Apt-file is a great command line tool for searching for required packages and dependencies. Unfortunately, the Debian and Ubuntu repositories cannot cover the entire span of Linux software, and especially for user-generated content such as plasma widgets for KDE4, many programs still need to be compiled from source. However, since they are not installed with a package manager, dependencies are not covered, and a user can encounter many errors due to missing files or libraries that are not installed. If one gets an error such as as no such file or directory: taskmanager.h, one can simply enter the command apt-file search taskmanager.h to search the repositories for packages that have that file and if they find the right package they can enter apt-get install correctpackage and if no more dependencies are encountered, the install should go smoothly. It should be included with build-essential or installed automatically because it is a great resource for those who may have to compile from source code.