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25.8.4 Functions that Expand Filenames

Expansion of a file name means converting a relative file name to an absolute one. Since this is done relative to a default directory, you must specify the default directory name as well as the file name to be expanded. Expansion also simplifies file names by eliminating redundancies such as ./ and name/../.

— Function: expand-file-name filename &optional directory

This function converts filename to an absolute file name. If directory is supplied, it is the default directory to start with if filename is relative. (The value of directory should itself be an absolute directory name or directory file name; it may start with ‘~’.) Otherwise, the current buffer's value of default-directory is used. For example:

          (expand-file-name "foo")
               ⇒ "/xcssun/users/rms/lewis/foo"
          (expand-file-name "../foo")
               ⇒ "/xcssun/users/rms/foo"
          (expand-file-name "foo" "/usr/spool/")
               ⇒ "/usr/spool/foo"
          (expand-file-name "$HOME/foo")
               ⇒ "/xcssun/users/rms/lewis/$HOME/foo"

If the part of the combined file name before the first slash is ‘~’, it expands to the value of the HOME environment variable (usually your home directory). If the part before the first slash is ‘~user’ and if user is a valid login name, it expands to user's home directory.

Filenames containing ‘.’ or ‘..’ are simplified to their canonical form:

          (expand-file-name "bar/../foo")
               ⇒ "/xcssun/users/rms/lewis/foo"

In some cases, a leading ‘..’ component can remain in the output:

          (expand-file-name "../home" "/")
               ⇒ "/../home"

This is for the sake of filesystems that have the concept of a “superroot” above the root directory /. On other filesystems, /../ is interpreted exactly the same as /.

Note that expand-file-name does not expand environment variables; only substitute-in-file-name does that.

Note also that expand-file-name does not follow symbolic links at any level. This results in a difference between the way file-truename and expand-file-name treat ‘..’. Assuming that ‘/tmp/bar’ is a symbolic link to the directory ‘/tmp/foo/bar’ we get:

          (file-truename "/tmp/bar/../myfile")
               ⇒ "/tmp/foo/myfile"
          (expand-file-name "/tmp/bar/../myfile")
               ⇒ "/tmp/myfile"

If you may need to follow symbolic links preceding ‘..’, you should make sure to call file-truename without prior direct or indirect calls to expand-file-name. See Truenames.

— Variable: default-directory

The value of this buffer-local variable is the default directory for the current buffer. It should be an absolute directory name; it may start with ‘~’. This variable is buffer-local in every buffer.

expand-file-name uses the default directory when its second argument is nil.

The value is always a string ending with a slash.

               ⇒ "/user/lewis/manual/"
— Function: substitute-in-file-name filename

This function replaces environment variable references in filename with the environment variable values. Following standard Unix shell syntax, ‘$’ is the prefix to substitute an environment variable value. If the input contains ‘$$’, that is converted to ‘$’; this gives the user a way to “quote” a ‘$’.

The environment variable name is the series of alphanumeric characters (including underscores) that follow the ‘$’. If the character following the ‘$’ is a ‘{’, then the variable name is everything up to the matching ‘}’.

Calling substitute-in-file-name on output produced by substitute-in-file-name tends to give incorrect results. For instance, use of ‘$$’ to quote a single ‘$’ won't work properly, and ‘$’ in an environment variable's value could lead to repeated substitution. Therefore, programs that call this function and put the output where it will be passed to this function need to double all ‘$’ characters to prevent subsequent incorrect results.

Here we assume that the environment variable HOME, which holds the user's home directory name, has value ‘/xcssun/users/rms’.

          (substitute-in-file-name "$HOME/foo")
               ⇒ "/xcssun/users/rms/foo"

After substitution, if a ‘~’ or a ‘/’ appears immediately after another ‘/’, the function discards everything before it (up through the immediately preceding ‘/’).

          (substitute-in-file-name "bar/~/foo")
               ⇒ "~/foo"
          (substitute-in-file-name "/usr/local/$HOME/foo")
               ⇒ "/xcssun/users/rms/foo"
               ;; /usr/local/ has been discarded.