Questions[package-management](ubuntu)

I want to know where I can find my installed application when I installed it on Ubuntu using package manager.

I installed RabbitMQ and ran locate rabbitmq which gave me following result:

/home/anupamg/.m2/repository/com/rabbitmq
/home/anupamg/.m2/repository/com/rabbitmq/amqp-client
/home/anupamg/.m2/repository/com/rabbitmq/amqp-client/1.7.2
/home/anupamg/.m2/repository/com/rabbitmq/amqp-client/1.7.2/_maven.repositories
/home/anupamg/.m2/repository/com/rabbitmq/amqp-client/1.7.2/amqp-client-1.7.2.jar
/home/anupamg/.m2/repository/com/rabbitmq/amqp-client/1.7.2/amqp-client-1.7.2.jar.lastUpdated
/home/anupamg/.m2/repository/com/rabbitmq/amqp-client/1.7.2/amqp-client-1.7.2.jar.sha1
/home/anupamg/.m2/repository/com/rabbitmq/amqp-client/1.7.2/amqp-client-1.7.2.pom
/home/anupamg/.m2/repository/com/rabbitmq/amqp-client/1.7.2/amqp-client-1.7.2.pom.lastUpdated
/home/anupamg/.m2/repository/com/rabbitmq/amqp-client/1.7.2/amqp-client-1.7.2.pom.sha1
/home/anupamg/Downloads/rabbitmq-server-generic-unix-2.4.0.tar.gz

I'd like to output a list of all installed packages into a text file so that I can review it and bulk-install on another system. How would I do this?

I get this error when trying to use apt-get:

E: Could not get lock /var/lib/dpkg/lock - open (11 Resource temporarily unavailable)
E: Unable to lock the administration directory (/var/lib/dpkg/) is another process using it?  

How can I fix this?

I just added a PPA repository for the development version of the GIMP, but I get this error:

$ apt-get update && apt-get upgrade
...
The following packages have been kept back:
  gimp gimp-data libgegl-0.0-0 libgimp2.0

Why and how can I solve it so that I can use the latest version instead of the one I have now?

Simple enough question: is there some shell command (or GUI method) I can use that, given the path to a file on my system, tells me what package put it there? Assuming the file did in fact come from a package, that is.

Bonus question: what if it's a file that isn't installed on my system? Is there, say, a website that will let me look up a file and see what packages, if any, provide it?