POSIX does actually define what a "text file" is, but the definition is a bit unusual:

See https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1...

> 3.387 Text File

> A file that contains characters organized into zero or more lines. The lines do not contain NUL characters and none can exceed {LINE_MAX} bytes in length, including the <newline> character.

So, if you have some non-printable characters like BEL/␇/ASCII 0x07, that's still a text file.

(and I believe what bytes count as a valid character depend on your `LC_CTYPE`).

But the moment you have a line longer than {LINE_MAX} bytes (which can depend on which POSIX environment you have), suddenly your text file is now a binary file.