Extending Bazaar with Hooks and Plugins

Bazaar offers a powerful extension mechanism for adding capabilities. In addition to offering full library API access to all of its structures, which can be useful for outside programs that would like to interact with Bazaar branches, Bazaar can also load plugins that perform specific tasks. These specific tasks are specified by hooks that run during certain steps of the version control process.

For full documentation on the available hooks, see bzr help hooks. Among those, some of the most significant hooks from an administration standpoint are pre_commit, post_commit and post_change_branch_tip. A pre_commit hook can inspect a commit before it happens and cancel it if some criteria are not met. This can be useful for enforcing policies about the code, such as line-endings or whitespace conventions. A post_commit hook can take actions based on the commit that just happened, such as providing various types of notifications. Finally, a post_change_branch_tip hook is a more general form of a post_commit hook which is used whenever the tip of a branch changes (which can happen in more ways than just committing). This too can be used for notification purposes, as well as for backups and mirroring.

Information on the whole range of Bazaar plugins is available at http://doc.bazaar.canonical.com/plugins/en/. For purposes of installation, plugins are simply python packages. They can be installed alongside Bazaar in the bzrlib.plugins package using each plugin’s setup.py. They can also be installed in the plugin path which is the user’s ~/.bazaar/plugins directory or can be specified with the BZR_PLUGIN_PATH environment variable. See bzr help configuration for more on specifying the location of plugins.

Email Notification

A common need is for every change made on a branch to send an email message to some address, most often a mailing list. These plugins provide that capability in a number of different ways.

The email plugin sends email from each individual developer’s computer. This can be useful for situations that want to track what each individual developer is working on. On the downside, it requires that every developer’s branches be configured individually to use the same plugin.

The next two plugins hookless-email and email-notifier address this concern by running on a central server whenever changes happen on centrally stored branches.


To configure this plugin, simply install the plugin and configure the post_commit_to option for each branch. This configuration can be done in the locations.conf file or individually in each branch’s branch.conf file. The sender’s email address can be specified as post_commit_sender if it is different than the email address reported by bzr whoami. The post_commit_mailer option specifies how the mail should be sent. If it isn’t set, email is sent via /usr/bin/mail. It can also be configured to communicate directly with an SMTP server. For more details on configuring this plugin, see http://doc.bazaar.canonical.com/plugins/en/email-plugin.html. As examples, consider the following two possible configurations. A minimal one (uses /usr/bin/mail)

post_commit_to = projectx-commits@example.com

and a more complicated one (using all of the options)

post_commit_url = http://www.example.com/code/projectx/trunk
post_commit_to = projectx-commits@example.com
post_commit_sender = donotreply@example.com
post_commit_mailer = smtplib
smtp_server = mail.example.com:587
smtp_username = bob
# smtp_password = 'not specified, will prompt'


This plugin is basically a server-side version of the email plugin. It is a program that runs either from the command line or as a daemon that monitors the branches specified on the command line for any changes. When a change occurs to any of the monitored branches, it will send an email to the specified address. Using our simple example, the following command would send an email to projectx-commits@example.com on any of the branches under /srv/bzr since the last time the command was run. (This command could be set up to run at regular intervals, for example from cron.)

$ bzr_hookless_email.py --email=projectx-commits@example.com \
--recurse /srv/bzr


This is a more elaborate version of the hookless-email plugin that can send templated HTML emails, render wiki-style markup in commit messages and update working copies on the server (similar to push_and_update). It can also send emails reporting the creation of new branches or the removal of branches under a specified directory (here /srv/bzr/projectx). As it is more complicated, its configuration is also more complicated and we won’t repeat its documentation here, but a simple configuration that will send emails on commits and creation/deletion of branches is


# If user is not provided then no authentication will be performed.


# The address to send commit emails to.

# A Cheetah template used to construct the subject of the email message.
subject=$relative_path: $revision_number $summary

subject=$relative_path: New branch created

subject=$relative_path: Branch removed

If this file is stored as /srv/bzr/email-notifier.conf, then the command

$ bzr-email-notifier.py --config=/srv/bzr/email-notifier.conf /srv/bzr/projectx

will watch all branches under the given directory for commits, branch creations and branch deletions.

