---
name: "filename"
section: "3erl"
platform: "linux"
package: "erlang-manpages"
version: "1:25.2.3+dfsg-1+deb12u4"
release: "Debian 12 (Bookworm)"
platforms: ["linux"]
url: "https://new.carta.tech/filename/3erl"
---

# filename(3erl)

Filename manipulation functions.

## NAME

filename - Filename manipulation functions.

## DESCRIPTION

This module provides functions for analyzing and manipulating filenames. These functions are designed so that the Erlang code can work on many different platforms with different filename formats. With filename is meant all strings that can be used to denote a file. The filename can be a short relative name like *foo.erl*, a long absolute name including a drive designator, a directory name like *D:\\usr/local\\bin\\erl/lib\\tools\\foo.erl*, or any variations in between.

In Windows, all functions return filenames with forward slashes only, even if the arguments contain backslashes. To normalize a filename by removing redundant directory separators, use *join/1*.

The module supports raw filenames in the way that if a binary is present, or the filename cannot be interpreted according to the return value of *file:native\_name\_encoding/0*, a raw filename is also returned. For example, *join/1* provided with a path component that is a binary (and cannot be interpreted under the current native filename encoding) results in a raw filename that is returned (the join operation is performed of course). For more information about raw filenames, see the *file* module.

**Note:**

Functionality in this module generally assumes valid input and does not necessarily fail on input that does not use a valid encoding, but may instead very likely produce invalid output.

File operations used to accept filenames containing null characters (integer value zero). This caused the name to be truncated and in some cases arguments to primitive operations to be mixed up. Filenames containing null characters inside the filename are now *rejected* and will cause primitive file operations to fail.

**Warning:**

Currently null characters at the end of the filename will be accepted by primitive file operations. Such filenames are however still documented as invalid. The implementation will also change in the future and reject such filenames.

## EXPORTS

```
absname(Filename) -> file:filename_all()
```

Types:

Filename = file:name\_all()\

Converts a relative *Filename* and returns an absolute name. No attempt is made to create the shortest absolute name, as this can give incorrect results on file systems that allow links.

*Unix examples:*

```
1> pwd().
"/usr/local"
2> filename:absname("foo").
"/usr/local/foo"
3> filename:absname("../x").
"/usr/local/../x"
4> filename:absname("/").
"/"
```

*Windows examples:*

```
1> pwd().
"D:/usr/local"
2> filename:absname("foo").
"D:/usr/local/foo"
3> filename:absname("../x").
"D:/usr/local/../x"
4> filename:absname("/").
"D:/"
```

```
absname(Filename, Dir) -> file:filename_all()
```

Types:

Filename = Dir = file:name\_all()\

Same as *absname/1*, except that the directory to which the filename is to be made relative is specified in argument *Dir*.

```
absname_join(Dir, Filename) -> file:filename_all()
```

Types:

Dir = Filename = file:name\_all()\

Joins an absolute directory with a relative filename. Similar to *join/2*, but on platforms with tight restrictions on raw filename length and no support for symbolic links, leading parent directory components in *Filename* are matched against trailing directory components in *Dir* so they can be removed from the result - minimizing its length.

```
basedir(PathType, Application) -> file:filename_all()
```

```
basedir(PathsType, Application) -> [file:filename_all()]
```

Types:

PathType = basedir\_path\_type()\
 PathsType = basedir\_paths\_type()\
 Application = string() \| binary()\

```
basedir_path_type() =
    user_cache | user_config | user_data | user_log
```

```
basedir_paths_type() = site_config | site_data
```

Equivalent to  basedir(PathType, Application, #{}) or  basedir(PathsType, Application, #{}).

```
basedir(PathType, Application, Opts) -> file:filename_all()
```

```
basedir(PathsType, Application, Opts) -> [file:filename_all()]
```

Types:

PathType = basedir\_path\_type()\
 PathsType = basedir\_paths\_type()\
 Application = string() \| binary()\
 Opts = basedir\_opts()\

```
basedir_path_type() =
    user_cache | user_config | user_data | user_log
```

```
basedir_paths_type() = site_config | site_data
```

```
basedir_opts() =
    #{author => string() | binary(),
      os => windows | darwin | linux,
      version => string() | binary()}
```

Returns a suitable path, or paths, for a given type. If *os* is not set in *Opts* the function will default to the native option, that is *'linux'*, *'darwin'* or *'windows'*, as understood by *os:type/0*. Anything not recognized as *'darwin'* or *'windows'* is interpreted as *'linux'*.

The options *'author'* and *'version'* are only used with *'windows'* option mode.

- \*

  *user\_cache*

  The path location is intended for transient data files on a local machine.

