g. Exercises II

Checking Understanding of git diff

Consider this command: git diff HEAD~9 mars.txt. What do you predict this command will do if you execute it? What happens when you do execute it? Why?

Try another command, git diff [ID] mars.txt, where [ID] is replaced with the unique identifier for your most recent commit. What do you think will happen, and what does happen?

Getting Rid of Staged Changes

git checkout can be used to restore a previous commit when unstaged changes have been made, but will it also work for changes that have been staged but not committed? Make a change to mars.txt, add that change, and use git checkout to see if you can remove your change.

Explore and Summarize Histories

Exploring history is an important part of Git, and often it is a challenge to find the right commit ID, especially if the commit is from several months ago.

Imagine the planets project has more than 50 files. You would like to find a commit that modifies some specific text in mars.txt. When you type git log, a very long list appeared. How can you narrow down the search?

Recall that the git diff command allows us to explore one specific file, e.g., git diff mars.txt. We can apply a similar idea here.

$ git log mars.txt

Unfortunately some of these commit messages are very ambiguous, e.g., update files. How can you search through these files?

Both git diff and git log are very useful and they summarize a different part of the history for you. Is it possible to combine both? Let’s try the following:

$ git log --patch mars.txt

You should get a long list of output, and you should be able to see both commit messages and the difference between each commit.

Question: What does the following command do?

$ git log --patch HEAD~9 *.txt

How to change your last commit

Sometimes you will find you might want to add things to your last commit. For example you might have broken a build or want to change formatting. Instead of creating a completely separate commit or rebasing you can simply run:

 $ git commit --ammend

It will take your existing staged changes and add them to the current commit by creating a whole new commit will all the changes combined. When you commit you are given the opporutnity to reword your commit message or you can simply leave it unchanged.

Make a change to mars.txt and ammend your last commit with a new message

$ echo "A small fix"  mars.txt

$ git commit --amend

$ git log

$ git diff HEAD HEAD~1