File comparison in
computingis the automatic comparing of data between files on a file system. The result of comparisons are typically displayed to the user, but can also be used to accomplish tasks in networks, file systems and revision control.
Examples of programs that complete the task include
diffand cmp. The ability to highlight the changes to a document or file is found in text editors and word processors.
Examples of file comparison utilities include FileMerge and
Microsoft File Compare. Diffutilsis a GNUpackage which includes the diffcommand among other utilities. Free software comparison tools that provide file comparison systems, include WinMergeand Meld.
Most file comparison tools find the
longest common subsequencebetween two files. However, other file comparison tools find the longest increasing subsequencebetween two files (US patent|7031972). The file comparison tool used in Bazaar uses "(insert name of algorithm here)" [http://bramcohen.livejournal.com/37690.html] . The rsyncprotocol uses a rolling hashfunction to compare two files on two distant computers with low communication overhead.
When the most absolute type of file comparison is necessary, one would use what is referred to as "byte-level" file comparison. This method compares two or more files byte by byte to find the actual detailed comparison. While algorithmic methods are very accurate indeed, nothing is ever as accurate as byte-level comparison. However, one must trade off speed for this type of accuracy. On the other hand, this reduction in speed is really only an issue during an initial scan of all files. It becomes fairly negligible when performing later, incremental scans or comparisons.
It is important to note that there are different reasons for one to use different types of comparison tools. When one wishes to compare binary files, byte-level is probably best. But if one wishes to compare text files, a side-by-side visual comparison is usually best. (Note that visual comparison is also necessary for program files that are based upon languages that are human-readable or that are script-based.) This gives the user the chance to decide which file is the preferred one to retain, if the files should be merged together to create one containing all of the differences, or perhaps to keep them both as-is for later reference, thru some form of "versioning" control. Versioning is also important for backup purposes.
File comparison is an important, and most likely integral, part of file synchronization and/or backup. Even in backup methodologies, the issue of corruption is an important one. Corruption occurs without warning and without our knowledge; at least usually until too late to recover the missing parts. Usually, the only way to know for sure if a file has become corrupted is when it is next used or opened. Barring that, one must use a comparison tool to at least recognize that a difference has occurred. Therefore, all file sync or backup programs must include file comparison if these programs are to be actually useful and trusted.
When used in automated processes, file comparison can be set to automatically perform the correct method of saving. Usually the default should be to create another version of the same file automatically so that the user does not have to monitor the process at that point in time. Review, for the sake of elimination of unneeded versions of files, can then occur later at a more convenient time.
Prior to file comparison, machines existed to compare magnetic tapes or punch cards. The
IBM 519 Card Reproducercould determine whether a deck of punched cardswere equivalent. In 1957, John Van Gardnerdeveloped a system to compare the check sums of loaded sections of Fortranprograms to debugcompilation problems on the IBM 704. [http://www.softwarepreservation.org/projects/FORTRAN/paper/John%20Van%20Gardner%20-%20Fortran%20And%20The%20Genesis%20Of%20Project%20Intercept.pdf]
Comparison of file comparison tools
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