Diff, Patch, and Friends

Near the beginning of this article, I promised to explain this line:

 -1,9 +1,9

that describes the chunk that diff found differences in. In each file, the chunk starts on line 1 and extends for 9 lines beyond the first line. However, with this small example, the chunk shown in the example contains the whole file. With larger files, only the lines around the changes, called the context, are shown.

In files 9 and 10, I've inserted a lot of blank lines in the middle of the paragraph, in order to show what multiple chunks look like. File 9 is damaged, file 10 is correct (except for the blank lines in the middle of the paragraph):

<h3>diff -u 9 10</h3>
--- 9   Mon Apr 22 15:46:37 1996
+++ 10  Mon Apr 22 15:46:14 1996
 -1,7 +1,7
 Ecce Eduardus Ursus scalis nunc tump-tump-tump
 occipite gradus pulsante post Christophorum
 Robinum descendens. Est quod sciat unus et solus
-modus gradibus desendendi, non nunquam autem
+modus gradibus descendendi, nonnunquam autem
 -33,7 +33,7
 sentit, etiam alterum modum exstare, dummodo
-pulsationibus desinere et de no modo meditari
+pulsationibus desinere et de eo modo meditari
 possit. Deinde censet alios modos non esse. En,
 nunc ipse in imo est, vobis ostentari paratus.
 Winnie ille Pu.

So you see that we have one seven-line chunk starting at line 1 and one seven-line chunk starting at line 33 are shown here.

You should notice several things here:

“Patches” (or “diffs”) are the output of the diff program. They include all the chunks of changes between the two files.

This only brushes the surface of diff. For one thing, the three lines of unchanged context is configurable. Instead of using the -u option, you can use the -U lines option to specify any reasonable number of lines of context. You can even specify -U 0 if you don't want to use any context at all, though that is rarely useful.

What does the -u (or -U lines) argument mean? It specifies the unified diff format, which is the particular format covered here. Other formats include:

You will almost never want to create context or normal diffs, but it may be useful to recognize them from time to time. Context diffs are marked by the use of the character ! to mark changes, and normal diffs are marked by the use of the characters < and > to mark changes.

Here are examples:

diff -c 1 2
*** 1   Sat Apr 20 22:11:53 1996
--- 2   Sat Apr 20 22:12:01 1996
***************
*** 1,9 ****
  Ecce Eduardus Ursus scalis nunc tump-tump-tump
  occipite gradus pulsante post Christophorum
  Robinum descendens. Est quod sciat unus et solus
! modus gradibus desendendi, non nunquam autem
  sentit, etiam alterum modum exstare, dummodo
! pulsationibus desinere et de no modo meditari
  possit. Deinde censet alios modos non esse. En,
  nunc ipse in imo est, vobis ostentari paratus.
  Winnie ille Pu.
--- 1,9 ----
  Ecce Eduardus Ursus scalis nunc tump-tump-tump
  occipite gradus pulsante post Christophorum
  Robinum descendens. Est quod sciat unus et solus
! modus gradibus descendendi, nonnunquam autem
  sentit, etiam alterum modum exstare, dummodo
! pulsationibus desinere et de eo modo meditari
  possit. Deinde censet alios modos non esse. En,
  nunc ipse in imo est, vobis ostentari paratus.
  Winnie ille Pu.
diff 1 2
4c4
< modus gradibus desendendi, non nunquam autem
---
> modus gradibus descendendi, nonnunquam autem
6c6
< pulsationibus desinere et de no modo meditari
---
< pulsationibus desinere et de eo modo meditari

There are a few other important things to note here: