.TXT is the uppercase version of the .txt file extension. It is essentially the same thing, and is a very common file extension. It is a plain text file. It is not associated with any programs, and can be used on any operating system.
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Operating system: Any (MS-DOS, DR-DOS, Windows, Windows 9x, Windows NT, Linux, GNU/Linux, Classic MacOS, MacOS, MacOS X, OS X, iOS, Android, ChromiumOS, ChromeOS, OS/2, Solaris, FreeBSD, NetBSD, OpenBSD, DragonflyBSD, etc.)
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Opens with: Gedit, Notepad++ (GNU/Linux) Notepad, Notepad++ (Windows) ???, Notepad++ (MacOS) ???, Notepad++ (BSD) HTML Viewer (Android) ??? (iOS/iPadOS/iPhoneOS) ??? (OS/2) ??? (MS-DOS) ??? (DR-DOS) ??? (Solaris) etc.
Shebang? Typically none, none required
Encoding: Any
File icon
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GNOME 1: 
GNOME 2: 
GNOME 3: 
GNOME 40: 
GNOME 41: 
KDE 1: 
KDE 2: 
KDE 3: 
KDE 4: 
KDE 5: 
XFCE: 
LXQT: 
CINNAMON: 
Common Desktop Environment (CDE): 
MacOS (Classic): 
MacOS (Mac OS X): 
MacOS (OS X): 
MacOS (MacOS 10.10-10.12): 
MacOS (Modern, pre-MacOS11): 
MacOS (Modern, MacOS11): 
WinRAR: 
Android: 
iOS: 
Windows (DOS): 
Windows (9x): 
Windows (NT): 
Solaris: 