By default, everything you check into a Fossil repository is shared to all clones of that repository. In Fossil, you don't push and pull individual branches; you push and pull everything all at once.
But sometimes users want to keep some private work that is not shared with others. This might be a preliminary or experimental change that needs further refinement before it is shared and which might never be shared at all. To do this in Fossil, simply commit the change with the --private command-line option:
fossil commit --private
The --private option causes Fossil to put the check-in in a new branch named "private". That branch will not participate in subsequent clone, sync, push, or pull operations. The branch will remain on the one local repository where it was created. Note that you only use the --private option for the first check-in that creates the private branch. Additional checkins into the private branch remain private automatically.
Publishing Private Changes
After additional work, one might desire to publish the changes associated with a private branch. The usual way to do this is to merge those changes into a public branch. For example:
fossil update trunk fossil merge private fossil commit
The private branch remains private. (There is no way to convert a private branch into a public branch.) But all of the changes associated with the private branch are now folded into the public branch and are hence visible to other users of the project.
Syncing Private Branches
A private branch normally stays on the one repository where it was originally created. But sometimes you want to share private branches with another repository. For example, you might be building a cross-platform application and have separate repositories on your Windows laptop, your Linux desktop, and your iMac. You can transfer private branches between these machines by using the --private option on the "sync", "push", "pull", and "clone" commands. For example, if you are running "fossil server" on your Linux box and you want to clone that repository to your Mac, including all private branches, use:
fossil clone --private http://user@linux.localnetwork:8080/ mac-clone.fossil
You'll have to supply a username and password in order for this to work. Fossil will not clone (or sync) private branches anonymously. Furthermore, you have to enable the "Private" capability (the "x" capability) in order to enable private branch syncing. The Admin ("a") and Superuser ("s") do not imply Private ("x"); you'll have to set "x" in addition to "a" or "s". This is a little extra work, but there is a purpose here: the interface is designed to make it very difficult to accidently sync a private branch to a public server. It is highly recommended that you leave the "x" capability turned off on all repositories used for collaboration (repositories to which many people push and pull) and only enable "x" for local repositories when you need to share private branches.
Private branch sync only works if you use the --private command-line option. Private branches are never synced via the auto-sync mechanism. Once again, this restriction is designed to make it hard to accidently push private branches beyond their intended audience.
Purging Private Branches
You can remove all private branches from a repository using this command:
fossil scrub --private
Note that the above is a permanent and irreversible change. You will be asked to confirm before continuing. Once the private branches are removed, they cannot be retrieved (unless you have synced them to another repository.) So be careful with the command.
Additional Notes
All of the features above apply to all private branches in a single repository at once. There is no mechanism in Fossil (currently) that allows you to push, pull, clone, sync, or scrub and individual private branch within a repository that contains multiple private branches.