10.16 Accessing Files And Directories—library(file_systems)

This module provides operations on files and directories, such as renaming, deleting, opening, checking permissions, accessing members of.

The following principles have been observed:

• An absolute distinction is drawn between files and directories. The set of operations one can usefully perform on a directory is different from the set one can perform on a file: for example, having write permission to a directory allows the user to create new files in it, not to rewrite the entire directory! If any routine in this package tells you that a “file” exists, you can be sure that it means a file and not a directory (and vice versa for “directory” exists).
• The directory scanning routines do not actually open the files they find. Thus finer discriminations, such as that between source and object code, are not made.
• All paths are expanded as if by absolute_file_name/3.
• Every predicate acts like a genuine logical relation insofar as it possibly can.
• If anything goes wrong, the predicates raise an error exception. Any time that a predicate fails quietly, it should mean “this question is meaningful, but the answer is no”.
• The directory scanning routines insist that the directory argument name a searchable directory.
• On Unix-like systems, symbolic links are followed by default and symbolic links that can not be followed are treated as non-existing. This means file_exists/1 will fail if passed such a “broken” link and that neither file_members_of_directory/1 nor directory_members_of_directory/1 et al. will return such a link.

  On Windows, symbolic links (and other reparse points) are not followed when enumerating directory contents with file_members_of_directory/1 nor directory_members_of_directory/1 et al. and are not returned for these predicates.

  The behavior for symbolic links (and reparse points) may change on all platforms in the future to ensure a well defined and consistent behavior on all platforms.

  To see all members of a directory you can use absolute_file_name/3 with a glob('*') option.

The “property” routines use the same simplistic access control model as that used by the absolute_file_name/3 access/1-option. See mpg-ref-absolute_file_name, for details.

Exported predicates:

rename_file(+OldName, +NewName)

OldName must identify an existing file, which will be renamed to NewName. The details of just when this can be done are operating-system dependent. Typically it is only possible to rename within the same file system.

rename_directory(+OldName, +NewName)

OldName must identify an existing directory, which will be renamed to NewName. The details of just when this can be done are operating-system dependent. Typically it is only possible to rename empty directories within the same file system.

delete_file(+OldName)

OldName must identify an existing file, which will be deleted.

delete_directory(+Directory)

delete_directory(+Directory, +Options)

Directory must identify an existing directory, which will be deleted, if empty. Options should be a list of at most one term of the form:

if_nonempty(Value)

Defines what to do if the directory is nonempty. One of:

ignore

The predicate simply succeeds, deleting nothing.

fail

The predicate simply fails, deleting nothing.

error

The predicate raises a permisison error.

delete

The predicate recursively deletes the directory and its contents.

directory_exists(+Directory)

directory_exists(+Directory, +Mode)

is true when Directory is an existing directory that is accessible according to Mode. Mode defaults to exist.

This is more or less equivalent to absolute_file_name(File, _, [file_type(directory),access([exist|Mode]),file_errors(fail)]).

make_directory(+Directory)

Directory is expanded, as if by absolute_file_name/3, and the resulting directory is created.

file_exists(+File)

file_exists(+File, +Mode)

is true when File is an existing file that is accessible according to Mode. Mode defaults to exist.

This is more or less equivalent to absolute_file_name(File, _, [access([exist|Mode]),file_errors(fail)]).

file_must_exist(+File)

file_must_exist(+File, +Mode)

is like file_exists(File[, Mode]) except that if the file is not accessible it reports an error.

This is more or less equivalent to absolute_file_name(File, _, [access([exist|Mode]),file_errors(error)]).

directory_must_exist(+File)

directory_must_exist(+File, +Mode)

is like file_must_exists(File[, Mode]), but for directories.

This is more or less equivalent to absolute_file_name(File, _, [file_type(directory),access([exists|Mode]),file_errors(error)]).

close_all_streams

closes all the streams (other than the standard streams) which are currently open. The time to call this is after an abort/0. Note that current_stream/3 does not notice the standard streams.

directory_member_of_directory(?BaseName, ?FullName)

is true when BaseName is the name of a subdirectory of the current directory (other than ’.’ or ’..’) and FullName is its absolute name.

This uses absolute_file_name/3 with the glob/1 option.

directory_member_of_directory(+Directory, ?BaseName, ?FullName)

is true when Directory is a name (not necessarily the absolute name) of a directory, BaseName is the name of a subdirectory of that directory (other than ’.’ or ’..’) and FullName is its absolute name.

This uses absolute_file_name/3 with the glob/1 option.

directory_member_of_directory(+Directory, +Pattern, ?BaseName, ?FullName)

is true when Directory is a name (not necessarily the absolute name) of a directory, BaseName is the name of a directory of that directory (other than ’.’ or ’..’) which matches the given Pattern, and FullName is the absolute name of the subdirectory.

