git - 如何读取 git diff 的输出？

Question

git-diff 的手册页相当长，解释了许多对于初学者来说似乎没有必要的情况。例如:

git diff origin/master

Answer 1

让我们看一下来自 git 历史的高级差异示例(在 commit 1088261f in git.git repository 中):

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] == '-') {

让我们逐行分析这个补丁。

• 第一行

  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.crename 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;

表示 cmd_http_fetch 被 main 替换，并且添加了 const char *prefix; 行。

换句话说，在更改之前，“builtin-http-fetch.c”文件的相应片段如下所示:

    #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;

更改后，现在“http-fetch.c”文件的这个片段看起来像这样:

    #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;

• 可能有

  \ No newline at end of file

  行存在(它不在示例 diff 中)。

作为Donal Fellows said最好在现实生活中的例子中练习阅读差异，你知道你改变了什么。

引用资料:

• git-diff(1) manpage ，“使用 -p 生成补丁”部分
• (diff.info)Detailed Unified node, 《统一格式详解》。