What’s the Difference – Use the Linux diff Command to Find Changes to Files

If you have worked with Linux or any version of Unix for a while, then you are likely aware of the diff command.  But you may not be aware of some of the siblings commands of diff and some of the more cool things that can be done with diff itself.

First, let’s discuss how I use the Linux diff Command to find and fix differences in files.

Use Diff to Compare Versions of a Configuration File

Let’s imagine a fairly common situation where we are planning to edit a configuration file such as the nginx.conf, my.cnd, sshd_config, or the httpd.conf file.  We typically backup the file.  Typically, we would backup the file so that we could put it back in place in the event that our changes didn’t work out so well.  Afterwards, using diff is an excellent way to review our changes.

Let’s imagine that we have a file with some text like the following:

$ cat file1

And let’s imagine that we have another file, or rather a copy of the file that has been updated a bit.

$ sed 's/line6/some changes here/g' file1 > file2
$ cat file2
some changes here

(And let's add another change)

$ echo "line11" >> file2

We can now use the diff command to review the changes that have been made.  The “-y” option allows us to compare the files side by side and to see the changes.

Example:  We will compare two files side by side using the diff command.

$ diff -y file1 file2
line1                                line1
line2                                line2
line3                                line3
line4                                line4
line5                                line5
line6                              | some changes here
line7                                line7
line8                                line8
line9                                line9
line10                               line10
                                   > line11

By the way, the “sdiff <file1> <file2>” command gives us the same output as “diff -y <file1> <file2>”.  The changed line is marked with the “|” and the new line is marked with the “>”.

Sometimes, the files are so long that it is hard to see the changes amongst all of the lines.  In this case, we can run diff on the files without any options to see just the changes from one file to another.

Example:  Using diff to find only the differences in two files.

$ diff file1 file2
< line6
> some changes here
> line11

The lines in the output is the ed command that would convert file1 to file2.  in this case, line 6 in the first file saw a change to line 6 in the second file.  This is followed by the lines that show the actual changes.  The lines in the first file are marked with a “<” while the lines from the second file are marked with a “>”.  The second change (the additional line) is marked with the 10a11 showing that there is an additional line.

Diff and Unified Output

The diff command can also show the changes in unified output using the “-u” option.

Example:  Displaying the unified output differences between 2 files

(First, before we run the unified diff, let's get some information about the files)

$ ls -lh
total 8.0K
-rw-rw-r-- 1 mary mary 61 Nov  6 22:53 file1
-rw-rw-r-- 1 mary mary 80 Nov  7 00:00 file2

(Now, let's run the unified diff.)
$ diff -u file1 file2
--- file1    2014-11-06 22:53:21.365002606 -0500
+++ file2    2014-11-07 00:00:04.078002667 -0500
@@ -3,8 +3,9 @@
+some changes here

The first two lines (or header) tell us about the files themselves.  We see the file name, Then we see the timestamp of the two files followed by the offset from GMT.  This header can be changed by using the –label=label option.

After the header comes the “hunk” which is delimited by the @@’s.  The hunk will look like ‘start,count’.

After the hunk comes the actual lines.  Lines with a ‘+’ show lines that have been added and lines with a ‘-‘ show lines that are removed.

All of these lines (header, hunk, data lines) can be used in the patch command to transform the first file into the second file.

Using the Patch Command

This is the point where I probably should stray for a moment from the diff command family to mention the “patch” command.  The patch command can take the unified output from the diff command to transform the original file to the modified file.  By the way, I had to add the patch command to my system with yum.

Example:  Find the differences between 2 files and make them match.

$ cp file1 file1-orig
$ diff -u file1 file2 > differences
$ patch file1 < differences
patching file file1
$ diff file1 file2

In the example above, the patch command took the unified output from “diff -u” to make the files exactly the same again.

Fixing Files with Vimdiff

With the vim-enhanced package installed, you can also use the vimdiff to review changes and fix differences between two files with the vim editor.

$ cp file1-orig file1
$ vimdiff file1 file2

Running the vimdiff command above will look like this:

Fix Differences Between Two Files with vimdiff

Fix Differences Between Two Files with vimdiff

The differences in the files will be highlighted and as you are in the vim editor, you can makes the changes to make them match.  As you fix and eliminate the differences, the highlighting will disappear.

There are other ways to compare file differences in Linux.  Gvimdiff is a graphical version of vimdiff and there are others still.  If you have a favorite method for comparing files for differences, please leave a comment below.

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Jeff has 20 years of professional IT experience, having done nearly everything in his roles of IT consultant, Systems Integrator, Systems Engineer, CNOC Engineer, Systems Administrator, Network Systems Administrator, and IT Director. If there is one thing he knows for sure, it is that there is always a simple answer to every IT problem and that downtime begins with complexity. Seasoned IT professional by day, Jeff hopes to help other IT professionals by blogging about his experiences at night on his blog: http://uptimemadeeasy.com. You can find Jeff on or LinkedIn at: LinkedIn or Twitter at: Twitter

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1 comment for “What’s the Difference – Use the Linux diff Command to Find Changes to Files

  1. Wendell Dingus
    November 7, 2014 at 8:26 am

    A good visual comparison tool is “meld”.

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