The first version control system I used was CSV, then I moved to Subversion as everyone else does. Later, people start to look for server-less solutions and I used Mercurial. I even made a cheatsheet about it that is now the top result on Google for searching “mercurial cheatsheet”. But when I was in my day job, I need to learn to use git instead. The popularity of Github compared to BitBucket seems made git the winner version control system. But can I use both? After all, I wish to keep my repositories hosted by multiple providers while keeping them in sync as well.

Bridging git and mercurial

The git command line is git and mercurial is hg. Google can tell you that. However, if you want to use both git and mercurial in the same repository, what it means is that you can make a commit in git and see it as a revision in mercurial. Or vice versa. What we want to avoid is to make a git commit and then we have to craft a mercurial revision separately.

There are two ways to do it: We ask git to copy the commit into mercurial revision or ask hg to copy a revision into git commit. There are multiple tools to do that but these two are quite official nowadays:

We will focus on the former here.

There are multiple projects of git-remote-hg, all in the same name. The one quoted above is semi-official one and most widely used. The architecture make use of a feature from git, the git-remote-helper. It defines a protocol for git to use when it talk remotely. This is how git knows how to handle ssh:// or https:// URLs. With this plugin, git can do:

git clone hg::ssh://hostname//path/to/repository

which it will convert a Mercurial repo located at /path/to/repository in a remote host to a local git repo. Afterwards, we can push and pull in the local git repo to synchronize with the remote mercurial.

Creating a local co-existing git and mercurial repo with git tool

It is best to demonstrate by example. Prerequisites: Allows SSH to localhost, with hg command line installed and accessible by non-login shells (i.e., set up $PATH in ~/.bashrc if needed)

# create a repo in a directory
$ mkdir testrepo
$ cd testrepo
$ git init
$ hg init

# make some files and create a git commit
$ echo .hg > .gitignore
$ echo .git > .hgignore
$ git add .hgignore .gitignore
Initialized empty Git repository in /path/to/testrepo/.git/
$ git commit -m 'ignores'
[master (root-commit) b289014] ignores
 2 files changed, 2 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 .hgignore
 
# set up mercurial as a "remote", but actually a local
$ git remote add localhg hg::ssh://localhost/`pwd`

# push, and set up remote master branch
$ git push --set-upstream localhg master
no changes found
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: added 1 changesets with 2 changes to 2 files
To hg::ssh://localhost//path/to/testrepo
 * [new branch]      master -> master
Branch 'master' set up to track remote branch 'master' from 'localhg'.

# verify the mercurial revision history
$ hg log
…

# Add another commit and revision
$ echo "something should be here" > README
$ git add README
$ git commit -m 'Add readme'
[master 67fda75] Add readme
 1 file changed, 1 insertion(+)
 create mode 100644 README

$ git push localhg
searching for changes
no changes found
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: added 1 changesets with 1 changes to 1 files
To hg::ssh://localhost//path/to/testrepo
   b289014..67fda75  master -> master

Pay attention that we need .gitignore to have .hg and .hgignore to have .git so that git will not mess with the mercurial metadata and mercurial not with git either.

Multihost repository

What I mean is to have the repository hosted by multiple providers and synchronized.

Git has a concept of remote, which we can see all remotes set up in a repo with the following:

$ git remote -v
bakkdoor  https://github.com/bakkdoor/grit (fetch)
bakkdoor  https://github.com/bakkdoor/grit (push)
cho45     https://github.com/cho45/grit (fetch)
cho45     https://github.com/cho45/grit (push)
defunkt   https://github.com/defunkt/grit (fetch)
defunkt   https://github.com/defunkt/grit (push)
koke      git://github.com/koke/grit.git (fetch)
koke      git://github.com/koke/grit.git (push)
origin    git@github.com:mojombo/grit.git (fetch)
origin    git@github.com:mojombo/grit.git (push)

The remote named origin is special that it is the default remote when you didn’t specify one in the git commands fetch, pull, and push.