  On Linux: Respects the os environment variable *XDG\_CACHE\_HOME*.

```
1> filename:basedir(user_cache, "my_application", #{os=>linux}).
"/home/otptest/.cache/my_application"
```

```
1> filename:basedir(user_cache, "my_application", #{os=>darwin}).
"/home/otptest/Library/Caches/my_application"
```

```
1> filename:basedir(user_cache, "My App").
"c:/Users/otptest/AppData/Local/My App/Cache"
2> filename:basedir(user_cache, "My App").
"c:/Users/otptest/AppData/Local/My App/Cache"
3> filename:basedir(user_cache, "My App", #{author=>"Erlang"}).
"c:/Users/otptest/AppData/Local/Erlang/My App/Cache"
4> filename:basedir(user_cache, "My App", #{version=>"1.2"}).
"c:/Users/otptest/AppData/Local/My App/1.2/Cache"
5> filename:basedir(user_cache, "My App", #{author=>"Erlang",version=>"1.2"}).
"c:/Users/otptest/AppData/Local/Erlang/My App/1.2/Cache"
```

- \*

  *user\_config*

  The path location is intended for persistent configuration files.

  On Linux: Respects the os environment variable *XDG\_CONFIG\_HOME*.

```
2> filename:basedir(user_config, "my_application", #{os=>linux}).
"/home/otptest/.config/my_application"
```

```
2> filename:basedir(user_config, "my_application", #{os=>darwin}).
"/home/otptest/Library/Application Support/my_application"
```

```
1> filename:basedir(user_config, "My App").
"c:/Users/otptest/AppData/Roaming/My App"
2> filename:basedir(user_config, "My App", #{author=>"Erlang", version=>"1.2"}).
"c:/Users/otptest/AppData/Roaming/Erlang/My App/1.2"
```

- \*

  *user\_data*

  The path location is intended for persistent data files.

  On Linux: Respects the os environment variable *XDG\_DATA\_HOME*.

```
3> filename:basedir(user_data, "my_application", #{os=>linux}).
"/home/otptest/.local/my_application"
```

```
3> filename:basedir(user_data, "my_application", #{os=>darwin}).
"/home/otptest/Library/Application Support/my_application"
```

```
8> filename:basedir(user_data, "My App").
"c:/Users/otptest/AppData/Local/My App"
9> filename:basedir(user_data, "My App",#{author=>"Erlang",version=>"1.2"}).
"c:/Users/otptest/AppData/Local/Erlang/My App/1.2"
```

- \*

  *user\_log*

  The path location is intended for transient log files on a local machine.

  On Linux: Respects the os environment variable *XDG\_CACHE\_HOME*.

```
4> filename:basedir(user_log, "my_application", #{os=>linux}).
"/home/otptest/.cache/my_application/log"
```

```
4> filename:basedir(user_log, "my_application", #{os=>darwin}).
"/home/otptest/Library/Logs/my_application"
```

```
12> filename:basedir(user_log, "My App").
"c:/Users/otptest/AppData/Local/My App/Logs"
13> filename:basedir(user_log, "My App",#{author=>"Erlang",version=>"1.2"}).
"c:/Users/otptest/AppData/Local/Erlang/My App/1.2/Logs"
```

- \*

  *site\_config*

  On Linux: Respects the os environment variable *XDG\_CONFIG\_DIRS*.

```
5> filename:basedir(site_config, "my_application", #{os=>linux}).
["/usr/local/share/my_application",
 "/usr/share/my_application"]
6> os:getenv("XDG_CONFIG_DIRS").
"/etc/xdg/xdg-ubuntu:/usr/share/upstart/xdg:/etc/xdg"
7> filename:basedir(site_config, "my_application", #{os=>linux}).
["/etc/xdg/xdg-ubuntu/my_application",
 "/usr/share/upstart/xdg/my_application",
 "/etc/xdg/my_application"]
8> os:unsetenv("XDG_CONFIG_DIRS").
true
9> filename:basedir(site_config, "my_application", #{os=>linux}).
["/etc/xdg/my_application"]
```

```
5> filename:basedir(site_config, "my_application", #{os=>darwin}).
["/Library/Application Support/my_application"]
```

- \*

  *site\_data*

  On Linux: Respects the os environment variable *XDG\_DATA\_DIRS*.

```
10> os:getenv("XDG_DATA_DIRS").
"/usr/share/ubuntu:/usr/share/gnome:/usr/local/share/:/usr/share/"
11> filename:basedir(site_data, "my_application", #{os=>linux}).
["/usr/share/ubuntu/my_application",
 "/usr/share/gnome/my_application",
 "/usr/local/share/my_application",
 "/usr/share/my_application"]
12> os:unsetenv("XDG_DATA_DIRS").
true
13> filename:basedir(site_data, "my_application", #{os=>linux}).
["/usr/local/share/my_application",
 "/usr/share/my_application"]
```

```
5> filename:basedir(site_data, "my_application", #{os=>darwin}).
["/Library/Application Support/my_application"]
```

```
basename(Filename) -> file:filename_all()
```

Types:

Filename = file:name\_all()\

Returns the last component of *Filename*, or *Filename* itself if it does not contain any directory separators.

