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Using hash values (SHA-1)

From: Git Essential Training

Video: Using hash values (SHA-1)

In this movie we're going to talk about the way that Git refers to its commits. Remember in the last movie where we talk about the workflow we had different changes that we moved from our working directory, to our staging index, and to the repository. And I just went ahead and gave those the very simple labels A, B, and C. Now that's not what Git calls them, and that's what we're going to looking at in this movie is how Git refers to each of these snapshots of changes. Now be careful don't mistakenly think that A, B, and C refer to a single file in anyway at all. In our example we were using a single file, but these are changed sets, sets of changes, and more often than not they wil refer to multiple files.

Using hash values (SHA-1)

In this movie we're going to talk about the way that Git refers to its commits. Remember in the last movie where we talk about the workflow we had different changes that we moved from our working directory, to our staging index, and to the repository. And I just went ahead and gave those the very simple labels A, B, and C. Now that's not what Git calls them, and that's what we're going to looking at in this movie is how Git refers to each of these snapshots of changes. Now be careful don't mistakenly think that A, B, and C refer to a single file in anyway at all. In our example we were using a single file, but these are changed sets, sets of changes, and more often than not they wil refer to multiple files.

So in a typical Git workflow, A would represent changes to five files, B would represent changes that were made to three files, C might be two new files that were added to the repository. So A, B, and C are snapshots of the changes that were made not anything to do with files or versions of files. So let's take a look at how Git does refer to these files. When we submit these changes to the repository at that point Git generates a checksum for each changed set. A checksum is a number that's generated by taking data and feeding it into an algorithm, so checksum algorithm converts data into a simple number, and we call that simple number a checksum.

The same data put into the algorithm always equals the same checksum coming out that's important because if we change the data going in we get a different checksum out. So one of the most common uses for checksums in computers is to make sure that the data didn't change, if the data changed well then the checksum will be different. And this data integrity is fundamentally built into Git that's very different from other version control systems, they don't use checksums to validate that the data hasn't change. Git does it makes sure that you can't change what's in a commit or else you'll change the checksum that comes out of it. Changing the data changes the checksum.

Now the way that Git generates this checksum is by using the SHA-1 hash algorithm. You don't need to know anything about that hash algorithm itself, but you do need to know that it's called that because you will often hear people refer to this checksum or hash as being the SHA, or S-H-A value. The number that the algorithm generates is always going to be a 40 character hexadecimal string. Hexadecimal means they can have the numbers 0 through 9 and the letters a through f. So an example might look something like this 5c15e8bd540 and so on, 40 characters long made up of those characters.

So what Git does is it takes the entire set of changes, runs them through in algorithm, and in the end comes out with this one 40 digit number, we've seen this number before. When we get our Git log command here is that ID that I told you to get uses to track each one of our commits, its right there this is the SHA, S-H-A value, or the commit ID, you can call it whatever you want really. But it is a number that will be unique to the changes that are in this commit. So the way they get actually attaches that information is that if we have those three snapshot those sets of changes it feeds them into its algorithm to come up with the S-H-A value, and then it attaches a bit of meta information to each one of those snapshots, it has that commit number at the top, it has the parent commit the commit that comes before it, the author of the commit, and then the commit message.

So here you can see how the series of those commits are linked together, you can see that the parent for each one refers to the SHA-1 value of the other one before the identifier that come before, and that's how it knows the sequence of those commits. And then each one of those, each bit of meta information, points at a snapshot a set of changes or a Git object. Understanding how Git generates these hash values is important, because it helps us understand how Git summarizes these snapshots, it illustrates the data integrity that's built into Git, and most importantly we're going to be using these SHA-1 hash values to refer to the commits.

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This video is part of

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Git Essential Training

89 video lessons · 28714 viewers

Kevin Skoglund
Author

 
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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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