dpkg/apt-cache: Useful commands
As I've mentioned in a couple of previous posts I've been playing around with creating a Vagrant VM that I can use for my neo4j hacking which has involved a lot of messing around with installing apt packages.
There are loads of different ways of working out what's going on when packages aren't installing as you'd expect so I thought it'd be good to document the ones I've been using so I can find them more easily next time.
Finding reverse dependencies
A couple of times I found myself wondering how a certain package had ended up on the VM because I hadn't specified that it should be installed so I wanted to know who had!
I wanted to find out the reverse dependency for the package. e.g. to find out who depended on make which we can find out with the following command:
$ apt-cache rdepends make make Reverse Depends: ... build-essential make:i386 libc6-dev:i386 open-vm-dkms mythbuntu-desktop broadcom-sta-source ...
The nice thing about 'rdepends' is that it will tell us reverse dependencies even for a package that we haven't installed. This was helpful here as I had forgotten to install 'build-essential' and this made it obvious.
Finding which version of a package is installed
I added one of the Brightbox repositories to get a more recent Ruby version and noticed that something weird was going on with the version of 'nginx-common' that puppet was trying to install.
It seemed like one one my dependencies was trying to pull in the 'latest' version of 'nginx-common' which I'd expected to be '1.1.19-1ubuntu0.1'.
By passing the 'policy' flag to apt-cache I was able to see that there was a recent version available via Brightbox:
$ apt-cache policy nginx-common nginx-common: Installed: 1.1.19-1ubuntu0.1 Candidate: 1:1.2.6-1~43~precise1 Version table: 1:1.2.6-1~43~precise1 0 500 http://ppa.launchpad.net/brightbox/ruby-ng/ubuntu/ precise/main amd64 Packages *** 1.1.19-1ubuntu0.1 0 500 http://us.archive.ubuntu.com/ubuntu/ precise-updates/universe amd64 Packages 100 /var/lib/dpkg/status 1.1.19-1 0 500 http://us.archive.ubuntu.com/ubuntu/ precise/universe amd64 Packages
Finding which versions of a package are available
Another flag that we can pass to apt-cache is 'madison' which shows us the available versions for a package but doesn't indicate which version is installed:
$ apt-cache madison nginx-common nginx-common | 1:1.2.6-1~43~precise1 | http://ppa.launchpad.net/brightbox/ruby-ng/ubuntu/ precise/main amd64 Packages nginx-common | 1.1.19-1ubuntu0.1 | http://us.archive.ubuntu.com/ubuntu/ precise-updates/universe amd64 Packages nginx-common | 1.1.19-1 | http://us.archive.ubuntu.com/ubuntu/ precise/universe amd64 Packages nginx | 1.1.19-1 | http://us.archive.ubuntu.com/ubuntu/ precise/universe Sources nginx | 1.1.19-1ubuntu0.1 | http://us.archive.ubuntu.com/ubuntu/ precise-updates/universe Sources nginx | 1:1.2.6-1~43~precise1 | http://ppa.launchpad.net/brightbox/ruby-ng/ubuntu/ precise/main Sources
Finding which package a file belongs to
At some stage I wanted to check which exact package was installing nginx which I was able to do with the following command:
$ dpkg -S `which nginx` nginx-extras: /usr/sbin/nginx
I had installed 'nginx-common' which I learn depends on 'nginx-extras' by using our 'rdepends' command:
$ apt-cache rdepends nginx-extras nginx-extras Reverse Depends: nginx-naxsi:i386 ... nginx-common
Finding the dependencies of a package
I wanted to check the dependencies of the 'ruby1.9.1' package to see whether or not I needed to explicitly install 'libruby1.9.1' or if that would be taken care of.
Passing the '-s' flag to dpkg let me check this:
$ dpkg -s ruby1.9.1 Package: ruby1.9.1 Status: install ok installed Architecture: amd64 Version: 1:184.108.40.2067-1bbox2~precise1 Replaces: irb1.9.1, rdoc1.9.1, rubygems1.9.1 Provides: irb1.9.1, rdoc1.9.1, ruby-interpreter, rubygems1.9.1 Depends: libruby1.9.1 (= 1:220.127.116.117-1bbox2~precise1), libc6 (>= 2.2.5) Suggests: ruby1.9.1-examples, ri1.9.1, graphviz, ruby1.9.1-dev, ruby-switch Conflicts: irb1.9.1 (<< 18.104.22.1688-2~), rdoc1.9.1 (<< 22.214.171.1248-2~), ri (<= 4.5), ri1.9.1 (<< 126.96.36.199-3~), ruby (<= 4.5), rubygems1.9.1 ...
These are the ones that I've found useful so far. I'd love to here other people's favourites though as I'm undoubtably missing some.