For this task we will need:
git cat-filesedgit hash-objectgit update-ref
Let's create a sample repository. This script sets things up so that we have repeatable commit ids and we don't run into any issues caused by weird local git configurations:
#!/bin/bash
set -e
HOME=$PWD
GIT_AUTHOR_NAME=Alice
GIT_AUTHOR_EMAIL=alice@example.com
GIT_AUTHOR_DATE="2023-01-01 00:00:00"
GIT_COMMITTER_NAME=$GIT_AUTHOR_NAME
GIT_COMMITTER_EMAIL=$GIT_AUTHOR_EMAIL
GIT_COMMITTER_DATE=$GIT_AUTHOR_DATE
export HOME GIT_{AUTHOR,COMMITTER}_{NAME,EMAIL,DATE}
workdir="$(mktemp -d "$PWD/gitXXXXXX")"
trap 'cd /; rm -rf $workdir' EXIT
cd "$workdir"
git config --global init.defaultBranch main
git init
for x in A C D; do
echo "file for commit $x" > file-$x
git add file-$x
git commit -m "$x"
done
git checkout --orphan feat
git reset
echo "file for commit B" > file-B
git add file-B
git commit -m 'B'
git checkout -f main
PS1="git$ " bash --norc
That gets us:
git$ git log --oneline
6352bde (HEAD -> main) D
25635b4 C
79a5602 A
git$ git log --oneline feat
db65aa0 (feat) B
We can use git cat-file -p to dump the structure of a commit. We want to add a new parent to commit C, which looks like:
git$ git cat-file -p 25635b4
tree dfa1779e5574c1b6f1c9c9071aa1a820b1e03680
parent 79a56022dc4511577b0281bb034b56e0352d2e36
author Alice <alice@example.com> 1672549200 -0500
committer Alice <alice@example.com> 1672549200 -0500
C
To make B a parent of this commit, we need to add a second parent line. We need the full commit id for commit B:
git$ git rev-parse feat
db65aa0d30cf551fdd25ad93d0c8e2f8da057572
We can add that as a parent of C using sed, like this:
git$ git cat-file -p 25635b4 | sed '/parent/ a\parent db65aa0d30cf551fdd25ad93d0c8e2f8da057572'
tree dfa1779e5574c1b6f1c9c9071aa1a820b1e03680
parent 79a56022dc4511577b0281bb034b56e0352d2e36
parent db65aa0d30cf551fdd25ad93d0c8e2f8da057572
author Alice <alice@example.com> 1672549200 -0500
committer Alice <alice@example.com> 1672549200 -0500
C
That looks right. Now we need to write that into the object database:
git$ git cat-file -p 25635b4 | sed '/parent/ a\parent db65aa0d30cf551fdd25ad93d0c8e2f8da057572' | git hash-object -t commit --stdin -w
a6db46299e550128fa8534dcc001f961ac4265c5
So now we have commit C' with commit id a6db46299e550128fa8534dcc001f961ac4265c5. We need to edit D to get D' with parent C', which is just a simple search/replace operation:
git$ git cat-file -p 6352bde | sed 's/25635b41c4e003279c17c0cc50bf1e565b36ecfb/a6db46299e550128fa8534dcc001f961ac4265c5/' | git hash-object -t commit --stdin -w
96b1aea0c6c75f53b0ae45658b8ffdb3921560c0
Lastly, we need to update the main branch to point to D' as the new HEAD:
git$ git update-ref refs/heads/main 96b1aea0c6c75f53b0ae45658b8ffdb3921560c0
Now let's see what we have:
git$ git log --graph --pretty='%h (%s)%n' --abbrev-commit --date=relative --branches --all --decorate
* 96b1aea (D)
|
* a6db462 (C)
|\
| |
| * db65aa0 (B)
|
* 79a5602 (A)
I think that's what you were after.
I wrote out all the steps here in detail which makes it look enormous compare to the solution from @ElpieKay, but when we distill it down to the crucial commands we get:
#!/bin/bash
git update-ref refs/heads/main "$(
git cat-file -p "$C" |
sed "/parent/ a\parent $B" |
git hash-object -t commit --stdin -w
)"
git update-ref refs/heads/main "$(
git cat-file -p "$D" |
sed "/parent/ s/parent.*/parent $(git rev-parse main)/" |
git hash-object -t commit --stdin -w
)"
Fill in $A through $D with the appropriate commit ids. If you're working with the sample repository created by the script earlier in the post, you can run (assuming you save the script to a file named reparent-w-hash-object.sh):
eval $(git log --oneline --branches --pretty="%s=%H") \
sh reparent-w-hash-object.sh
