Python Fabric: Getting File from Host as String

When using fabric for deployments you will sometimes want check an existing file for the presence of a value before applying an update. A common example I run into is checking if an apt-source has already been added before adding it again. This is a little clunky in fabric, but thankfully you can write a simple helper which takes case of it for you.

def _get_remote(fileloc):
    '''Pulls back a file contents from connection as string'''
    from StringIO import StringIO

    fd = StringIO()
    get(fileloc, fd)
    content = fd.getvalue()
    return content

Usage is fairly simple. Say we want to install the latest version of Varnish Cache on an Ubuntu server. Usage like so works,

if ' trusty varnish-4.0' not in _get_remote('/etc/apt/sources.list.d/varnish-cache.list'):
    sudo('curl | sudo apt-key add -')
    sudo('''sudo sh -c 'echo "deb trusty varnish-4.0" >> /etc/apt/sources.list.d/varnish-cache.list' ''')
    sudo('apt-get -y update')

At this point you should be able to install the latest version of varnish with a simple apt-get.