You basically just have to know the tricks of inputting binary data through the shell. Learn how the ASCII table works, what the ctrl and shift keys do (setting/unsetting bits 5-7, in particular), and how the Alt key enables the 8-bit.
You could even go a little further. the printf command should be a built-in with just about any shell, so even lacking the `cat` program, you can output arbitrary data to a file.
Almost seems cheating to just hand-craft an ELF executable from the keyboard, but who's making the rules?
If you have a shell or standard POSIX tools you'll have printf, which supports hex and octal formatting, and often C modifiers, so you can encode it as one long-ass integer and save keystrokes.
You could even go a little further. the printf command should be a built-in with just about any shell, so even lacking the `cat` program, you can output arbitrary data to a file.
Almost seems cheating to just hand-craft an ELF executable from the keyboard, but who's making the rules?