Author: Dave McKay / Source: How-To Geek
Need to see the differences between two revisions of a text file? Then diff is the command you need. This tutorial shows you how to use diff on Linux and macOS, the easy way.
Diving into diff
The diff command compares two files and produces a list of the differences between the two.
diff. The diff command was designed to find differences between source code files and to produce an output that could be read and acted upon by other programs, such as the patch command. In this tutorial, we’re going to look at the most useful human-friendly ways to use diff. Let’s dive right in and analyze two files. The order of the files on the command line determines which file diff considers to be the ‘first file’ and which it considers to be the “second file.” In the example below alpha1 is the first file, and alpha2 is the second file. Both files contain the phonetic alphabet but the second file, alpha2, has had some further editing so that the two files are not identical.
We can compare the files with this command. Type diff, a space, the name of the first file, a space, the name of the second file, and then press Enter.
diff alpha1 alpha2
How do we dissect that output? Once you know what to look for it’s not that bad. Each difference is listed in turn in a single column, and each difference is labeled. The label contains numbers either side of a letter, like 4c4. The first number is the line number in alpha1, and the second number is the line number in alpha2. The letter in the middle can be:
- c: The line in the first file needs to be changed to match the line in the second file.
- d: The line in the first file must be deleted to match the second file.
- a: Extra content must be added to the first file to make it match the second file.
The 4c4 in our example tell us that line four of alpha1 must be changed to match line four of alpha2. This is the first difference between the two files that diff found.
Lines that begin with < refer to the first file, in our example alpha1, and lines that start with > refer to the second file, alpha2. The line < Delta tells us that the word Delta is the content of line four in alpha1. The line > Dave tells us that the word Dave is the content of line four in alpha2. To summarise then, we need to replace Delta with Dave on line four in alpha1, to make that line match in both files.
The next change is indicated by the 12c12. Applying the same logic, this tells us that line 12 in alpha1 contains the word Lima, but line 12 of alpha2 contains the word Linux.
The third change refers to a line that has been deleted from alpha2. The label 21d20 is deciphered as “line 21 needs to be deleted from the first file to make both files synchronize from line 20 onwards.” The < Uniform line shows us the content of the line which needs to be deleted from alpha1.
The fourth difference is labeled 26a26,28. This change refers to three extra lines that have been added to alpha2. Note the 26,28 in the label. Two-line numbers separated by a comma represents a range of line numbers. In this example, the range is from line 26 to line 28. The label is interpreted as “at line 26 in the first file, add lines 26 to 28 from the second file.” We are shown the three lines in alpha2 that need to be added to alpha1. These contain the words Quirk, Strange, and Charm.
Snappy One-Liners
If you all you want to know is whether two files are the same, use the -s (report identical files) option.
diff -s alpha1 alpha3
You can use the -q (brief) option to get an equally terse statement about two files being different.
diff -q alpha1 alpha2
One thing to watch out for is that with two identical files the-q (brief) option completely clams up and doesn’t report anything at all.
An Alternative View
The -y (side by side) option uses a different layout to describe the file differences. It is often convenient to use the -W (width) option with the side by side view, to limit the number of columns that are displayed. This avoids ugly wrap-around lines that make the output difficult to read. Here we have told diff to produce a side by side display and to limit the output to 70 columns.
diff -y -W 70 alpha1 alpha2
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