How to view a binary file

This Super User question: Why don't you see binary code when you open a binary file with text editor? addresses your first point quite well.

Binary and text data aren't separated: They are simply data. It depends on the interpretation that makes them one or the other. If you open binary data (such as an image file) in a text editor, much of it won't make sense, because it does not fit your chosen interpretation (as text).

Files are stored as zeros and ones (e.g. voltage/no voltage on memory, magnetization/no magnetization on hard drive). You don't see zeros and ones when cat ing the files because the 0/1 sequences won't be of much use to an human; characters make more sense, and an hexdump is better for most purposes (try hexdump on a file).

Executable files do have a header that describes parameters such as the architecture for which the program was built, and what sections of the file are code and data. This is what file uses to identify the characteristics of your binary file.

Finally: yes, you can write programs in assembly language using CPU opcodes directly. Take a look at Introduction to UNIX assembly programming and the Intel x86 documentation for a starting point.

It appears to be the presence of the null character in the file.(displayed ^@ usually) I entered various control characters into a text file(like delete, ^?, for example), and only the null character caused grep to consider it a binary. This was only tested for grep. The less and diff commands, for instance, may have different methods. Control characters in general don't appear except in binaries. The exceptions are the whitespace characters: newline(^M), tab(^I), formfeed(^L), vertical tab(^K), and return(^J).

However, foreign characters, like arabic or chinese letters, are not standard ascii, and perhaps could be confused with control characters. Perhaps that's why it's only the null character.

You can test it out for yourself by insterting control characters into a text file using the text editor vim. Just go to insert mode, press control-v, and then the control character.