If a File Extension Is Not in the File Name?

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If a file extension is not in the file name, and not because it is hidden, such as a .pdf, how do you determine its file type?

Caleb@debianbox.~$ echo test > test caleb@debianbox.~$ cat test test caleb@debianbox.~$ file test test. ASCII text I wouldn’t say that there’s explicitly a type associated with a file without an extension, but most programs will treat it as plain text. If you rename a file (say, a PDF or Word document) to remove the extension and then attempt to open it, it may not show up in the dialog anymore unless you hit All Files (at that point, the PDF viewer/Microsoft Word is trusting that you know better than it does which types of files it can open). However, the file will still open without any problems. Linux and macOS executables (and probably other binaries on other architectures as well) are also stored with no extension, but are usually stored with the executable bit on so that the default method of opening it is executing it. The Linux kernel itself, when compiled, also has no extension (vmlinux or vmlinuz).

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The Windows NT and Windows 95 operating systems are probably the same, although NT might even have no extension. Mac OS X (which actually is an instance of UNIX) does have a way to explicitly associate an extension with some files. It will tell the operating system that the file should have that particular extension (using its icon) and will look for the file in a certain directory. When a Macintosh user wants to read data from or write to one of those data files (say, a Word document), or a Linux user wants to access or open a Linux data file on Mac OS X, the operating system will first look in the directory where the data file is stored. If the data file is not in that directory, the operating system will look elsewhere for it. If it is in the proper directory and the data file.