When an input file ends in a non-newline character, its last line is called an incomplete line because its last character is not a newline. All other lines are called full lines and end in a newline character. Incomplete lines do not match full lines unless differences in white space are ignored (see section Suppressing Differences in Blank and Tab Spacing).
An incomplete line is normally distinguished on output from a full line
by a following line that starts with `\'. However, the RCS format
(see section RCS Scripts) outputs the incomplete line as-is, without any trailing
newline or following line. The side by side format normally represents
incomplete lines as-is, but in some cases uses a `\' or `/'
gutter marker; See section Showing Differences Side by Side. The if-then-else line format
preserves a line's incompleteness with `%L', and discards the
newline with `%l'; See section Line Formats. Finally, with the
ed and forward ed output formats (see section diff Output Formats)
diff cannot represent an incomplete line, so it pretends there
was a newline and reports an error.
For example, suppose `F' and `G' are one-byte files that contain just `f' and `g', respectively. Then `diff F G' outputs
1c1 < f \ No newline at end of file --- > g \ No newline at end of file
(The exact message may differ in non-English locales.) `diff -n F G' outputs the following without a trailing newline:
d1 1 a1 1 g
`diff -e F G' reports two errors and outputs the following:
1c g .