Editing COM file (section)

==Execution preference==
In DOS, if a directory contains both a COM file and an [[EXE]] file with same name, when no extension is specified the COM file is preferentially selected for execution. For example, if a directory in the [[Path (variable)|system path]] contains two files named <code>foo.com</code> and <code>foo.exe</code>, the following would execute <code>foo.com</code>:

 C:\>foo

A user wishing to run <code>foo.exe</code> can explicitly use the complete filename:

 C:\>foo.exe

Taking advantage of this default behaviour, [[computer virus|virus]] writers and other malicious programmers have used names like <code>notepad.com</code> for their creations, hoping that if it is placed in the same directory as the corresponding EXE file, a command or batch file may accidentally trigger their program instead of the text editor [[Notepad (Windows)|<code>notepad.exe</code>]]. Again, these .com files may in fact contain a .exe format executable.

On [[Windows NT]] and derivatives ([[Windows 2000]], [[Windows XP]], [[Windows Vista]], and [[Windows 7]]), the {{mono|PATHEXT}} variable is used to override the order of preference (and acceptable extensions) for calling files without specifying the extension from the command line. The default value still places <code>.com</code> files before <code>.exe</code> files. This closely resembles a feature previously found in JP Software's line of extended command line processors [[4DOS]], [[4OS2]], and [[Take Command Console|4NT]].