What does a/ and b/ stands for in patch header?

Question

The first line of patches always has this kind of code:

diff --git a/i18nviews.inc b/i18nviews.inc

or

diff --git a/plugins/views_plugin_localization.inc b/plugins/views_plugin_localization.in

What does the a and b stand for?

Answer 1

A' and the 'B' are prefixes to differentiate source and destination.

You can change the prefixes with

--src-prefix=<prefix>
Show the given source prefix instead of "a/".

--dst-prefix=<prefix>
Show the given destination prefix instead of "b/".

From http://git-scm.com/docs/git-format-patch

Answer 2

As a rule of thumb, just consider a/ to refer to the "old" file (with filename indicated following the a/), and b/ to refer to the 'new file (with filename indicated following the b/).

As an illustration, consider the patch attached to comment nr 15 in issue nr 1632898, which looks like so:

--- a/panels_ipe/plugins/display_renderers/panels_renderer_ipe.class.php
+++ b/panels_ipe/plugins/display_renderers/panels_renderer_ipe.class.php
@@ -137,7 +137,10 @@ class panels_renderer_ipe extends panels_renderer_editor {
   function render_pane_content(&$pane) {
     $content = parent::render_pane_content($pane);
     // Ensure that empty panes have some content.
-    if (empty($content) || empty($content->content)) {
+    if (empty($content) || !is_object($content) || empty($content->content)) {
+      if (!is_object($content)) {
+        $content = new StdClass();
+      }
       // Get the administrative title.
       $content_type = ctools_get_content_type($pane->type);
       $title = ctools_content_admin_title($content_type, $pane->subtype, $pane->configuration, $this->display->context);

In regular English you could say "Remove that 1 line from a/ (indicated with '-', located at line 141 in a/) and insert again these 4 lines fromb/` (indicated with '+', starting from line 141)".

Some more details about the -137,7 +137,10 part of this patch example: if you'd browse the a/ (old) file, this is how these 7 lines look like, starting from line 137,

class panels_renderer_ipe extends panels_renderer_editor {
   function render_pane_content(&$pane) {
     $content = parent::render_pane_content($pane);
     // Ensure that empty panes have some content.
     if (empty($content) || empty($content->content)) {
       // Get the administrative title.
       $content_type = ctools_get_content_type($pane->type);
       $title = ctools_content_admin_title($content_type, $pane->subtype, $pane->configuration, $this->display->context);

So to know what the content of a/ actually looks like (by just looking at the patch), ignore the lines with a "+", and replace any "-" with a blank.

And if you'd browse the b/ (new) file, this is how these 10 lines look like, starting from line 137:

class panels_renderer_ipe extends panels_renderer_editor {
   function render_pane_content(&$pane) {
     $content = parent::render_pane_content($pane);
     // Ensure that empty panes have some content.
    if (empty($content) || !is_object($content) || empty($content->content)) {
      if (!is_object($content)) {
        $content = new StdClass();
      }
       // Get the administrative title.
       $content_type = ctools_get_content_type($pane->type);
       $title = ctools_content_admin_title($content_type, $pane->subtype, $pane->configuration, $this->display->context);

So to know what the content of b/ actually looks like (by just looking at the patch), ignore the lines with a "-", and replace any "+" with a blank.