How to check if upstream's HEAD/latest commit is present in forked repo? - bash

Question

My thought is that, in order to automatically fetch updates only if upstream has pushed some commits, I should check its hashes.
2 ways, I could do is that to check the logs or use rev-list something like below:
# this fetches top 30 commits from forked repo
git rev-list -n 30 main
# this also does the same but not limited to 30
git log --format='%H'
I'm finding it difficult to compare the hashes as I'm failing to fetch more than 1 commit from forked repo.
Explained my approach below:
git remote add upstream https://github.com/upstream/upstream.git
git fetch upstream --tags
upstream_commit="$(git rev-parse upstream/main)"
echo "Upstream commit: ${upstream_commit}"
# This should store 30 recent commit hashes as an array AFAIK
forked_commits=($(git rev-list -n 30 main))
# another I tried to put forloop is this:
# for forked_commit in "$forked_commits[#]"; do
for forked_commit in $(git log --format='%H'); do
# but prints only the top most element and exits the loop
echo "$forked_commit"
if [ "$upstream_commit" == "$forked_commit" ]; then
has_new_commits=false
else
# since I've added my commits on top, if condition fails and this always returns true
has_new_commits=true
fi
done
if [ "${has_new_commits}" ]; then
git checkout main
git rebase upstream/main
git push -f origin main
echo "Rebase successful!"
else
echo "No commits to be synced!"
fi

Answer 1

Ok, I did many mistake that were minor.
Array stuff didn't work. I was supposed to put them directly within the forloop.
Next mistake I never validated them properly. i.e., even if the hash was found, it never exited.
Using [ $has_new_commits ]; then is wrong. I should be using [ $has_new_commits == "true"]; then instead.
Corrected code:
git remote add upstream https://github.com/upstream_repo/upstream.git
git fetch upstream --tags
upstream_commit="$(git rev-parse upstream/main)"
for forked_commit in $(git rev-list -n 20 main); do
if [ $upstream_commit != $forked_commit ]; then
has_new_commits=true
continue
else
has_new_commits=false
break
fi
done
if [ $has_new_commits == "true" ]; then
echo "New commits found!"
else
echo "No commits to be synced!"
fi