Version Control with Git: Discussion

Frequently Asked Questions

People often have questions about Git beyond the scope of the core material. Students who have completed the rest of the lessons might find value in looking through the following topics.

Note that since this material isn’t essential for basic Git usage, it won’t be covered by the instructor.

More Advanced Git Configuration

In Setting Up Git, we used git config --global to set some default options for Git. It turns out that these configuration options get stored in your home directory in a plain text file called .gitconfig.

$ cat ~/.gitconfig

[user]
	name = Vlad Dracula
	email = vlad@tran.sylvan.ia
[color]
	ui = true
[core]
	editor = nano

This file can be opened in your preferred text editor. (Note that it is recommended to continue using the git config command, as this helps avoid introducing syntax errors.)

Eventually, you will want to start customizing Git’s behaviour. This can be done by adding more entries to your .gitconfig. The available options are described in the manual:

$ git config --help

In particular, you might find it useful to add aliases. These are like shortcuts for longer Git commands. For example, if you get sick of typing git checkout all the time, you could run the command:

$ git config --global alias.co checkout

Now if we return to the example from Exploring History where we ran:

$ git checkout f22b25e mars.txt

we could now instead type:

$ git co f22b25e mars.txt

Styling Git’s Log

A good target for customization is output from the log. The default log is quite verbose but gives no graphical hints such as information about which commits were done locally and which were pulled from remotes.

You can use git log --help and git config --help to look for different ways to change the log output. Try the following commands and see what effect they have:

$ git config --global alias.lg "log --graph"
$ git config --global log.abbrevCommit true
$ git config --global format.pretty oneline
$ git lg

If you don’t like the effects, you can undo them with:

$ git config --global --unset alias.lg
$ git config --global --unset log.abbrevCommit
$ git config --global --unset format.pretty

Undoing Git Configuration Changes

You can use the --unset flag to delete unwanted options from .gitconfig. Another way to roll back changes is to store your .gitconfig using Git.

For hints on what you might want to configure, go to GitHub and search for “gitconfig”. You will find hundreds of repositories in which people have stored their own Git configuration files. Sort them by the number of stars and have a look at the top few. If you find some you like, please check that they’re covered by an open source license before you clone them.

Non-text Files

Recall when we discussed Conflicts there was a challenge that asked, “What does Git do when there is a conflict in an image or some other non-textual file that is stored in version control?”

We will now revisit this in more detail.

Many people want to version control non-text files, such as images, PDFs and Microsoft Office or LibreOffice documents. It is true that Git can handle these filetypes (which fall under the banner of “binary” file types). However, just because it can be done doesn’t mean it should be done.

Much of Git’s magic comes from being able to do line-by-line comparisons (“diffs”) between files. This is generally easy for programming source code and marked up text. For non-text files, a diff can usually only detect that the files have changed but can’t say how or where.

This has various impacts on Git’s performance and will make it difficult to compare different versions of your project.

For a basic example to show the difference it makes, we’re going to go see what would have happened if Dracula had tried using outputs from a word processor instead of plain text.

Create a new directory and go into it:

$ mkdir planets-nontext
$ cd planets-nontext

Use a program such as Microsoft Word or LibreOffice Writer to create a new document. Enter the same text that we began with before:

Cold and dry, but everything is my favorite color

Save the document into the planets-nontext directory with the name of mars.doc. Back in the terminal, run the usual commands for setting up a new Git repository:

$ git init
$ git add mars.doc
$ git commit -m "Starting to think about Mars"

Then make the same changes to mars.doc that we (or Vlad) previously made to mars.txt.

Cold and dry, but everything is my favorite color
The two moons may be a problem for Wolfman

Save and close the word processor. Now see what Git thinks of your changes:

$ git diff

diff --git a/mars.doc b/mars.doc
index 53a66fd..6e988e9 100644
Binary files a/mars.doc and b/mars.doc differ

Compare this to the earlier git diff obtained when using text files:

diff --git a/mars.txt b/mars.txt
index df0654a..315bf3a 100644
--- a/mars.txt
+++ b/mars.txt
@@ -1 +1,2 @@
 Cold and dry, but everything is my favorite color
+The two moons may be a problem for Wolfman

Notice how plain text files give a much more informative diff. You can see exactly which lines changed and what the changes were.

An uninformative git diff is not the only consequence of using Git on binary files. However, most of the other problems boil down to whether or not a good diff is possible.

This isn’t to say you should never use Git on binary files. A rule of thumb is that it’s OK if the binary file won’t change very often, and if it does change, you don’t care about merging in small differences between versions.

We’ve already seen how a word processed report will fail this test. An example that passes the test is a logo for your organization or project. Even though a logo will be stored in a binary format such as jpg or png, you can expect it will remain fairly static through the lifetime of your repository. On the rare occasion that branding does change, you will probably just want to replace the logo completely rather than merge little differences in.

