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 White Space).
An incomplete line is normally distinguished on output from a full line by a following line that starts with ‘\’. However, the RCS format (see RCS) 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 Side by Side. The if-then-else line format preserves a line's incompleteness with ‘%L’, and discards the newline with ‘%l’. See Line Formats. Finally, with the ed and forward ed output formats (see 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
.