Ubuntu – Is it possible to know from apt if the package was installed explicitly or as a dependency of another package

What you're experiencing is the problem with apt / apt-get not being as smart as you think it is.

This problem happens when trying to downgrade your package(s) or install an older package version than the version the repositories have as the latest candidate (with regard to your apt priority pinning and other policies regarding repository priorities). When you downgrade your package, you actually have to specify for each individual dependency which version you're downgrading to, or in this case which specific version you actually want to install.

In the case of the nginx packages, where nginx-full and nginx-common depend on each other, you have to explicitly tell apt to install each of the packages of the specified version(s). This is because 1.7.6-1+trusty1 supersedes 1.7.5-1+trusty1 by version number. As a result, you have to specifically say "Only install the package of this specific version" because of the superseded version(s) existing, i.e. apt-get install nginx-full=1.7.5-1+trusty1 nginx-common=1.7.5-1+trusty1

_{Not relevant to your question, but this also happens when you install from a repository that has a lower apt pinning priority than another version, in which case you have to specify the versions and/or source(s) to install from manually, i.e. sudo apt-get install nginx-full/trusty-proposed nginx-common/trusty-proposed being a prime example of trying to install a package and dependencies from the proposed repository, which has a much lower apt priority than PPAs or the main repositories.}

1. Compare the log of /var/log/apt/term.log, which is the log file generated by apt and /var/log/dpkg.log, which is the log file generated by dpkg. If you note a package name which can be found in /var/log/dpkg.log but not in /var/log/apt/term.log, it is very likely that you installed the package by dpkg that time.

2. Please remember apt is a frontend for dpkg. Thus, it is recommended to remove any package by apt rather than dpkg, because apt will handle the dependency to remove those packages which will not be used by any other packages (, or mark them as "not use anymore" to let you remove them later. It depends on the exact apt options you actually use). However, it is no harm to you to remove your package by dpkg if you want. You will just likely leave many packages that you won't use anymore.