How to use custom APT configuration

The Pro Client CLI is able, through the security-status command, to show information about installed packages in the system, their origins, and potential updates that may be available for them. This information is also available through the packages updates API and the packages summary API calls, for example.

The information presented on the outputs mentioned above is taken directly from the local APT caches. Packages, origins, versions, and candidates are all calculated using the python3-apt library, which in turn relies on the knowledge APT has about in a particular machine.

You may need to configure APT to have some specific behaviour. An example could be to pass custom .list or .sources files to APT, whether to limit or to increase knowledge about different package sources.

The Pro Client will respect the configuration set directly on APT, at execution time. If system level configuration is present, it will be used by default. To configure APT without changing the system, there is the option of passing the config file as an environment variable.

Use an APT config file

For example, consider that the /etc/apt/apt.conf.d/50-custom file exists, and has this content:

$ cat /etc/apt/apt.conf.d/50-custom
# setting custom sources.list and unsetting the parts directory
Dir::Etc::Sourcelist "/my/personal/configured/sources.list";
Dir::Etc::Sourceparts "nonexisting-dir";

APT will use this configuration by default when running all operations. The Pro Client will do the same - all outputs will take the custom .list file into consideration, and no parts directory will be loaded (unless the directory set there actually exists).

Use an environment variable

Now, if the user does not want to change their system, the APT_CONFIG environment variable can be set when running APT commands, pointing to the desired configuration file.

For example, let’s say that instead of apt.conf.d, the config file is created in /tmp:

$ cat /tmp/custom-config
# setting custom sources.list and unsetting the parts directory
Dir::Etc::Sourcelist "/my/personal/configured/sources.list";
Dir::Etc::Sourceparts "nonexisting-dir";

APT will respect this configuration if found in the aforementioned environment variable:

$ APT_CONFIG=/tmp/custom-config apt list --installed
$ APT_CONFIG=/tmp/custom-config sudo apt update

If the variable is set when executing the Pro Client commands that depend on APT, it will also be respected:

$ APT_CONFIG=/tmp/custom-config pro security-status
$ APT_CONFIG=/tmp/custom-config pro api u.pro.packages.updates.v1

In the python API integration

Changing the environment also works for the integration with the Pro Client’s APIs. Even after import time, it is just a matter of setting the value in os.environ before calling the function.

import os
from uaclient.api.u.pro.packages.updates.v1 import updates

# call updates using the system APT configuration
updates()

# set the custom configuration, so updates will respect it, then set it
# back to the original value so it respects the system config again
try:
    old_env_apt_config = os.getenv("APT_CONFIG")
    os.environ["APT_CONFIG"] = "/tmp/custom-config"
    updates()
# (...)
finally:
    os.environ["APT_CONFIG"] = old_env_apt_config