filename

A filename is the name assigned to a file in a filesystem to identify it and to allow users and programs to refer to the file. It may be used together with a directory path to locate the file on storage media. In many contexts the filename includes an extension, which is a suffix separated by a dot that often indicates the file’s type or intended program.

In many operating systems, a filename consists of a base name and an extension, such as document.txt. If a file has multiple dots, the convention is to treat the portion after the last dot as the extension. In Unix-like systems the extension is not strictly significant to the system, but many applications rely on it to determine how to handle the file. In Windows, the extension is commonly used to associate the file with a program and to determine behavior in user interfaces.

Platform differences and restrictions affect what can appear in a filename. Windows forbids characters such as < > : " / \ | ? * and reserves names like CON, PRN, AUX, NUL, and COM1 through COM9 and LPT1 through LPT9. Filenames are typically limited to about 255 characters per component, while full path length limits (such as MAX_PATH) constrain the total path. Linux and other Unix-like systems generally allow slashes only as directory separators and permit a wide range of characters, though certain characters may cause issues in shells or scripts.

Best practices for portability include avoiding leading or trailing spaces, avoiding trailing dots, using safe characters, and treating extensions as a hint rather than a guarantee of file type.