the contents of the buffer.  But you can do this in any
buffer you can edit; and when you write code or documentation (such as
this book), this feature is very useful.