What is a Git commit ID?

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A Git commit ID is a SHA-1 hash of every important thing about the commit. I’m not going to list them all, but here’s the important ones…

  • The content, all of it, not just the diff.
  • Commit date.
  • Committer’s name and email address.
  • Log message.
  • The ID of the previous commit(s).

Change any of that and the commit ID changes. And yes, the same commit with the same properties will have the same ID on a different machine. This serves three purposes. First, it means the system can tell if a commit has been tampered with. It’s baked right into the architecture.

Second, one can rapidly compare commits just by looking at their IDs. This makes Git’s network protocols very efficient. Want to compare two commits to see if they’re the same? Don’t have to send the whole diff, just send the IDs.

Third, and this is the genius, two commits with the same IDs have the same history. That’s why the ID of the previous commits are part of the hash. If the content of a commit is the same but the parents are different, the commit ID must be different. That means when comparing repositories (like in a push or pull) once Git finds a commit in common between the two repositories it can stop checking. This makes pushing and pulling extremely efficient. For example…

origin
A - B - C - D - E [master]

A - B [origin/master]

The network conversation for git fetch origin goes something like this…

  • local Hey origin, what branches do you have?
  • origin I have master at E.
  • local I don’t have E, I have your master at B.
  • origin B you say? I have B and it’s an ancestor of E. That checks out. Let me send you C, D and E.

This is also why when you rewrite a commit with rebase, everything after it has to change. Here’s an example.

A - B - C - D - E - F - G [master]

Let’s say you rewrite D, just to change the log message a bit. Now D can no longer be D, it has to be copied to a new commit we’ll call D1.

A - B - C - D - E - F - G [master]
         \
          D1

While D1 can have C as its parent (C is unaffected, commits do not know their children) it is disconnected from E, F and G. If we change E’s parent to D1, E can’t be E anymore. It has to be copied to a new commit E1.

A - B - C - D - E - F - G [master]
         \
          D1 - E1

And so on with F to F1 and G to G1.

A - B - C - D - E - F - G
         \
          D1 - E1 - F1 - G1 [master]

They all have the same code, just different parents (or in D1’s case, a different commit message).

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