How do I synchronise two remote Git repositories?

Git branches do not have "heads" in the Mercurial sense. There is only one thing called HEAD, and it's effectively a symlink to the commit you currently have checked out. In the case of hosted repositories like GitHub, there is no commit checked out—there's just the repository history itself. (Called a "bare" repo.)

The reason for this difference is that Git branch names are completely arbitrary; they don't have to match between copies of a repository, and you can create and destroy them on a whim.[1] Git branches are like Python variable names, which can be shuffled around and stuck to any value as you like; Mercurial branches are like C variables, which refer to fixed preallocated memory locations you then fill with data.

So when you pull in Mercurial, you have two histories for the same branch, because the branch name is a fixed meaningful thing in both repositories. The leaf of each history is a "head", and you'd normally merge them to create a single head.

But in Git, fetching a remote branch doesn't actually affect your branch at all. If you fetch the master branch from origin, it just goes into a branch called origin/master.[2] git pull origin master is just thin sugar for two steps: fetching the remote branch into origin/master, and then merging that other branch into your current branch. But they don't have to have the same name; your branch could be called development or trunk or whatever else. You can pull or merge any other branch into it, and you can push it to any other branch. Git doesn't care.

Which brings me back to your problem: you can't push a "second" branch head to a remote Git repository, because the concept doesn't exist. You could push to branches with mangled names (bitbucket_master?), but as far as I'm aware, you can't update a remote's remotes remotely.

I don't think your plan makes a lot of sense, though, since with unmerged branches exposed to both repositories, you'd either have to merge them both, or you'd merge one and then mirror it on top of the other... in which case you left the second repository in a useless state for no reason.

Is there a reason you can't just do this:

1. Pick a repository to be canonical—I assume BitBucket. Clone it. It becomes origin.

2. Add the other repository as a remote called, say, github.

3. Have a simple script periodically fetch both remotes and attempt to merge the github branch(es) into the origin branches. If the merge fails, abort and send you an email or whatever. If the merge is trivial, push the result to both remotes.

Of course, if you just do all your work on feature branches, this all becomes much less of a problem. :)

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[1] It gets even better: you can merge together branches from different repositories that have no history whatsoever in common. I've done this to consolidate projects that were started separatedly; they used different directory structures, so it works fine. GitHub uses a similar trick for its Pages feature: the history of your Pages is stored in a branch called gh-pages that lives in the same repository but has absolutely no history in common with the rest of your project.

[2] This is a white lie. The branch is still called master, but it belongs to the remote called origin, and the slash is syntax for referring to it. The distinction can matter because Git has no qualms about slashes in branch names, so you could have a local branch named origin/master, and that would shadow the remote branch.

Here's a tested solution for the issue: http://www.tikalk.com/devops/sync-remote-repositories/

The commands to run:

#!/bin/bash

# REPO_NAME=<repo>.git
# ORIGIN_URL=git@<host>:<project>/$REPO_NAME
# REPO1_URL=git@<host>:<project>/$REPO_NAME

rm -rf $REPO_NAME
git clone --bare $ORIGIN_URL
cd $REPO_NAME
git remote add --mirror=fetch repo1 $REPO1_URL
git fetch origin --tags ; git fetch repo1 --tags
git push origin --all ; git push origin --tags
git push repo1 --all ; git push repo1 --tags

For something similar I use this simple code trigerred by webhook in both repositories to sync GitLab and Bitbucket master branch:

git pull origin master
git pull gitlab master
git push origin master
git push gitlab master

It propably is not what you need in question, but it could be helpful for somebody else who needs to sync just one branch.