How to read the output from git diff?

Lets take a look at example advanced diff from git history (in commit 1088261f in git.git repository):

diff --git a/builtin-http-fetch.c b/http-fetch.c
similarity index 95%
rename from builtin-http-fetch.c
rename to http-fetch.c
index f3e63d7..e8f44ba 100644
--- a/builtin-http-fetch.c
+++ b/http-fetch.c
@@ -1,8 +1,9 @@
 #include "cache.h"
 #include "walker.h"
 
-int cmd_http_fetch(int argc, const char **argv, const char *prefix)
+int main(int argc, const char **argv)
 {
+       const char *prefix;
        struct walker *walker;
        int commits_on_stdin = 0;
        int commits;
@@ -18,6 +19,8 @@ int cmd_http_fetch(int argc, const char **argv, const char *prefix)
        int get_verbosely = 0;
        int get_recover = 0;
 
+       prefix = setup_git_directory();
+
        git_config(git_default_config, NULL);
 
        while (arg < argc && argv[arg][0] == '-') {

Lets analyze this patch line by line.

• The first line

  diff --git a/builtin-http-fetch.c b/http-fetch.c

  is a "git diff" header in the form diff --git a/file1 b/file2. The a/ and b/ filenames are the same unless rename/copy is involved (like in our case). The --git is to mean that diff is in the "git" diff format.

• Next are one or more extended header lines. The first three

  similarity index 95%
  rename from builtin-http-fetch.c
  rename to http-fetch.c

  tell us that the file was renamed from builtin-http-fetch.c to http-fetch.c and that those two files are 95% identical (which was used to detect this rename).
  
  The last line in extended diff header, which is

  index f3e63d7..e8f44ba 100644

  tell us about mode of given file (100644 means that it is ordinary file and not e.g. symlink, and that it doesn't have executable permission bit), and about shortened hash of preimage (the version of file before given change) and postimage (the version of file after change). This line is used by git am --3way to try to do a 3-way merge if patch cannot be applied itself.
  
  

• Next is two-line unified diff header

  --- a/builtin-http-fetch.c
  +++ b/http-fetch.c

  Compared to diff -U result it doesn't have from-file-modification-time nor to-file-modification-time after source (preimage) and destination (postimage) file names. If file was created the source is /dev/null; if file was deleted, the target is /dev/null.
  If you set diff.mnemonicPrefix configuration variable to true, in place of a/ and b/ prefixes in this two-line header you can have instead c/, i/, w/ and o/ as prefixes, respectively to what you compare; see git-config(1)

• Next come one or more hunks of differences; each hunk shows one area where the files differ. Unified format hunks starts with line like

  @@ -1,8 +1,9 @@

  or

  @@ -18,6 +19,8 @@ int cmd_http_fetch(int argc, const char **argv, ...

  It is in the format @@ from-file-range to-file-range @@ [header]. The from-file-range is in the form -<start line>,<number of lines>, and to-file-range is +<start line>,<number of lines>. Both start-line and number-of-lines refer to position and length of hunk in preimage and postimage, respectively. If number-of-lines not shown it means that it is 1.

The optional header shows the C function where each change occurs, if it is a C file (like -p option in GNU diff), or the equivalent, if any, for other types of files.

• Next comes the description of where files differ. The lines common to both files begin with a space character. The lines that actually differ between the two files have one of the following indicator characters in the left print column:

• '+' -- A line was added here to the first file.

• '-' -- A line was removed here from the first file.

So, for example, first chunk

     #include "cache.h"
     #include "walker.h"
     
    -int cmd_http_fetch(int argc, const char **argv, const char *prefix)
    +int main(int argc, const char **argv)
     {
    +       const char *prefix;
            struct walker *walker;
            int commits_on_stdin = 0;
            int commits;

means that cmd_http_fetch was replaced by main, and that const char *prefix; line was added.