Feed Generation

A related concept to sending out emails when branches change is the generation of news feeds from changes on each branch. Interested parties can then choose to follow those news feeds in order to see what is happening on a branch.


This plugin creates an ATOM feed for every branch on every branch change (commit, etc.). It stores these files as .bzr/branch/branch.atom inside each branch. Currently, it includes the 20 most recent changes in each feed. To use it, simply install the plugin and set your feed reader to follow the branch.atom files.

In addition, there are other tools that are not plugins for creating news feeds from Bazaar branches. See http://wiki.bazaar.canonical.com/FeedGenerators for more on those tools.


Sometimes it is useful to ensure that one branch exists as an exact copy of another. This can be used to provide simple backup facilities or redundancy (see Back-up and restore for more details on backups). One way to do this using Bazaar’s workflows is to make the branch where changes happen into a bound branch of the mirror branch. Then, when commits happen on the working branch, they will also happen on the mirror branch. Note that commits to bound branches do not update the mirror branch’s working copy, so if the mirror branch is more than just a backup of the complete history of the branch, for example if it is being served as a web page, then additional plugins are necessary.


This plugin updates Bazaar’s push command to also update the remote working copy. It can only work over connections that imply filesystem or SSH access to the remote working copy (bzr+ssh://, sftp:// and file://). Also, it is only useful when the remote branch is updated with an explicit push command.


This plugin is similar to push_and_update in that it updates the working copy of a remote branch. The difference is that this plugin is designed to update the remote branch on every change to the working branch. To configure this, set the post_commit_mirror = URL option on a branch. This option can include multiple branch URLs separated by commas to create multiple mirrors. For example, if we want to mirror our /srv/bzr/projectx/trunk branch to the URL sftp://www.example.com/var/www/projectx (for example if ProjectX were a web project that we wanted to access at http://www.example.com/projectx), then we could include

post_commit_mirror = sftp://www.example.com/var/www/branches/trunk

in the file /srv/bzr/projectx/trunk/.bzr/branch/branch.conf.

Other Useful Plugins

pqm (plugin)

Facilitating interaction with PQM, this plugin provides support for submitting merge requests to a remote Patch Queue Manager. PQM provides a way to automatically run the test suite before merging changes to the trunk branch.


Sometimes referred to as the poor man’s PQM, this plugin runs a single command on the updated revision (in a temporary directory) and if the command returns 0, then the revision can be committed to that branch. For example, if the testsuite is run with the command nosetests in the root of the branch (which returns 0 if the test suite passes and 1 if it doesn’t pass), then one can set

pre_change_branch_tip_test_command = nosetests

in .bzr/branch/branch.conf.


This plugin is an example of a pre_commit hook that checks the revision being committed for meeting some policy. In this case, it checks that all of the files have the specified line endings. It uses a configuration file .bzreol in the root of the working tree (similar to the .bzrignore file). This configuration file has sections for line feed endings (LF), carriage return/line-feed endings (CRLF) and carriage return endings (CR). For an unusual example that specifies different line endings for different files, that file might look like




or if you simply want to enforce a single line ending convention on the branch you can use


This plugin needs to be installed on the server where the branch updates will happen, and the .bzreol file must be in each branch where line ending policies will be enforced. (Adding it to the branch with bzr add .bzreol is an easy way to ensure this, although it means that branches on the server must have working trees.)


This plugin is a more advanced version of checkeol that can check such coding style guidelines such as trailing whitespace, long lines and files that don’t end with a newline. It is configured using Bazaar’s built in rules specification in BZR_HOME/rules (see bzr help rules for more information. For different types of undesired changes, you can specify different types of actions. For example


[name *.py]

will prevent changes from adding new trailing whitespace to the specified files and keep all python source files free of tabs and lines over 78 characters. To commit while violating these rules, one can pass the --text-check-warn-only option to commit.