*Examples:*

```
5> filename:basename("foo").
"foo"
6> filename:basename("/usr/foo").
"foo"
7> filename:basename("/").
[]
```

```
basename(Filename, Ext) -> file:filename_all()
```

Types:

Filename = Ext = file:name\_all()\

Returns the last component of *Filename* with extension *Ext* stripped. This function is to be used to remove a (possible) specific extension. To remove an existing extension when you are unsure which one it is, use *rootname(basename(Filename))*.

*Examples:*

```
8> filename:basename("~/src/kalle.erl", ".erl").
"kalle"
9> filename:basename("~/src/kalle.beam", ".erl").
"kalle.beam"
10> filename:basename("~/src/kalle.old.erl", ".erl").
"kalle.old"
11> filename:rootname(filename:basename("~/src/kalle.erl")).
"kalle"
12> filename:rootname(filename:basename("~/src/kalle.beam")).
"kalle"
```

```
dirname(Filename) -> file:filename_all()
```

Types:

Filename = file:name\_all()\

Returns the directory part of *Filename*.

*Examples:*

```
13> filename:dirname("/usr/src/kalle.erl").
"/usr/src"
14> filename:dirname("kalle.erl").
"."
```

```
5> filename:dirname("\\usr\\src/kalle.erl"). % Windows
"/usr/src"
```

```
extension(Filename) -> file:filename_all()
```

Types:

Filename = file:name\_all()\

Returns the file extension of *Filename*, including the period. Returns an empty string if no extension exists.

*Examples:*

```
15> filename:extension("foo.erl").
".erl"
16> filename:extension("beam.src/kalle").
[]
```

```
flatten(Filename) -> file:filename_all()
```

Types:

Filename = file:name\_all()\

Converts a possibly deep list filename consisting of characters and atoms into the corresponding flat string filename.

```
join(Components) -> file:filename_all()
```

Types:

Components = \[file:name\_all()\]\

Joins a list of filename *Components* with directory separators. If one of the elements of *Components* includes an absolute path, such as *"/xxx"*, the preceding elements, if any, are removed from the result.

The result is "normalized":

- \*

  Redundant directory separators are removed.

- \*

  In Windows, all directory separators are forward slashes and the drive letter is in lower case.

*Examples:*

```
17> filename:join(["/usr", "local", "bin"]).
"/usr/local/bin"
18> filename:join(["a/b///c/"]).
"a/b/c"
```

```
6> filename:join(["B:a\\b///c/"]). % Windows
"b:a/b/c"
```

```
join(Name1, Name2) -> file:filename_all()
```

Types:

Name1 = Name2 = file:name\_all()\

Joins two filename components with directory separators. Equivalent to *join(\[Name1, Name2\])*.

```
nativename(Path) -> file:filename_all()
```

Types:

Path = file:name\_all()\

Converts *Path* to a form accepted by the command shell and native applications on the current platform. On Windows, forward slashes are converted to backward slashes. On all platforms, the name is normalized as done by *join/1*.

*Examples:*

```
19> filename:nativename("/usr/local/bin/"). % Unix
"/usr/local/bin"
```

```
7> filename:nativename("/usr/local/bin/"). % Windows
"\\usr\\local\\bin"
```

```
pathtype(Path) -> absolute | relative | volumerelative
```

Types:

Path = file:name\_all()\

Returns the path type, which is one of the following:

- ***absolute*:**

  The path name refers to a specific file on a specific volume.

  Unix example: */usr/local/bin*

  Windows example: *D:/usr/local/bin*

- ***relative*:**

  The path name is relative to the current working directory on the current volume.

  Example: *foo/bar, ../src*

- ***volumerelative*:**

  The path name is relative to the current working directory on a specified volume, or it is a specific file on the current working volume.

  Windows example: *D:bar.erl, /bar/foo.erl*

```
rootname(Filename) -> file:filename_all()
```

```
rootname(Filename, Ext) -> file:filename_all()
```

Types:

Filename = Ext = file:name\_all()\

Removes a filename extension. *rootname/2* works as *rootname/1*, except that the extension is removed only if it is *Ext*.

*Examples:*

```
20> filename:rootname("/beam.src/kalle").
"/beam.src/kalle"
21> filename:rootname("/beam.src/foo.erl").
"/beam.src/foo"
22> filename:rootname("/beam.src/foo.erl", ".erl").
"/beam.src/foo"
23> filename:rootname("/beam.src/foo.beam", ".erl").
"/beam.src/foo.beam"
```

```
split(Filename) -> Components
```

Types:

Filename = file:name\_all()\
 Components = \[file:name\_all()\]\

Returns a list whose elements are the path components of *Filename*.

*Examples:*

```
24> filename:split("/usr/local/bin").
["/","usr","local","bin"]
25> filename:split("foo/bar").
["foo","bar"]
26> filename:split("a:\\msdev\\include").
["a:/","msdev","include"]
```