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Git Essential Training

Referencing commits


From:

Git Essential Training

with Kevin Skoglund

Video: Referencing commits

In this chapter, we are going to learn how to navigate the repositories commit tree. In order to do that, we need to start off by talking about the ways that we can reference commits in Git. We have covered a few basic ways already, but there are others that are going to be really useful to know. We will start off by introducing a new concept in Git called tree-ish. It's kind of a funny word, we've already talked about what a tree is in Git. It's the structure of files in the Git repository. It's similar to a directory in your file system. In Git, tree-ish means something that references part of the tree. It's ish because what that something is can vary widely.
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  1. 2m 46s
    1. Introduction
      1m 7s
    2. How to use the exercise files
      1m 39s
  2. 20m 24s
    1. Understanding version control
      4m 48s
    2. The history of Git
      7m 58s
    3. About distributed version control
      5m 4s
    4. Who should use Git?
      2m 34s
  3. 26m 12s
    1. Installing Git on a Mac
      3m 44s
    2. Installing Git on Windows
      5m 37s
    3. Installing Git on Linux
      1m 30s
    4. Configuring Git
      7m 29s
    5. Exploring Git auto-completion
      5m 35s
    6. Using Git help
      2m 17s
  4. 15m 49s
    1. Initializing a repository
      1m 58s
    2. Understanding where Git files are stored
      2m 34s
    3. Performing your first commit
      2m 4s
    4. Writing commit messages
      5m 22s
    5. Viewing the commit log
      3m 51s
  5. 17m 44s
    1. Exploring the three-trees architecture
      3m 57s
    2. The Git workflow
      3m 15s
    3. Using hash values (SHA-1)
      4m 7s
    4. Working with the HEAD pointer
      6m 25s
  6. 25m 52s
    1. Adding files
      5m 59s
    2. Editing files
      3m 56s
    3. Viewing changes with diff
      3m 35s
    4. Viewing only staged changes
      2m 28s
    5. Deleting files
      5m 29s
    6. Moving and renaming files
      4m 25s
  7. 19m 18s
    1. Introducing the Explore California web site
      2m 2s
    2. Initializing Git
      3m 48s
    3. Editing the support phone number
      6m 20s
    4. Editing the backpack file name and links
      7m 8s
  8. 38m 45s
    1. Undoing working directory changes
      3m 49s
    2. Unstaging files
      2m 37s
    3. Amending commits
      4m 50s
    4. Retrieving old versions
      4m 7s
    5. Reverting a commit
      3m 12s
    6. Using reset to undo commits
      3m 44s
    7. Demonstrating a soft reset
      4m 8s
    8. Demonstrating a mixed reset
      4m 7s
    9. Demonstrating a hard reset
      5m 8s
    10. Removing untracked files
      3m 3s
  9. 27m 22s
    1. Using .gitignore files
      8m 23s
    2. Understanding what to ignore
      4m 47s
    3. Ignoring files globally
      4m 49s
    4. Ignoring tracked files
      5m 26s
    5. Tracking empty directories
      3m 57s
  10. 26m 51s
    1. Referencing commits
      4m 52s
    2. Exploring tree listings
      3m 46s
    3. Getting more from the commit log
      7m 38s
    4. Viewing commits
      4m 4s
    5. Comparing commits
      6m 31s
  11. 39m 35s
    1. Branching overview
      4m 56s
    2. Viewing and creating branches
      2m 57s
    3. Switching branches
      2m 58s
    4. Creating and switching branches
      4m 53s
    5. Switching branches with uncommitted changes
      3m 26s
    6. Comparing branches
      4m 28s
    7. Renaming branches
      2m 28s
    8. Deleting branches
      4m 18s
    9. Configuring the command prompt to show the branch
      9m 11s
  12. 28m 32s
    1. Merging code
      3m 11s
    2. Using fast-forward merge vs. true merge
      6m 49s
    3. Merging conflicts
      7m 26s
    4. Resolving merge conflicts
      7m 5s
    5. Exploring strategies to reduce merge conflicts
      4m 1s
  13. 14m 34s
    1. Saving changes in the stash
      4m 5s
    2. Viewing stashed changes
      2m 39s
    3. Retrieving stashed changes
      4m 24s
    4. Deleting stashed changes
      3m 26s
  14. 1h 5m
    1. Using local and remote repositories
      6m 38s
    2. Setting up a GitHub account
      5m 39s
    3. Adding a remote repository
      4m 0s
    4. Creating a remote branch
      4m 3s
    5. Cloning a remote repository
      4m 26s
    6. Tracking remote branches
      4m 5s
    7. Pushing changes to a remote repository
      5m 8s
    8. Fetching changes from a remote repository
      5m 47s
    9. Merging in fetched changes
      4m 50s
    10. Checking out remote branches
      3m 22s
    11. Pushing to an updated remote branch
      2m 6s
    12. Deleting a remote branch
      3m 8s
    13. Enabling collaboration
      3m 40s
    14. A collaboration workflow
      8m 43s
  15. 16m 23s
    1. Setting up aliases for common commands
      5m 14s
    2. Using SSH keys for remote login
      2m 56s
    3. Exploring integrated development environments
      1m 4s
    4. Exploring graphical user interfaces
      4m 32s
    5. Understanding Git hosting
      2m 37s
  16. 55s
    1. Goodbye
      55s

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Git Essential Training
6h 26m Beginner Aug 24, 2012

Viewers: in countries Watching now:

The course shows how to use Git, the popular open-source version control software, to manage changes to source code and text files. Using a step-by-step approach, author Kevin Skoglund presents the commands that enable efficient code management and reveals the fundamental concepts behind version control systems and the Git architecture. Discover how to track changes to files in a repository, review previous edits, and compare versions of a file; create branches to test new ideas without altering the main project; and merge those changes into the project if they work out. The course begins by demonstrating version control in a single-user, standalone context, before exploring how remote repositories allow users to collaborate on projects effectively.

