Package sources are usually specified in /etc/apt/sources.list and can be of many different types, like http, ftp, file, cdrom, …​ (see man sources.list). In a default Qlustar installation this file is empty, since all the Qlustar package sources are defined in the file /etc/apt/sources.list.d/qlustar.list. If your system has access to the Internet either directly or through a http proxy the file will look like this:

This enables access to the Qlustar 11.0 software repository.

dpkg (see man dpkg) is the basic package management tool for Ubuntu/Debian, comparable to rpm (Red Hat Package Manager). It is not capable of automatically resolving package dependencies.

apt is the high-level package management tool for Ubuntu/Debian. apt (man apt) with its sub-commands provides all the functionality needed to maintain an Ubuntu/Debian system. A seem-less and fast upgrade of an Ubuntu/Debian system is typically performed running the two commands

Detailed upgrade instructions for a Qlustar system can be found in the Qlustar Update Section.

New packages can be installed by running apt install <package name>. If <package name> depends on, or conflicts with other packages those will be automatically installed or removed upon confirmation.

The possibility to concurrently run different versions of the same application on a single cluster is often crucial. In principle, this is achievable in a couple of ways, each one requiring more or less handwork depending on the type of application in question. Fortunately, Ubuntu/Debian provides the built-in alternatives mechanism to manage software versions in automated form. It has the additional advantage that it works appropriately for any kind of application.

Let us consider the case of the GNU C compiler gcc as an example of the situation described above. Simply installing the gcc package via apt is all you need to do in this case. The alternatives are automatically configured for you.

This tells us that currently we are running the version 5.4.0 of the GNU C compiler. Let us inspect the cc binary.

As we tried to locate the cc command, we found that it was being executed from the /usr/bin path. However we also discovered, that it’s a symbolic link pointing to /etc/alternatives/cc. The directory /etc/alternatives is the place, where all the software alternatives are configured in Ubuntu/Debian. Let us inspect a little further.

We have another symbolic link, this time referring to /usr/bin/gcc and a little digging afterward reveals that this is a link to the real gcc executable. If alternative versions for a program are available, the alternatives system will create a link with the name of the program in the default path pointing to the appropriate file in /etc/alternatives. This will finally link to the executable, we actually want to use. Instead of manually manipulating these links, choosing a different default version for a program should be done using the command update-alternatives.

Using update-alternatives we can quickly figure out which alternatives are currently configured for a certain executable. Let us look at the current setup of cc.

As you can see the current link points to /usr/bin/gcc as we already discovered. There might be another alternative, with a different priority. The line 'gcc - auto mode’ means that the alternatives system will look for the package with the highest priority in order to use that executable.

The last line in the above output says that the current best priority link is /usr/bin/gcc. If we had installed an alternative for cc and would want to want to use it instead, we could tell update-alternatives to use this variant as follows:

A further useful feature of update-alternatives is the possibility of creating groups of files having a relation with each other. These so called slaves will not only allow you to update the link to the desired executable, but also any other information related to it like man pages, documentation, etc. as shown for the cc example above. Please consult man update-alternatives for more information on the capabilities of this powerful software versioning method.