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Commands to Compare Files in Linux

written by: Kristen Grubb•edited by: J. F. Amprimoz•updated: 7/5/2011

Text files make up a large portion of the Linux operating system. Source code, configuration files, and log files are all simple text files. Thus, Unix developers in their infinite wisdom, have provided us with three commands that allow us to compare two text files and discover the differences.

  • slide 1 of 4

    The cmp command

    The cmp (compare) command compares two files, character (byte) by character, and provides the location of the differences to the screen.

    Used with no options, cmp will only provide the location of the first discrepancy in the form of “char, line". However, there are options that will provide you with more information. The “-l" option will provide you with a list of the byte number and the ASCII octal value of the differing bytes.

    The cmp command is the best command for simply telling you if two files are different, but unless you want to count each character number, it's not a very good tool for finding the differences.

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    The diff Command

    The diff command provides you with the line numbers of one of the files that need to be changed to make the files identical. The output looks like:

    10c10

    < 1. Create a script or determine which command you want to be executed regularly and determine how often it should be run.

    ---

    > 1. Create a script or determine which executed regularly and determine how often it should be run.

    24,26d23

    < Any field that is not applicable to your job should have an asterisk (*) as a place holder. If you want to specify more than one value for any field, separate the values with a comma and no whitespace. The days of the week are indicated by a number with Sunday being 0.

    < For example:

    The diff command uses special symbols to tell you how to change the first file to make it identical to the second file. “10c10" replace line 10 of the old file, which is indicated by the “<" symbol and replace it with line 10 of the second file, which is indicated by the “>" symbol. “24,26d23" says to delete lines 24 through 26 of the old file and the two files will be identical. One other symbol is in the form of 5a6, which says append the line that follows to line 5.

    Once you make the changes, you can use the cmp command to ensure that they are exactly the same.

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    The comm Command

    If you have alphabetically sorted files, or files that you can use the “sort" command on, you can find the commonalities and differences using the comm command. The output is comprised of 3 columns, the first column containing the lines unique to the first file, the second column being the lines common to both files, the third column being the lines unique to the second file.

    The comm command also has options that allow you to selectively print each column by itself. By placing the minus sign (-) and the column number after the command, you will “drop" that column. So, if you wanted only the lines that are the same for each file, you would issue the command: comm -13.

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    Conclusion

    As you can see, there are different ways to compare two text files to find the differences. Each of the three commands has a specific purpose, but they are all very useful when you are evaluating programming code, shell scripts, configuration files or reports.