eintr: Buffer Names
2.1 Buffer Names
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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 in Info, the value returned by evaluating ‘(buffer-name)’
is ‘"*info*"’, and the value returned by evaluating ‘(buffer-file-name)’
is ‘nil’.
On the other hand, while I am writing this document, the value
returned by evaluating ‘(buffer-name)’ is ‘"introduction.texinfo"’, and
the value returned by evaluating ‘(buffer-file-name)’ is
‘"/gnu/work/intro/introduction.texinfo"’.
The former is the name of the buffer and the latter is the name of
the file. In Info, the buffer name is ‘"*info*"’. Info does not point
to any file, so the result of evaluating ‘(buffer-file-name)’ is ‘nil’.
The symbol ‘nil’ is from the Latin word for “nothing”; in this case, it
means that the buffer is not associated with any file. (In Lisp, ‘nil’
is also used to mean “false” and is a synonym for the empty list, ‘()’.)
When I am writing, the name of my buffer is ‘"introduction.texinfo"’.
The name of the file to which it points is
‘"/gnu/work/intro/introduction.texinfo"’.
(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. 
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, a
‘*scratch*’ buffer does not visit any file. Similarly, a ‘*Help*’
buffer is not associated with any file.
In the old days, when you lacked a ‘~/.emacs’ file and started an
Emacs session by typing the command ‘emacs’ alone, without naming any
files, Emacs started with the ‘*scratch*’ buffer visible. Nowadays, you
will see a splash screen. You can follow one of the commands suggested
on the splash screen, visit a file, or press the spacebar to reach the
‘*scratch*’ buffer.
If you switch to the ‘*scratch*’ buffer, type ‘(buffer-name)’,
position the cursor after it, and then type ‘C-x C-e’ to evaluate the
expression. The name ‘"*scratch*"’ will be returned and will appear in
the echo area. ‘"*scratch*"’ is the name of the buffer. When you type
‘(buffer-file-name)’ in the ‘*scratch*’ buffer and evaluate that, ‘nil’
will appear in the echo area, just as it does when you evaluate
‘(buffer-file-name)’ in Info.
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.
Info Catalog
eintr: How to Evaluate
eintr: Practicing Evaluation
eintr: Getting Buffers