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Day 3

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Schmoozerd edited this page Oct 6, 2011 · 1 revision

In Day 2 we showed what commits are, how to navigate in them, and how to create them.

This part is now about the content of a commit. Remember the (meta) structure of a commit:

  • Commit hash

  • Commit date

  • Author information

  • Commit message

  • Content changes

Today we will talk about the Content changes in each commit.

First Part:

The diff format

One of the natural ways in programming to display changes between two files (or two states), is a so called "diff".

Usually this might look like this: (pointless example to emphasise how to read it)

diff --git a/README b/README        (1)
index bba7c29..79dc7e6 100644
--- a/README
+++ b/README
@@ -22,7 +23,7 @@ that comes with MaNGOS ( http://www.getmangos.com ), written in C++ and is        (2)
 compatible with Windows and Linux. SQL needed for database support both                            (3)
 MySQL and PostgreSQL.                                                                              (3)

-This script library provides unique scripts for NPCs, gameobjects, events              (4)
+This script library provides unique and great scripts for NPCs, gameobjects, events    (5)
 and other that need unique implementation.                                                         (3)

 Once ScriptDev2 is compiled it is automatically run by MaNGOS on server                            (3)

You can read such a (unified) patch format like this:

  1. diff --git a/path/to/file b/path/to/file This line tells you that you have a diff created with git, and it has the changes in path/to/file (two versions, a and b)

  2. @@ -A,B +C,D <someString> This line tells you where the hunk you have changes with is located, and how big it is.

    • A is the line where the content did begin in the first version

    • B is the length of the hunk before the change in the first version

    • C is the line where the content begins in the second version

    • D is the length of the hunk in the second version

  3. A content (unchanged) line always starts with a blanc

  4. A line starting with a minus sign indicates that this line is removed

  5. A line starting with a plus sign indicates that this line is added

So, now to our example diff:

the first line tells us, that we look at a change for the file README (located in main ScriptDev2 directory)

Open it and look into line 22 (A).

line 22 currently is compatible with Windows and Linux. SQL needed for database support both

so we know that we are in the right place to apply our changes.

A few lines later, we see the line This script library provides unique scripts for NPCs, gameobjects, events which out diff replaces with the line This script library provides unique and great scripts for NPCs, gameobjects, events.

(Clearly: if you delete a line, and insert at the same place another line, you actually do replace this line)

Note that a diff can consist of changes for multiple files, and each file changes can consist of multiple hunks

Tip
 Further documentation including various similar diff formats are also mentioned there, perhaps just look into it when reading the second time

Creating a diff with Git

git diff

This is one of the most important commands in Git.

With git diff you see the change set as unified diff between the current working tree and the HEAD

For memory: HEAD is the topmost commit in the current branch

Exercise
Exercise 1 - Use git diff

Edit a few files and see what happens with git diff
Exercise
Exercise 2 - Compare to diff above

Edit the README in a way to get the same unified diff I posted above - did I cheat?
Tip
 Further documentation his will show additional more advanced uses of git diff, but you should be able to understand them!

Other important commands to see diffs

You already should know git show - this commands also shows the changes of the specified commit as unified diff (after some meta information of the commit)

However with this command, you can only see the changes of a commit, not between current working tree and index

Another tool I like very very much to see diffs of multiple commits is

git format-patch

  • However this tool creates by default a bunch of diff-files, to prevent this you can use the --stdout option.

  • Also you should specify from which point on you want to see the commits.

  • Also you can specify to which point you want to see the commits.

I usually use it this way:

> git format-patch <startCommit>[..<endCommit>] --stdout

With the meaning:

<startCommit>

Commit, from which the changes will be shown (see day2 how a commit can be referenced)
..<endCommit>

Optional commit, to which the changes will be shown
--stdout

print the output to the console instead of creating files for each commit
Exercise
Exercise 3 - Basic example for git format-patch

Use format-patch to see the changes between master and icc_project, try

git format-patch master --stdout
Exercise
Exercise 4 - git format-patch for different branches

Do Day 2 - Special Exercise 8 if you have not already done, and look at the format-patch output between the created testing branch and the icc_project branch

Cleanup what happened today

Assuming you are on the icc_project branch, and you have no "valuable" custom changes on there, use

git reset --hard origin/icc_project

to reset all changes of tracked files from your index and working tree.

Check if everything is nice with git status ;)

End of First Part

This Part should have shown how to use the commands

git diff
git show
git format-patch

to look into changes of commits, or between commits and the working-tree

Basic concepts:

  • You should have a rough feeling of how to "read" unified patches.

What comes next:

Add a custom footer
Add a custom sidebar

Clone this wiki locally