an alias companion virus is a type of companion virus that takes advantage of the command aliasing functionality of operating systems that support it in order to get executed instead of the program the user intended to execute.
command aliasing is a feature whereby long, complex commands can be given simple, easy to type aliases in order to enhance the usability of the system. MS-DOS didn't support this functionality, nor is there much demand for it in the face of graphical user interfaces, but some alternative DOS command shells did, and various flavours of *nix (including linux) do as well.
if a viral program creates an alias that points to itself but gives that alias the same name as some other program (the so-called host program), and if the operating system in question gives execution precedence to aliases (which they often do), then alias (or more specifically the viral program it points to) will get executed instead of the host program the alias got it's name from.
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