Commits in git are immutable. This is a fundamental feature of the tool. The oid of a commit (often referred to as the “hash” or the “sha” of the commit, since the two primary implementations of computing an oid use either the sha-1 or the sha-256 algorithm to compute the value) is a hash of all of the information stored in the commit. As such, a commit cannot be modified.

You will often see the phrase “re-write history”, and this phrase is patently absurd. You cannot rewrite history. Neither in the real world (unless you are a politician) nor in git. History is fixed, commits are immutable. Period. When you rebase a commit, your are not rewriting history!!

Branches, however, are not immutable. They constantly change, but changing the referrant of a branch is not rewriting history.

Consider:

$ d=git-example; rm -rf $d; mkdir $d; cd $d; git init --template=/dev/null > /dev/null 2>&1
$ git config user.email bob@foo.org; git config user.name bob
$ for i in a b c; do echo $i > $i; git add $i; git commit -m "Append line to $i" > /dev/null; done
$ git log --pretty=oneline
cb8b457eb38041da256e0fa0b26ee74444bfa4de (HEAD -> main) Append line to c
373eef38f6c8f1fabc2ea5acf09c47f27039a97f Append line to b
50f3819a8e57fa449e4faf2f4d3e2fd1184228e9 Append line to a
$ GIT_SEQUENCE_EDITOR='sh -c "cat << EOF > $1
> p 373eef38f
> p cb8b457e
> p 50f3819a8e5
> EOF
> "' git rebase -i --root
Successfully rebased and updated refs/heads/main.
$ git log --pretty=oneline
b3a020f9c6f3e08ed22322144d98f63c38237dc9 (HEAD -> main) Append line to a
8f8ee615a2a382b442aaa984d72217842b629db7 Append line to c
c9b6d7316bf36c446ed4d526618083310d81f9fb Append line to b

At first glance, a naive user will say that history has been re-written. The main branch used to look like A-B-C, but now it looks like B’-C’-A’! It has changed, history has been re-written!! Nonsense. The ref “main” used to refer to the immutable commit cb8b457eb, but now it refers to the immmutable commit b3a020f9c6f. Nothing has changed. cb8b457eb still exists and is accessible, and if you want “main” to refer to it you can change it to do so.

Stop abusing language, and stop talking about re-writing history. Or, perhaps more importantly, stop expecting branches to be something other than what they are. A branch is a reference to a particular commit and the commit a branch refers to can (and is expected to!) change. Commits are immutable, but refs are not (nor should they be). But when you change the referent commit of a branch, you are not “re-writing history”.

In other words, don’t be afraid to rebase. Your data is safe. With one minor caveat; git does garbage collect, so although commits are immutable and can never be changed, they certainly can get deleted. Make sure any commit you care about has at least one ref to it to prevent it from being discarded.