Comparing Files

Joey Bernard — Mon, 29 Mar 2010 14:54:22 +0000

You often may need to compare one version of a file to an earlier one or check one file against a reference file. Linux provides several tools for doing this, depending on how deep a comparison you need to make.

The most common task involves comparing two text files. The tool of choice for this task is diff. With diff, you can compare two files line by line. By default, diff notices any differences between the two text files, no matter how small. This could be as simple as a space character being changed into a tab character from one file to the next. The file will look the same to a user, but diff will find that difference. The real power of diff comes from the options available to ignore certain kinds of differences between files. In the above example, you could ignore that change from a space character to a tab character by using the -b or --ignore-space-change options, which tell diff to ignore any differences in the amount of whitespace from one file to the next.

What about blank lines? The -B or --ignore-blank-lines options tell diff to ignore any changes in the number of blank lines from one file to the next. In this way, diff effectively looks only at the actual characters when comparing the files, narrowing diff’s focus to the actual content.

What if that’s not good enough for your situation? You may need to compare files where one was entered with Caps Lock turned on for some reason, or maybe the terminal being used was misconfigured. You may not want diff to report simple differences in case as “real” differences. In this situation, use the -i or --ignore-case options.

What if you’re working with files from a Windows box? Everyone who works on both Linux and Windows has run into the issue with line endings on text files. Linux expects only a single newline character, while Windows uses a carriage return and a newline character. diff can ignore this with the --strip-trailing-cr option.

diff’s output can take a few different formats. The default contains the line that is different, along with a number of lines right before and after the line in question. These extra lines are called context and can be set with the “-c”, “-C” or “--context=” options and the number of lines to use for context. This default output can be used by the patch program to change one file into the other. In this way, you can create source code patches to upgrade code from one version to the next. diff also can output differences between files that can be used by ed as a script with the -e or --ed options. diff also will output an RCS-format diff with the option -n or --rcs. Another option is to print out the differences in two columns, side by side, with the -y or --side-by-side options.

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Comparing Files