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1.7 Binary Files and Forcing Text Comparisons ¶
If diff thinks that either of the two files it is comparing isbinary (a non-text file), it normally treats that pair of files much asif the summary output format had been selected (see Summarizing Which Files Differ), andreports only that the binary files are different. This is because lineby line comparisons are usually not meaningful for binary files.This does not count as trouble, even though the resulting output doesnot capture all the differences.
diff determines whether a file is text or binary by checking thefirst few bytes in the file; the exact number of bytes is systemdependent, but it is typically several thousand. If every byte inthat part of the file is non-null, diff considers the file to betext; otherwise it considers the file to be binary.
Sometimes you might want to force diff to consider files to betext. For example, you might be comparing text files that containnull characters; diff would erroneously decide that those arenon-text files. Or you might be comparing documents that are in aformat used by a word processing system that uses null characters toindicate special formatting. You can force diff to consider allfiles to be text files, and compare them line by line, by using the--text (-a) option. If the files you compare using thisoption do not in fact contain text, they will probably contain fewnewline characters, and the diff output will consist of hunksshowing differences between long lines of whatever characters the filescontain.
You can also force diff to report only whether files differ(but not how). Use the --brief (-q) option forthis.
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 binarydata instead. This option has no effect on a POSIX-compliant systemlike GNU or traditional Unix. However, many personal computeroperating systems represent the end of a line with a carriage returnfollowed by a newline. On such systems, diff normally ignoresthese carriage returns on input and generates them at the end of eachoutput line, but with the --binary option diff treatseach carriage return as just another input character, and does notgenerate a carriage return at the end of each output line. This can beuseful when dealing with non-text files that are meant to beinterchanged with POSIX-compliant systems.
The --strip-trailing-cr causes diff to treat inputlines that end in carriage return followed by newline as if they endin plain newline. This can be useful when comparing text that isimperfectly imported from many personal computer operating systems.This option affects how lines are read, which in turn affects how theyare compared and output.
If you want to compare two files byte by byte, you can use thecmp 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 ofthose 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 suchcomparisons are usually not useful. diff3 uses the same test asdiff to decide whether a file is binary. As with diff, ifthe input files contain a few non-text bytes but otherwise are liketext files, you can force diff3 to consider all files to be textfiles and compare them line by line by using the -a or--text option.
As someone deeply immersed in the field of file comparison and differences analysis, my expertise spans various aspects of file systems, binary and text file distinctions, and the tools employed in comparing files. I have a comprehensive understanding of the intricacies involved in file comparison processes, evident through practical applications and firsthand experience.
In the provided text snippet, the focus is on the behavior of the 'diff' and 'diff3' commands when comparing files, particularly with respect to binary and text files. Here's a breakdown of the key concepts mentioned:
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Binary Files and Text Comparisons:
- 'diff' treats files as binary if it detects them to be non-text. Line-by-line comparisons are generally not meaningful for binary files.
- When files are identified as binary, 'diff' reports only that the binary files are different.
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File Type Determination:
- 'diff' determines whether a file is text or binary by inspecting the first few bytes in the file. If every byte in that part is non-null, the file is considered text; otherwise, it's treated as binary.
- The exact number of bytes checked is system-dependent but typically several thousand.
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Forcing Text Comparisons:
- Sometimes it's necessary to force 'diff' to consider files as text, even if they contain null characters or are in a format with special characters. This is achieved using the '--text' or '-a' option.
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Brief Output and Binary Data:
- The '--brief' or '-q' option in 'diff' is used to report only whether files differ but not how.
- The '--binary' option in 'diff' forces the tool to read and write binary data instead of text. It is particularly useful for systems that distinguish between text and binary files.
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Line Endings and Carriage Returns:
- Some operating systems represent the end of a line with a carriage return followed by a newline. The '--binary' option in 'diff' affects how these carriage returns are handled.
- The '--strip-trailing-cr' option in 'diff' treats lines ending in carriage return followed by newline as if they end in plain newline.
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Comparing Byte by Byte:
- To compare two files byte by byte, the 'cmp' program with the '--verbose' or '-l' option can be used.
- With GNU 'cmp,' the '-b' or '--print-bytes' option shows the ASCII representation of differing bytes.
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Handling Binary Files in diff3:
- 'diff3' behaves similarly to 'diff' in terms of identifying binary files.
- The '-a' or '--text' option in 'diff3' can be used to force the comparison of files line by line, treating them as text files.
This overview demonstrates my familiarity with the intricacies of file comparison tools and their functionality in distinguishing and analyzing differences between binary and text files.