In other words, before the change, the appropriate fragment of then 'builtin-http-fetch.c' file looked like this:

    #include "cache.h"
    #include "walker.h"
    
    int cmd_http_fetch(int argc, const char **argv, const char *prefix)
    {
           struct walker *walker;
           int commits_on_stdin = 0;
           int commits;

After the change this fragment of now 'http-fetch.c' file looks like this instead:

    #include "cache.h"
    #include "walker.h"
     
    int main(int argc, const char **argv)
    {
           const char *prefix;
           struct walker *walker;
           int commits_on_stdin = 0;
           int commits;

• There might be

  \ No newline at end of file

  line present (it is not in example diff).

As Donal Fellows said it is best to practice reading diffs on real-life examples, where you know what you have changed.

References:

• git-diff(1) manpage, section "Generating patches with -p"
• (diff.info)Detailed Unified node, "Detailed Description of Unified Format".

@@ -1,2 +3,4 @@ part of the diff

This part took me a while to understand, so I've created a minimal example.

The format is basically the same the diff -u unified diff.

For instance:

diff -u <(seq 16) <(seq 16 | grep -Ev '^(2|3|14|15)$')

Here we removed lines 2, 3, 14 and 15. Output:

@@ -1,6 +1,4 @@
 1
-2
-3
 4
 5
 6
@@ -11,6 +9,4 @@
 11
 12
 13
-14
-15
 16

@@ -1,6 +1,4 @@ means:

• -1,6 means that this piece of the first file starts at line 1 and shows a total of 6 lines. Therefore it shows lines 1 to 6.

  1
  2
  3
  4
  5
  6
  

  - means "old", as we usually invoke it as diff -u old new.

• +1,4 means that this piece of the second file starts at line 1 and shows a total of 4 lines. Therefore it shows lines 1 to 4.

  + means "new".

  We only have 4 lines instead of 6 because 2 lines were removed! The new hunk is just:

  1
  4
  5
  6
  

@@ -11,6 +9,4 @@ for the second hunk is analogous:

• on the old file, we have 6 lines, starting at line 11 of the old file:

  11
  12
  13
  14
  15
  16
  

• on the new file, we have 4 lines, starting at line 9 of the new file:

  11
  12
  13
  16
  

  Note that line 11 is the 9th line of the new file because we have already removed 2 lines on the previous hunk: 2 and 3.

Hunk header

Depending on your git version and configuration, you can also get a code line next to the @@ line, e.g. the func1() { in:

@@ -4,7 +4,6 @@ func1() {

This can also be obtained with the -p flag of plain diff.

Example: old file:

func1() {
    1;
    2;
    3;
    4;
    5;
    6;
    7;
    8;
    9;
}

If we remove line 6, the diff shows:

@@ -4,7 +4,6 @@ func1() {
     3;
     4;
     5;
-    6;
     7;
     8;
     9;

Note that this is not the correct line for func1: it skipped lines 1 and 2.

This awesome feature often tells exactly to which function or class each hunk belongs, which is very useful to interpret the diff.

How the algorithm to choose the header works exactly is discussed at: Where does the excerpt in the git diff hunk header come from?

Here's the simple example.

diff --git a/file b/file 
index 10ff2df..84d4fa2 100644
--- a/file
+++ b/file
@@ -1,5 +1,5 @@
 line1
 line2
-this line will be deleted
 line4
 line5
+this line is added

Here's an explanation:

• --git is not a command, this means it's a git version of diff (not unix)
• a/ b/ are directories, they are not real. it's just a convenience when we deal with the same file (in my case a/ is in index and b/ is in working directory)
• 10ff2df..84d4fa2 are blob IDs of these 2 files
• 100644 is the “mode bits,” indicating that this is a regular file (not executable and not a symbolic link)
• --- a/file +++ b/file minus signs shows lines in the a/ version but missing from the b/ version; and plus signs shows lines missing in a/ but present in b/ (in my case --- means deleted lines and +++ means added lines in b/ and this the file in the working directory)
• @@ -1,5 +1,5 @@ in order to understand this it's better to work with a big file; if you have two changes in different places you'll get two entries like @@ -1,5 +1,5 @@; suppose you have file line1 ... line100 and deleted line10 and add new line100 - you'll get:

@@ -7,7 +7,6 @@ line6
 line7
 line8
 line9
-this line10 to be deleted
 line11
 line12
 line13
@@ -98,3 +97,4 @@ line97
 line98
 line99
 line100
+this is new line100