Editing Filename extension (section)

== Executable programs ==
{{cleanup|section|reason=intractable construction|date=November 2015}}

The use of a filename extension in a command name appears occasionally, usually as a side effect of the command having been implemented as a script, e.g., for the [[Bourne shell]] or for [[Python (programming language)|Python]], and the interpreter name being suffixed to the command name, a practice common on systems that rely on associations between filename extension and interpreter, but sharply deprecated<ref>[http://www.talisman.org/~erlkonig/documents/commandname-extensions-considered-harmful Commandname Extensions Considered Harmful]</ref> in [[Unix-like]] systems, such as [[Linux]], [[Oracle Solaris]], [[BSD]]-based systems, and Apple's [[macOS]], where the interpreter is normally specified as a header in the script ("[[Shebang (Unix)|shebang]]").

On association-based systems, the filename extension is generally mapped to a single, system-wide selection of interpreter for that extension (such as ".py" meaning to use Python), and the command itself is runnable from the command line even if the extension is omitted (assuming appropriate setup is done). If the implementation language is changed, the command name extension is changed as well, and the OS provides a consistent [[API]] by allowing the same extensionless version of the command to be used in both cases. This method suffers somewhat from the essentially global nature of the association mapping, as well as from developers' incomplete avoidance of extensions when calling programs, and that developers can not force that avoidance. Windows is the only remaining widespread employer of this mechanism.

On systems with [[interpreter directive]]s, including virtually all versions of Unix, command name extensions have no special significance, and are by standard practice not used, since the primary method to set interpreters for scripts is to start them with a single line specifying the interpreter to use. In these environments, including the extension in a command name unnecessarily exposes an implementation detail which puts all references to the commands from other programs at future risk if the implementation changes. For example, it would be perfectly normal for a shell script to be reimplemented in Python or Ruby, and later in C or C++, all of which would change the name of the command were extensions used. Without extensions, a program always has the same extension-less name, with only the [[interpreter directive]] or [[magic number (programming)|magic number]] changing, and references to the program from other programs remain valid.