What is the significance of "Incomplete last line" when opening a file?

Question

I occasionally open a file with vim and see something like this:

"<file name>" [Incomplete last line][dos format] 71 lines, 2912 characters

I understand all of that information except Incomplete last line. I realize this just means that there isn't a line termination character on the last line, and I know if I resave the file and reopen it, that message goes away, which implies that vim automatically adds the line termination character if it isn't already present. My question is: why does this matter, and why does vim bother to tell me about it?

Answer 1

The Unix convention is that a line is a sequence of zero or more characters ending with a newline character. A text file is a sequence of such lines. This is just a convention, but adhering to that convention allows text-processing tools to work together. It avoids problems of ambiguity in commands such as cat foo bar when the last character of foo is not a newline.

A sequence of characters that does not end with a newline is not a "line", in this sense. It is incomplete and some tools will not process it as the user might expect. Vim warns you about it because it is a potential problem and fixes it because Vim attempts to write proper files.