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Predicate expand_file_name/2
Availability:built-in
expand_file_name(+WildCard, -List)
Unify List with a sorted list of files or directories matching WildCard. The normal Unix wildcard constructs‘?’,‘*’,‘[ ...’and‘{...}’are recognised. The interpretation of‘{...}’is slightly different from the C shell (csh(1)). The comma-separated argument can be arbitrary patterns, including‘{...}’patterns. The empty pattern is legal as well:‘{.pl,}’matches either‘.pl’or the empty string.

If the pattern contains wildcard characters, only existing files and directories are returned. Expanding a‘pattern’without wildcard characters returns the argument, regardless of whether or not it exists.

Before expanding wildcards, the construct $\arg{var} is expanded to the value of the environment variable var, and a possible leading ~ character is expanded to the user's home directory.161On Windows, the home directory is determined as follows: if the environment variable HOME exists, this is used. If the variables HOMEDRIVE and HOMEPATH exist (Windows-NT), these are used. At initialisation, the system will set the environment variable HOME to point to the SWI-Prolog home directory if neither HOME nor HOMEPATH and HOMEDRIVE are defined.

Examples

Using absolute_file_name/3 to find files

Using absolute_file_name/3 to find files

With swipl started from /home/janw/src/swipl-devel/library/:

?- absolute_file_name('lists.pl', Abs, [access(read)]).
Abs = '/home/janw/src/swipl-devel/library/lists.pl'.

Using a search path from any directory. Using file_type(prolog) sets the extensions to search for.

?- absolute_file_name(library(lists)', Abs,
                      [access(read), file_type(prolog)]).
Abs = '/home/janw/src/swipl-devel/library/lists.pl'.
?- absolute_file_name(path(ls), Abs, [access(execute)]).
Abs = '/bin/ls'.

We can create our own search path using file_search_path/2. In this example we define two places to search for data files. Search happens in the order in which file_search_path/2 generates answers.

:- multifile user:file_search_path/2.

user:file_search_path(data, MyData) :-
    expand_file_name('~/data', [MyData]).
user:file_search_path(data, '/usr/local/share/data').
?- absolute_file_name(data(myfile), Abs, [access(read)]).
Abs = '/home/janw/data/myfile'.
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Wouter Beek said (2017-10-28T20:00:15):1 upvotes 1 0 downvotes
Picture of user Wouter Beek.

A specific way in which the {...} wildcard behaves differently from the one in Bash is that Bash does not perform brace expansion for 0 or 1 arguments, whereas expand_file_name/2 does perform expansion in these cases.

Bash reference: "A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged."

LogicalCaptain said (2020-09-20T21:07:26):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

References

Note that this is based on "csh" filename expansion. Here is the manual page for the tcsh (an improved csh):

https://nature.berkeley.edu/~casterln/tcsh/Filename_substitution.html

The bash (an improved sh) filename expansion is somewhat different:

https://www.gnu.org/software/bash/manual/html_node/Filename-Expansion.html

Description needs a fix

Contrary to the description, you can ask for the home directory of another user:

% Mine

?-  expand_file_name("~", Home).
Home = ['/home/paquette'].

% Not mine

?- expand_file_name("~root", Home).
Home = ['/root'].

?- expand_file_name("~postfix", Home).
Home = ['/var/spool/postfix'].

Erroneous text

`$\arg{var}` is a typo.

The environment expansion is done by just giving $NAME :

% Yes

?- expand_file_name("$MAVEN_HOME", H).
H = ['/usr/local/java/maven'].

% No!

?- expand_file_name("${MAVEN_HOME}", H).
H = [].

It can alternatively be read by getenv/2

?- getenv('MAVEN_HOME',X).
X = '/usr/local/java/maven'.

Expansion problem

Problem: How do I deal with a filename that contains a '$' (perfectly legal)

?- expand_file_name('$MAVEN_HOME', H).
H = ['/usr/local/java/maven'].

?- expand_file_name('$$MAVEN_HOME', H).
H = ['$/usr/local/java/maven'].

?- expand_file_name('\$MAVEN_HOME', H).
ERROR: Syntax error: Unknown character escape in quoted atom or string: `\$'

?- expand_file_name('\\$MAVEN_HOME', H).
H = ['\\/usr/local/java/maven'].

Needs a fix?

Expanding, then removing the prefix

?- expand_file_name('/usr/lib64/security/*.so',List),maplist(file_base_name,List,Files).

Gives:

List = ['/usr/lib64/security/pam_access.so','/usr/lib64/security/pam_cap.so','/usr/lib64/security/pam_chroot.so','/usr/lib64/security/pam_console.so'|...],
Files = ['pam_access.so','pam_cap.so','pam_chroot.so','pam_console.so'|...].