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1.7 Binary Files and Forcing Text Comparisons

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 section 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.

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 ‘-a’ or ‘--text’ 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 consider all files to be binary files, and report only whether they differ (but not how). Use the ‘-q’ or ‘--brief’ option for this.

Differing binary files are considered to cause trouble because the resulting diff output does not capture all the differences. This trouble causes diff to exit with status 2. However, this trouble cannot occur with the ‘--a’ or ‘--text’ option, or with the ‘-q’ or ‘--brief’ option, as these options both cause diff to treat binary files like text files.

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 ‘-l’ option to show the values of each differing byte in the two files. With GNU cmp, you can also use the ‘-b’ option to show the ASCII representation of those bytes. See section 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’ options.

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