The interesting feature in git is that, we can set multiple URLs for a remote. When it is set, we can push to all URLs in one push command:

This is how we set up multiple URLs in one remote (origin in this case):

# assume we do not have any remote and we add remote origin
$ git remote add origin hg::ssh://localhost//path/to/myrepo

# with first URL set for the remote, we add more
$ git remote set-url origin --add ssh://git@github.com/myhandle/myrepo.git
$ git remote set-url origin --add ssh://git@gitlab.com/myhandle/myrepo.git
$ git remote set-url origin --add hg::ssh://hg@bitbucket.org/myhandle/myrepo

# check the setting
$ git remote -v
origin	hg::ssh://localhost//path/to/myrepo (fetch)
origin	hg::ssh://localhost//path/to/myrepo (push)
origin	ssh://git@github.com/myhandle/myrepo.git (push)
origin	ssh://git@gitlab.com/myhandle/myrepo.git (push)
origin	hg::ssh://hg@bitbucket.org/myhandle/myrepo (push)

or if this is clearer, the .git/config will look like the following:

[core]
	repositoryformatversion = 0
	filemode = true
	bare = false
	logallrefupdates = true
	ignorecase = true
	precomposeunicode = true
[remote "origin"]
	url = hg::ssh://localhost//path/to/myrepo
	url = ssh://git@gitlab.com/myhandle/myrepo.git
	url = ssh://git@github.com/myhandle/myrepo.git
	url = hg::ssh://hg@bitbucket.org/myhandle/myrepo
	fetch = +refs/heads/*:refs/remotes/origin/*
[branch "master"]
	remote = origin
	merge = refs/heads/master

And this is what happen when we push, note that each location is pushed one by one:

$ git push
searching for changes
no changes found
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: added 1 changesets with 1 changes to 1 files
To hg::ssh://localhost//path/to/myrepo
   571ca86..60a40a0  master -> master
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 369 bytes | 369.00 KiB/s, done.
Total 3 (delta 1), reused 0 (delta 0)
To ssh://gitlab.com/myhandle/myrepo.git
   571ca86..60a40a0  master -> master
searching for changes
no changes found
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: added 1 changesets with 1 changes to 1 files
To hg::ssh://hg@bitbucket.org/myhandle/myrepo
   60a40a0..60a40a0  master -> master
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 369 bytes | 369.00 KiB/s, done.
Total 3 (delta 1), reused 0 (delta 0)
remote: Resolving deltas: 100% (1/1), completed with 1 local object.
To ssh://git@github.com/myhandle/myrepo.git
   571ca86..60a40a0  master -> master

using SSH to access remote repositories

SSH is supported by github, gitlab, and bitbuckets. I would prefer to set a specific SSH key to use them, e.g.,

ssh-keygen -t ecdsa -f id_ecdsa.github

If we have multiple accounts on github et al, we can generate one key for each account. The repo service will remember each SSH key you assign to an account. No two accounts are allowed to share a key. This way, we can set up our ~/.ssh/config as follows:

Host myname.bitbucket.org
    HostName bitbucket.org
    User hg
    PreferredAuthentications publickey
    IdentityFile ~/.ssh/id_ecdsa.bitbucket.priv
    IdentitiesOnly yes

Host myname.github.com
    HostName github.com
    User git
    PreferredAuthentications publickey
    IdentityFile ~/.ssh/id_ecdsa.github.priv
    IdentitiesOnly yes

and in .git/config, we use the following:

[core]
repositoryformatversion = 0
filemode = true
bare = false
logallrefupdates = true
ignorecase = true
precomposeunicode = true
[remote "origin"]
url = hg::ssh://myname.bitbucket.org/myhandle/myrepo
fetch = +refs/heads/*:refs/remotes/origin/*
[branch "master"]
remote = origin
merge = refs/heads/master

The url part will use myname.bitbucket.org instead of bitbucket.org because we defined such host in SSH config. It does not need to be an existing DNS name. But because of the config, SSH will know to authenticate with a particular key so the remote knows it is you. We also omitted the login name git or hg because it is defined in SSH’s config.

If you did not clone the repository yet, you can create one by:

git clone ssh://myname.github.com/mygithubname/myproject

or

git clone hg::ssh://myname.bitbucket.com/mygithubname/myproject

which will have the above “origin” pre-set.