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parallel-ssh @ 00c87fe

Night Bus: Simple SSH-based build automation

Night Bus is a minimalist build automation tool. It requires nothing on the build machines but a shell, an SSH server and your build dependencies. It's intended for automating slow processes that need to run on multiple machines and was written initially for building and testing the GNU toolchain on various architectures1.

Night Bus is written in Python 3, and reads configuration from YAML format files. It's built on top of the ParallelSSH library. It's heavily inspired by the Ansible configuration management tool, with the main difference being that Night Bus provides real-time logging of the tasks as they execute.

1. including the old proprietary platforms where dependency hell is still a thing and where lots of modern tools don't even work....


Right now the software should be used directly from Git. It's early days.

# Install dependencies if needed... `pip3 install --user gevent paramiko pyyaml`
git clone --recursive git://


Now set up the configuration that you want. We recommend doing this in a separate directory or perhaps a Git repo.


First you need to describe how to access all of the build machines. This is done using a file named hosts. Here's an example:

  user: automation
  private_key: ssh/automation.key

  proxy_user: jenny
  proxy_private_key: ssh/jenny.key

The parameters are passed as in the ParallelSSHClient constructor, except for pkey -> private_key.

You can test your host configuration by running a test command:

../nightbus/ --command 'echo "Hello from $(hostname)"'


Now you describe what tasks need to be run. This is done using a file named tasks which contains an ordered list of tasks.

Here's a simple task file with one pointless task.

- name: counting-example
  commands: |
    echo "Counting to 20."
    for i in `seq 1 20`; do
      echo "Hello $i"
      sleep 1

When you run this it'll count to 20 on every host. You can see the results on stdout or in the log files in the log directory.

Here's a more advanced example, with some inline comments explaining it.

  # Night Bus comes with a library of shell functions that allow you to write
  # clearer tasks. You can include anything you want here and it'll be
  # prepended to each task.
    - nightbus/

- name: gcc-rebuild
  description: Run a build of GCC, reusing an existing build tree if present.
  commands: |
    set -e


    echo "Updating clone of $repo branch $track"

    # This helper function comes from nightbus/ and is documented
    # there. It will clone the repo if necessary and update it if necessary.
    # It returns false if there were no new changes.
    if ensure_uptodate_git_branch_checkout "$sourcedir" "$remote" "$repo" "$track" \
            || [ "$force" = "yes" ] ; then
        # This only runs if there were new changes in the repo, or if the user
        # passed `--force` to Night Bus.
        cd $sourccedir
        configure_args="--enable-languages=c,c++ --disable-bootstrap"

        echo "Running build"
        mkdir -p build
        cd build
        ../configure $configure_args
        echo "No changes in remote Git repo, exiting."
        exit 0

- name: gcc-test
  description: Runs the GCC test suite.
  commands: |
    set -e

    cd $sourcedir/build

    echo "Running test suite"
    gmake check

A task set executes one at a time. So first 'gcc-rebuild' runs on all hosts. If any hosts fail, those hosts are removed from the list. Once all the task has completed on all hosts, the hosts which succeeded will then start the next task 'gcc-test'. If all hosts fail, the sequence is aborted.

We need a directory to store logs for the tasks. For testing, create one in the current directory, and then run the tasks:

mkdir logs
../nightbus/ --log-directory=./logs

You should see files like this inside the ./logs directory:


You can tail -f these to see how your build is going.

There are some commandline options to help you debug tasks:

  • --command: run a single command on all hosts
  • --ignore-errors: continue running tasks even if some have failed
  • --force: adds force=yes as the first line of the task.
  • --tasks: select a subset of tasks to be executed


To make logs browsable outside the machine running Night Bus, install a web server and set the log directory to somewhere inside /var/www.

To start your builds at a specific time, use Cron or a systemd .timer unit to execute the script appropriately.

Advanced features

Night Bus supports parameterization of tasks. This is inspired by similar features in other tools such as pytest.

This input will result in three tasks being generated:

- name: example
    person: ['Eleanor', 'Matthew', 'Bill']
  commands: |
    echo "Hello, $person"

You can run nightbus --list and see them for example:

  • example-Eleanor
  • example-Matthew
  • example-Bill

Multiple parameters can be specified, but beware that the code will probably not scale well if you start generating hundreds of tasks this way! Instead of passing a string for the value, you can pass a dict like this:

{ repr: default, value: '' }

The 'repr' value is used in the task name, while the 'value' is what gets used in the task. This is useful if the values you're working with contain characters that aren't valid in task names for example.


We like ...

  • ... a small, tidy codebase
  • ... a clean, convenient commandline interface
  • ... no special requirements on the build machines
  • ... shell scripts which are smaller than 10 lines and can be read at a glance

Common issues

Debugging errors is hard

Add set -x to the top of your task so that the shell prints out each command it runs before running it.

The tasks don't use the correct PATH, so some programs aren't found

By default, Bash ignores your .profile and .bashrc files when running scripts. If you find that annoying, you can set this option for a task:

`shell: bash -i -l -c`

This tells Bash to behave the same way when running that task as it does would in your interactive SSH session. Read more here.

Known problems

We use a fork of Parallel-SSH, due to needing a better fix for:

Why not use...

Ansible or Fabric?

Ansible can't give us live output from the tasks, it just sends the whole log over once the task completes. For jobs that can take hours and hours like running the GCC test suite this is a major limitation.

At time of writing the Ansible team have looked at adding this feature and decided against it:

Fabric also appears not to have such a feature, and no discussion of it.

cdist also has nice aspects but its design really ties it to doing configuration management rather than build+test automation.


I would recommend BuildBot as the "next step up" from Night Bus.

Gitlab CI?

Gitlab CI requires a client program written in Go on each build machine. Go is not supported on every platform, for example AIX.


Copyright 2017 Codethink Ltd.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.