Do file extensions have any purpose in Linux?

Linux determines a file's type via code in the file's header. This process doesn't depend on file extensions to know which software to use for opening the file.

(That's what I remember from my education. Please correct me in case I'm wrong!)

Working a bit with Ubuntu systems recently: I see a lot of files on the systems which have extensions like .sh, .txt, .o, .c.

Now I'm wondering: Is the purpose of these extensions to merely help people understand what sort of file they happen to be looking at? Or do they have some purpose for the operating system also?

Solution 1:

There is no 100% black or white answer here.

Usually Linux does not rely on file names (and file extensions i.e. the part of the file name after the normally last period) and instead determines the file type by examining the first few bytes of its content and comparing that to a list of known magic numbers.

For example all Bitmap image files (usually with name extension .bmp) must start with the letters BM in their first two bytes. Scripts in most scripting languages like Bash, Python, Perl, AWK, etc. (basically everything that treats lines starting with # as comment) may contain a shebang like #!/bin/bash as first line. This special comment tells the system with which application to open the file.

So normally the operating system relies on the file content and not its name to determine the file type, but stating that file extensions are never needed on Linux is only half of the truth.

Applications may of course implement their file checks however they want, which includes verifying the file name and extension. An example is the Eye of Gnome (eog, standard picture viewer) which determines the image format by the file extension and throws an error if it does not match the content. Whether this is a bug or a feature can be discussed...

However, even some parts of the operating system rely on file name extensions, e.g. when parsing your software sources files in /etc/apt/sources.list.d/ - only files with the *.list extension get parsed all others are ignored. It's maybe not mainly used to determine the file type here but rather to enable/disable parsing of some files, but it's still a file extension that affects how the system treats a file.

And of course the human user profits most from file extensions as that makes the type of a file obvious and also allows multiple files with the same base name and different extensions like site.html, site.php, site.js, site.css etc. The disadvantage is of course that file extension and the actual file type/content do not necessarily have to match.

Additionally it's needed for cross-platform interoperability, as e.g. Windows will not know what to do with a readme file, but only a readme.txt.

Solution 2:

Linux determines the type of a file via a code in the file header. It doesn't depend on file extensions for to know with software is to use for opening the file.

That's what I remember from my education. Please correct me in case I'm wrong!

• correctly remembered.

Are these extensions are meant only for humans?

• Yes, with a but.

When you interact with other operating systems that do depend on extensions being what they are it is the smarter idea to use those.

In Windows, opening software is attached to the extensions.

Opening a text file named "file" is harder in Windows than opening the same file named "file.txt" (you will need to switch the file open dialog from *.txt to *.* every time). The same goes for TAB and semi-colon separated text files. The same goes for importing and exporting e-mails (.mbox extension).

In particular when you code software. Opening a file named "software1" that is an HTML file and "software2" that is a JavaScript file becomes more difficult compared to "software.html" and "software.js".

If there is a system in place in Linux where file extensions are important, I would call that a bug. When software depends on file extensions, that is exploitable. We use an interpreter directive to identify what a file is ("the first two bytes in a file can be the characters "#!", which constitute a magic number (hexadecimal 23 and 21, the ASCII values of "#" and "!") often referred to as shebang,").

The most famous problem with file extensions was LOVE-LETTER-FOR-YOU.TXT.vbs on Windows. This is a visual basic script being shown in file explorer as a text file.

In Ubuntu when you start a file from Nautilus you get a warning what it is going to do. Executing a script from Nautilus where it wants to start some software where it is supposed to open gEdit is obvious a problem and we get a warning about it.

In command line when you execute something, you can visually see what the extension is. If it ends on .vbs I would start to become suspicious (not that .vbs is executable on Linux. At least not without some more effort ;) ).