Topics include:
  • Exploring the history of version control
  • Installing Git on Mac, Windows, and Linux
  • Initializing a repository
  • Writing useful commit messages
  • Understanding the Git three-tree architecture
  • Tracking when files are added, edited, deleted, or moved
  • Viewing change sets and comparing versions
  • Undoing changes and rolling back to previous versions
  • Ignoring changes to select files
  • Creating and working with code branches
  • Merging branches and resolving merge conflicts
  • Stashing changes for later
  • Working with hosted repositories and remote branches
  • Developing an effective collaboration workflow
Subject:
Developer
Software:
Git GitHub
Author:
Kevin Skoglund

Referencing commits

In this chapter, we are going to learn how to navigate the repositories commit tree. In order to do that, we need to start off by talking about the ways that we can reference commits in Git. We have covered a few basic ways already, but there are others that are going to be really useful to know. We will start off by introducing a new concept in Git called tree-ish. It's kind of a funny word, we've already talked about what a tree is in Git. It's the structure of files in the Git repository. It's similar to a directory in your file system. In Git, tree-ish means something that references part of the tree. It's ish because what that something is can vary widely.

The suffix ish indicates that it's like something but maybe in a vague way. Tree-ish is an important term to know, because it does show up in the Git documentation. You will be looking up how to use a command, and it will tell you that you can pass in a tree-ish as an argument to that command. Now in its simplest terms, a tree-ish is a reference to a commit because that commit then in turn references the tree, the Git repository and all the files that are in there at that point. So if you have a hard time thinking about all the things that a tree-ish can be, the simplest version is that it's just something that points out a commit.

So how can you reference a commit? The easiest way to do it is to use the SHA-1 hash, to use the entire 40 character string in order to reference the commit. Git will always know which commit we mean because each one of those 40 character strings is unique, and as a side note, if you were wondering, how Git make sure that all those are unique, it doesn't, but the odds of it not being unique are astronomical. We are talking you would need billions and billions of objects in order to have a chance of having two objects that have the same SHA-1 hash.

It so highly unlikely that, for practical day-to-day purposes we assume that they are unique. The other way we have seen that we can refer to a commit is using a shortened version of that hash. We don't have to have the whole thing, we can just use a portion of it, and because those numbers are so unique, Git can use that small portion and still find the object that we were looking for. We do have to provide at least four characters, and we have to provide enough, that it is unambiguous. Now being unambiguous depends on the size of our project. If we have a project that only has five commits, well then 4 characters is enough for it to know which one of those 5 commits that we're talking about.

If we are working on a mid-size project, you typically want to have something like 8-10 characters, and if you are working on a really large project, one that had millions of objects committed into it, something like the Linux Kernel, well at that point you want to have 12, 13, maybe even 15 characters to make sure that Git can find the object that you are looking for. My advice is to stick to about 8- 10 characters unless you need more. And we have also seen that we can reference a commit by using the HEAD pointer. The HEAD pointer remember always points to the commit that's at the tip of the currently checked out branch.

So that's going to be a tree-ish that points to part of the tree. We can also use a branch reference. If we use a branch reference, then we are referring to the tip of the branch. Now it doesn't have to be the currently checked out branch, it can be a branch that's not checked out. We will talk a lot more about working with branches in the next chapter. We also can use a tag reference. We are not going to cover tagging in this tutorial, other than just to mention that it is another way that you can refer to commits. And the last way that we can refer to commits is by using any one of these methods and then referring to that object's ancestry.

Let me show you how we do that. So if we want to refer to the parent commit of something, we first provide the reference for what we want to focus on, and then we say, find its parent by using the caret. So that caret character comes right after it, you can think of it is pointing up, pointing up to the parent on the tree. If you think of like a genealogy chart where the grandparents at the top and the children come down, we are moving up. So we are moving up to the parent, and that's what the up arrow nature of the caret suggest to us. We can also use a tilde notation, that is to have a reference to the commit and then a tilde followed by the number of generations we want to go up.

So HEAD going back one, we can also leave off the one, and it's just assumed. Most people tend to use the caret in this case instead of the tilde, but they do the same thing. If our HEAD commit is acf87504 then all five of these, all refer to the same thing. They all refer to the parent commit of acf87504. Now in addition to the parent commit, we can refer to the grandparent commit. Of course, if we want the parent of the parent, well then we just provide two up arrows. We say start at the HEAD and go up twice, that will give us the grandparent.

This is where the tilde notation becomes a lot more useful because now we can say HEAD~2, and it's especially apparent when we have the great grandparent commit because then we start having a whole lot of caret characters coming after our commits, whereas the tilde notation is much more compact and clear as to exactly what we were doing. We are going from the HEAD, we are moving back three commits to the great grandparent. So these are some useful ways that we can refer to commits that are in the commit tree. In the next movie, let's put these into practice and actually try using them.

Find answers to the most frequently asked questions about Git Essential Training.


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Q: In the Chapter 10 movie "Configuring the command prompt to show the branch," when I type the function "__git_ps1," I do not get the expected result.
A: The function "__git_ps1" was recently moved to a new file, .git-prompt.sh, as described here: https://github.com/git/git/commit/af31a456b4cd38f2630ed8e556e23954f806a3cc.

We will update the video. In the meantime, you may do the same steps you do for .git-completion.bash, but a second time using ".git-prompt.sh" as shown here: https://github.com/git/git/blob/master/contrib/completion/git-prompt.sh.
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