Removing a File

Adding and modifying files are not the only actions one might take when working on a project. It might be required to remove a file from the repository.

Create a new file for the planet Nibiru:

$ echo "This is another name for fake planet X" > nibiru.txt

Now add to the repository like you have learned earlier:

$ git add nibiru.txt
$ git commit -m 'adding info on nibiru'
$ git status

On branch master
nothing to commit, working directory clean

Nibiru is not a real planet. That was a silly idea. Let us remove it from the disk and let Git know about it:

$ git rm nibiru.txt
$ git status

On branch master
Changes to be committed:
   (use "git reset HEAD <file>..." to unstage)

   deleted:    nibiru.txt

The change has been staged. Now commit the removal, and remove the file from the repository itself. Note that the file will be removed in the new commit. The previous commit will still have the file, if you were to retrieve that specific commit.

$ git commit -m 'Removing info on Nibiru.  It is not a real planet!'

Removing a File with Unix

Sometimes we might forget to remove the file through Git. If you removed the file with Unix rm instead of using git rm, no worries, Git is smart enough to notice the missing file. Let us recreate the file and commit it again.

$ echo "This is another name for fake planet X" > nibiru.txt
$ git add nibiru.txt
$ git commit -m 'adding nibiru again'

Now we remove the file with Unix rm:

$ rm nibiru.txt
$ git status

On branch master
Changes not staged for commit:
   (use "git add/rm <file>..." to update what will be committed)
   (use "git checkout -- <file>..." to discard changes in working directory)

    deleted:    nibiru.txt

no changes added to commit (use "git add" and/or "git commit -a")

See how Git has noticed that the file nibiru.txt has been removed from the disk. The next step is to “stage” the removal of the file from the repository. This is done with the command git rm just as before.

$ git rm nibiru.txt
$ git status

On branch master
Changes to be committed:
   (use "git reset HEAD <file>..." to unstage)

   deleted:    nibiru.txt

The change that was made in Unix has now been staged and needs to be committed.

$ git commit -m 'Removing info on Nibiru, again!'

Renaming a File

Another common change when working on a project is to rename a file.

Create a file for the planet Krypton:

$ echo "Superman's home planet" > krypton.txt

Add it to the repository:

$ git add krypton.txt
$ git commit -m 'Adding planet Krypton'

We all know that Superman moved to Earth. Not that he had much choice. Now his home planet is Earth.

Rename the file krypton.txt to earth.txt with Git:

$ git mv krypton.txt earth.txt
$ git status

# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#	renamed:    krypton.txt -> earth.txt
#

The final step is commit our change to the repository:

$ git commit -m 'Superman's home is now Earth'

Renaming a File with Unix

If you forgot to use Git and you used Unix mv instead of git mv, you will have a touch more work to do but Git will be able to deal with it. Let’s try again renaming the file, this time with Unix mv. First, we need to recreate the krypton.txt file:

$ echo "Superman's home planet" > krypton.txt
$ git add krypton.txt
$ git commit -m 'Adding planet Krypton again.'

Let us rename the file and see what Git can figured out by itself:

$ mv krypton.txt earth.txt
$ git status

On branch master
Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

        deleted:    krypton.txt

Untracked files:
  (use "git add <file>..." to include in what will be committed)

    earth.txt

no changes added to commit (use "git add" and/or "git commit -a")

Git has noticed that the file krypton.txt has disappeared from the file system and a new file earth.txt has showed up.

Add those changes to the staging area:

$ git add krypton.txt earth.txt
$ git status

On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

    renamed:    krypton.txt -> earth.txt

Notice how Git has now figured out that the krypton.txt has not disappeared - it has simply been renamed.

The final step, as before, is to commit our change to the repository:

$ git commit -m 'Superman's home is Earth, told you before.'

Further .gitignore concepts

For additional documentation on .gitignore, please reference the official git documentation.

In the ignore exercise, learners were presented with two variations of ignoring nested files. Depending on the organization of your repository, one may suit your needs over another. Keep in mind that the way that Git travels along directory paths can be confusing.

Sometimes the ** pattern comes in handy, too, which matches multiple directory levels. E.g. **/results/plots/* would make git ignore the results/plots directory in any root directory.

Ignoring Nested Files: Challenge Problem

Given a directory structure that looks like:

results/data
results/plots
results/run001.log
results/run002.log

And a .gitignore that looks like:

*.dat

How would you track all of the contents of results/data/, including *.dat files, but ignore the rest of results/?

Solution

To do this, your .gitignore would look like this:

*.dat                 # ignore the .dat files
results/*             # ignore the files in the results directory
!results/data/        # do not ignore the files in results/data
!results/data/*       # do not ignore the .dat files in reults/data