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(emacs)File Names

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File Names
==========

   Most Emacs commands that operate on a file require you to specify the
file name.  (Saving and reverting are exceptions; the buffer knows which
file name to use for them.)  You enter the file name using the
minibuffer (Note: Minibuffer).  "Completion" is available (Note:
Completion) to make it easier to specify long file names.  When
completing file names, Emacs ignores those whose file-name extensions
appear in the variable `completion-ignored-extensions'; see Note:
Completion Options.

   For most operations, there is a "default file name" which is used if
you type just <RET> to enter an empty argument.  Normally the default
file name is the name of the file visited in the current buffer; this
makes it easy to operate on that file with any of the Emacs file
commands.

   Each buffer has a default directory which is normally the same as the
directory of the file visited in that buffer.  When you enter a file
name without a directory, the default directory is used.  If you specify
a directory in a relative fashion, with a name that does not start with
a slash, it is interpreted with respect to the default directory.  The
default directory is kept in the variable `default-directory', which
has a separate value in every buffer.

   For example, if the default file name is `/u/rms/gnu/gnu.tasks' then
the default directory is `/u/rms/gnu/'.  If you type just `foo', which
does not specify a directory, it is short for `/u/rms/gnu/foo'.
`../.login' would stand for `/u/rms/.login'.  `new/foo' would stand for
the file name `/u/rms/gnu/new/foo'.

   The command `M-x pwd' displays the current buffer's default
directory, and the command `M-x cd' sets it (to a value read using the
minibuffer).  A buffer's default directory changes only when the `cd'
command is used.  A file-visiting buffer's default directory is
initialized to the directory of the file that is visited in that
buffer.  If you create a buffer with `C-x b', its default directory is
copied from that of the buffer that was current at the time.

   The default directory actually appears in the minibuffer when the
minibuffer becomes active to read a file name.  This serves two
purposes: it _shows_ you what the default is, so that you can type a
relative file name and know with certainty what it will mean, and it
allows you to _edit_ the default to specify a different directory.
This insertion of the default directory is inhibited if the variable
`insert-default-directory' is set to `nil'.

   Note that it is legitimate to type an absolute file name after you
enter the minibuffer, ignoring the presence of the default directory
name as part of the text.  The final minibuffer contents may look
invalid, but that is not so.  For example, if the minibuffer starts out
with `/usr/tmp/' and you add `/x1/rms/foo', you get
`/usr/tmp//x1/rms/foo'; but Emacs ignores everything through the first
slash in the double slash; the result is `/x1/rms/foo'.  Note:
Minibuffer File.

   `$' in a file name is used to substitute environment variables.  For
example, if you have used the shell command `export FOO=rms/hacks' to
set up an environment variable named `FOO', then you can use
`/u/$FOO/test.c' or `/u/${FOO}/test.c' as an abbreviation for
`/u/rms/hacks/test.c'.  The environment variable name consists of all
the alphanumeric characters after the `$'; alternatively, it may be
enclosed in braces after the `$'.  Note that shell commands to set
environment variables affect Emacs only if done before Emacs is started.

   You can use the `~/' in a file name to mean your home directory, or
`~USER-ID/' to mean the home directory of a user whose login name is
`user-id'.  (On DOS and Windows systems, where a user doesn't have a
home directory, Emacs substitutes `~/' with the value of the
environment variable `HOME'; see Note: General Variables.)

   To access a file with `$' in its name, type `$$'.  This pair is
converted to a single `$' at the same time as variable substitution is
performed for a single `$'.  Alternatively, quote the whole file name
with `/:' (Note: Quoted File Names).  File names which begin with a
literal `~' should also be quoted with `/:'.

   The Lisp function that performs the substitution is called
`substitute-in-file-name'.  The substitution is performed only on file
names read as such using the minibuffer.

   You can include non-ASCII characters in file names if you set the
variable `file-name-coding-system' to a non-`nil' value.  Note: Specify
Coding.