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Day 4

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Schmoozerd edited this page Oct 7, 2011 · 4 revisions

From Day 3 you should have learnt these things:

  • How to read unified diffs

  • How to use git diff, git show and git format-patch to create diff between different commits or the working tree

Today now is about how to

  • Create patch files, that is files which contain the output from the above commands

  • How to apply them

First Part:

A very neat way to create the patch files within a bash (ie Git bash) is to use the output redirection

Excursion: Some notes about working with a console

Reasons to still use a console environment for some things

  • You definitively will feel like a geek!

    (Do we actually need more reasons?)

  • Using a console is for some things very fast (and very slow for others)

  • Reasonable to learn, as *nix systems won’t proper work without (Windows systems won’t work proper without using cmd now and then, only that the cmd sucks)

Most useful commands in a bash console

OK, to clarify: When you type commands into a console, there is actually a program that interprets these commands.

Some examples are the cmd (from (win)do(w)s), or bash which got popular on Linux

ls

List the content of the current directory (on Windows this was called dir)
cd

Change your directory. If you invoke cd .. then you will change into the parent directory
cat

Can be used to get text from a file onto the console
cp A B

Copy a file from A to B
mv A B

Move a file from A to B (if A and B are in the same directory, than this renames)
rm A

Remove (this means delete) file A
mkdir

Create a directory
rmdir

Remove a directory

Very useful things:

Auto completion (Bash can do this)

If you want to type something, and press the tab key, the bash will try to auto complete a word for you.

Example: Type (in the ScriptDev2-directory) into the bash: ls to and then press the tab key.

Copy and Paste in the bash (For Windows users)

If you click on the top-left corner of your console window, you can select "Edit" and there "Mark" and "Paste". Also you can use the INSERT key to paste the content of your clipboard

(Marking is finished with enter, the marked text will then be in your clipboard)

Using previously typed commands

With the arrow keys (up, down) you can reuse previous commands easily.

Excursion: Bash output/ input redirection

For most console based systems it is possible to use these types of output redirection

>

Forward the output into a file, the file will be created if it does not exist, and truncated before refilled if it exists
>>

Forward the output into a file, but append if the file already exists
|

(Called pipe) Forward the output from the program as input to the next program :: A few more exists which I won’t cover!

Remember the nice tools like git diff or git format-patch --stdout from Day 3?

They were working nice, but only displayed their stuff in the console.

The trick to get their output into files is to use output redirection.

Exercise
Exercise 1 - Use output redirection to create patch files

  • Do Exercise 1 from Day3, but redirect the output of git diff with git diff > temp_diff.patch

  • Do Exercise 3 from Day3, but redirect the output of git diff with git diff > temp_format.patch

And look at these created files with a patch tool (if you have one installed), an editor and cat. (To use cat do: cat temp_diff.patch and cat temp_format.patch)

Congratulation, you have created your first patch files!
Exercise
Exercise 2 - Use output redirection to create patch files

Do some more edits and test bash auto completion with git diff > te <press tab key> d <press tab key>

Sometimes you might not want to get all changes directly into a file, but would like to view the output of ie git format-patch --stdout slowly.

Remember the pager 'less' from Day 2?

Exercise
Exercise 3 - Fais la pipe (My French is bad, maybe it was Utilize)

Use git format-patch master --stdout | less

End of First Part

Tip

If you still cannot get enough about using bash, look here

For more bash programming purposes look here

If anybody finds a neat beginners guide suitable for this topic, please inform!

Second Part:

Now we want to apply a few patch files.

You will need the patch files you created in Part 1 - Exercise 1!

Also you should copy the content of SD2-Commit 2302 into a file in your ScriptDev2 directory, expected name sd2_2302.patch

Applying patches with git apply and git am

Both commands apply a patch from a filename with appropriate format if possible.

git am

Git am applies a patch created with format-patch to the 'index'

As a formatted patch contains some meta information of the commit(s) from which it was created, it will directly commit the change sets from the commits, and thereby using the provided meta information in the patch file.

Hence a patch file for git am must contain this information in a defined way.

git apply

Git apply applies a patch created with git show or git diff to the 'working tree'

These patches contain no additional meta information, and hence won’t be committed automatically.

To apply the following examples, be sure to be on clear icc_project, to do so use

git reset --hard origin/icc_project
Exercise
Exercise 4 - Apply a patch with git apply

Apply the patch created in Exercise 1 from file temp_diff.patch

git apply temp_diff.patch

Check what this did! Use git status, git diff and git log to see what this did!

Reset your changes with git reset --hard
Exercise
Exercise 5 - Apply a patch with git am

Apply the patch from file sd2_2302.patch

git am sd2_2302.patch

Check what this did!

Reset your changes with git reset --hard origin/icc_project
Exercise
Exercise 6 - Why why why?

Why did I suggest to use different ways to git reset --hard in exercises 4 and 5?
Exercise
Special Exercise 7 - Some first conflicts

Try to apply the changes in temp_format.patch with git am

This gives errors - Why?

To abort the attempt to apply this with git am, use

git am --abort
Exercise
Special Exercise 8 - Apply to a conflict free point

Find a point in the history where the changes from temp_format.patch could be applied!

Checkout to this point, and apply there

git checkout -b apply-safe <ThisPoint>
git am temp_format.patch

Now you have applied the patch file. Check what happened with the usual suspects :)
Exercise
Special Exercise 9 - Merging conflicts

Checkout to your icc_project branch

Try to merge the apply-safe branch into your current branch, try git merge apply-safe

This will give the same conflicts as you had in Exercise 7, but now you have the possibility to use the powerful tools of git to merge. Actually for me merge went without problems!

However if you still get conflicts, abort the merge with git reset --hard

Now, there might have been many changes, and it is time to cleanup:

git checkout icc_project
git reset --hard origin/icc_project (1)
git branch -D apply-safe    (2)
rm sd2_2302.patch    (3)
rm temp_format.patch
rm temp_diff.patch

  1. Do you already know why this time the reset --hard version with origin/icc_project was used?

  2. git branch -D will delete a branch

  3. Remove a few untracked files - check for more with git status

End of Second Part

Today we have seen

  • how to use a console environment

  • how to use dark console powers to create patch files

  • how to apply patch files

  • how using Git the right way conflict solving can become very easy! (Special exercises)

So, now we have the tools in place to actually start working on ICC code.

Expect the first steps toward a C++ script in the next days!

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