git commit --amend - commit hash is changed when no changes are made

Question

I open for some correction of my understanding of this, but I am not sure why this happens.

When using git, I understand that if I make a change to a file and then commit it I get a new hash because the file has changed.

My understanding of git commit --amend is that I can make changes to the last commit.

Now I can understand that if I change the commit message, the hash will change.

But when I do not make any changes and just save and exit out of the editor (I may have changed my mind on needing a change) why does the hash change? All my files and everything is the same, but I have saved it. When I of a git log the time of the commit hasn't changed, it just appears twice in the log with the same time, the same message, same files except a different commit hash.

Why does it change if no alterations have been done??

Answer 1

Git hashes are calculated using a number of items, including the author name/date, commit name/date commit message, tree, and parent SHA, among others. When you amend a commit, the commit name and date are updated. (You generally don't see the commit name and date unless you pass additional formatting options to git log.) Because that has been updated, the commit hash will change when a commit is amended.