If diff thinks that either of the two files it is comparing is
binary (a non-text file), it normally treats that pair of files much as
if the summary output format had been selected (see Summarizing Which Files Differ), and
reports only that the binary files are different. This is because line
by line comparisons are usually not meaningful for binary files.
This does not count as trouble, even though the resulting output does
not capture all the differences.
diff determines whether a file is text or binary by checking the
first few bytes in the file; the exact number of bytes is system
dependent, but it is typically several thousand. If every byte in
that part of the file is non-null, diff considers the file to be
text; otherwise it considers the file to be binary.
Sometimes you might want to force diff to consider files to be
text. For example, you might be comparing text files that contain
null characters; diff would erroneously decide that those are
non-text files. Or you might be comparing documents that are in a
format used by a word processing system that uses null characters to
indicate special formatting. You can force diff to consider all
files to be text files, and compare them line by line, by using the
--text (-a) option. If the files you compare using this
option do not in fact contain text, they will probably contain few
newline characters, and the diff output will consist of hunks
showing differences between long lines of whatever characters the files
contain.
You can also force diff to report only whether files differ
(but not how). Use the --brief (-q) option for
this.
In operating systems that distinguish between text and binary files,
diff normally reads and writes all data as text. Use the
--binary option to force diff to read and write binary
data instead. This option has no effect on a POSIX-compliant system
like GNU or traditional Unix. However, many personal computer
operating systems represent the end of a line with a carriage return
followed by a newline. On such systems, diff normally ignores
these carriage returns on input and generates them at the end of each
output line, but with the --binary option diff treats
each carriage return as just another input character, and does not
generate a carriage return at the end of each output line. This can be
useful when dealing with non-text files that are meant to be
interchanged with POSIX-compliant systems.
The --strip-trailing-cr causes diff to treat input
lines that end in carriage return followed by newline as if they end
in plain newline. This can be useful when comparing text that is
imperfectly imported from many personal computer operating systems.
This option affects how lines are read, which in turn affects how they
are compared and output.
If you want to compare two files byte by byte, you can use the
cmp program with the --verbose (-l)
option to show the values of each differing byte in the two files.
With GNU cmp, you can also use the -b or
--print-bytes option to show the ASCII representation of
those bytes. See Invoking cmp, for more information.
If diff3 thinks that any of the files it is comparing is binary
(a non-text file), it normally reports an error, because such
comparisons are usually not useful. diff3 uses the same test as
diff to decide whether a file is binary. As with diff, if
the input files contain a few non-text bytes but otherwise are like
text files, you can force diff3 to consider all files to be text
files and compare them line by line by using the -a or
--text option.