This uses absolute_file_name/3 with a glob(Pattern) option.

directory_members_of_directory(-Set)

is true when Set is a set of BaseName-FullName pairs being the relative and absolute names of subdirectories of the current directory.

This uses absolute_file_name/3 with the glob/1 option.

directory_members_of_directory(+Directory, -Set)

is true when Set is a set of BaseName-FullName pairs being the relative and absolute names of subdirectories of the given Directory. Directory need not be absolute; the FullNames will be regardless.

This uses absolute_file_name/3 with the glob/1 option.

directory_members_of_directory(+Directory, +Pattern, -Set)

is true when Set is a set of BaseName-FullName pairs being the relative and absolute names of subdirectories of the given Directory, such that each BaseName matches the given Pattern.

This uses absolute_file_name/3 with a glob(Pattern) option.

file_member_of_directory(?BaseName, ?FullName)

is true when BaseName is the name of a file in the current directory and FullName is its absolute name.

This uses absolute_file_name/3 with the glob/1 option.

file_member_of_directory(+Directory, ?BaseName, ?FullName)

is true when Directory is a name (not necessarily the absolute name) of a directory, BaseName is the name of a file in that directory, and FullName is its absolute name.

This uses absolute_file_name/3 with the glob/1 option.

file_member_of_directory(+Directory, +Pattern, ?BaseName, ?FullName)

is true when Directory is a name (not necessarily the absolute name) of a directory, BaseName is the name of a file in that directory which matches the given Pattern, and FullName is its absolute name.

This uses absolute_file_name/3 with a glob(Pattern) option.

file_members_of_directory(-Set)

is true when Set is a set of BaseName-FullName pairs being the relative and absolute names of the files in the current directory.

This uses absolute_file_name/3 with the glob/1 option.

file_members_of_directory(+Directory, -Set)

is true when Set is a set of BaseName-FullName pairs being the relative and absolute names of the files in the given Directory. Directory need not be absolute; the FullNames will be regardless.

This uses absolute_file_name/3 with the glob/1 option.

file_members_of_directory(+Directory, +Pattern, -Set)

is true when Set is a set of BaseName-FullName pairs being the relative and absolute names of subdirectories of the given Directory, such that each BaseName matches the given Pattern.

This uses absolute_file_name/3 with a glob(Pattern) option.

directory_property(+Directory, ?Property)

is true when Directory is a name of a directory, and Property is a boolean property which that directory possesses, e.g.

    directory_property(., searchable).

The current set of file and directory properties include:

readable

writable

executable

searchable

Tries to determine whether the process has permission to read, write, execute (only for files) or search (only for directories) the file.

size_in_bytes

The size, in bytes, of the file. Not available for directories.

create_timestamp

modify_timestamp

access_timestamp

The time of creation, last modification or last access expressed as a timestamp. A timestamp is an integer expressing the time interval, in seconds, since the “Epoch”. The Epoch is the time zero hours, zero minutes, zero seconds, on January 1, 1970 Coordinated Universal Time (UTC).

The timestamp is what should be used when comparing information between files since it is independent of locale issues like time zone and daylight savings time etc.

create_localtime

modify_localtime

access_localtime

The same as the corresponding …_timestamp values passed through system:datime/2, i.e. expressed as local time and split up in the components year, month, day, hour, minute, seconds.

set_user_id

set_group_id

save_text

True if the set-uid, set-group-id, save-text bits, respectively, are set for the file. Always false on Windows.

who_can_read

who_can_write

who_can_execute

who_can_search

A list containing the subset of [user,group,other] for the process classes that can, respectively, read, write, execute (only for files) or search (only for directories.

owner_user_id

owner_group_id

The id of the owner and group of the file. The id is an integer on UNIX and an atom (expressed as a string security identifier) on Windows.

owner_user_name

owner_group_group

The atom containing the name of the files owner and group respectively. On Windows a name like 'DOMAIN\NAME' will be used.

If for some reason the name cannot be found it will fall back to using the same value as owner_user_id and owner_group_id.

Other properties may be added in the future. You can backtrack through the available properties by calling file_property/3 or directory_property/3 with an uninstantiated Property argument.

directory_property(+Directory, ?Property, ?Value)

is true when Directory is a name of a directory, Property is a property of directories, and Value is Directory’s Property Value. See directory_property/2, above, for a list of properties.

file_property(+File, ?Property)

is true when File is a name of a file, and Property is a boolean property which that file possesses, e.g.

    file_property('foo.txt', readable).

See directory_property/2, above, for a list of properties.

file_property(+File, ?Property, ?Value)

is true when File is a name of a file, Property is a property of files, and Value is File’s Property Value. See directory_property/2, above, for a list of properties.

current_directory(-Directory)

current_directory(-Directory, +NewDirectory)

Directory is unified with the current working directory and the working directory is set to NewDirectory.