Unix as a Second Language

Linux command: Side by side diff

Every now and then I come across a feature of some Linux command that works so well that I have trouble understanding how I managed without it for so long. The one that I most recently discovered is the side by side option for the diff command.

Sure, the designations that diff offers to identify differences between two files are useful. But I don't find them particularly intuitive.


# diff frogs frogs2
7,8c7,8
< Canyon Treefrog
< (Hyla arenicolor)
---
> Florida Cricket Frog
> (Acris gryllus dorsalis)
28,30d27
< Western Chorus Frog
< (Pseudacris triseriata triseriata)
<
33a31,33
> Southern Chorus Frog
> (Pseudacris nigrita nigrita)
>
35a36,38
>
> Little Grass Frog
> (Pseudacris ocularis)


The standard diff output is meant to be terse. It captures the differences between two files in such a way as to make it possible to convert one file to the other if you only end up with one of them. The "7,8c7,8" string, for example, captures the fact that lines 7 and 8 are different between the two files. If you want to turn frogs into frogs2, you exchange the following two lines with two following the "---". To turn frogs2 into frogs, you do the reverse. One way to keep many generations of an important file without duplicating most of the content over and over again would to be to save the diffs.

If you are only interested in whether the files are different, you can throw away the output from the diff command (i.e., send it to /dev/null) and look at $? (the return code). If the files are different, $? will be set to 1. Alternately, you could use the simpler cmp command which reports the first difference between the two files and then stops.

The side by side equivalent of this output is more wordy, but I instantly understand the nature and extent of the differences. The vertical bar ("|") characters point out differences between the two files while the "<" and ">" characters note where content appears in only one or the other of the two files.


# diff -y -W 80 frogs frogs2
Blanchard's Cricket FrogBlanchard's Cricket Frog
(Acris crepitans blanchardi)(Acris crepitans blanchardi)

Cuban TreefrogCuban Treefrog
(Osteopilus septentrionalis)(Osteopilus septentrionalis)


A diff command with the side by side feature can be added to Solaris systems via the diffutils package (diffutils-2.8.1-sol10-sparc-local.gz). When you want a human-friendly listing of file differences that doesn't give you anything you will need to decode, this is the command to use.