%general-entities; ]> The installation of Qt presents several challenges and choices. Complicating the Qt installation is the fact that there are two versions, Qt4 and Qt5, that have executable programs with identical names. If both packages are installed on one system, the only methods to manage which set of programs is used is to either control the users' PATH or to rename files either directly or via symbolic links. Both packages cannot be installed in the same directory. If you are only going to install one of the Qt versions, the choice of installation methods is easier. You can either install the components in the several directories of the /usr hierarchy or install the entire package in a separate directory of your choice. We refer to these options below as "Method 1: Installing in /usr" and "Method 2: Installing in /opt". If both versions of Qt are to be installed, one or both versions need to be installed in a separate directory. For the purposes here we use the /opt directory. The selection of which version is being used by individual users on a system is controlled by the PATH variable. Other systems can be designed, but the BLFS editors find the PATH method easiest. The advantage of installing in /usr is that no updates to the /etc/ld.so.conf or /etc/man_db.conf files are required. The package files are distributed within several subdirectories of the /usr hierarchy. This is the method that most commercial distributions use. The disadvantage for BLFS users is that this Qt instance cannot be upgraded while it is in use. For instance, it cannot be upgraded from a running KDE environment. It also precludes having multiple versions of Qt on your system and does not allow reverting to an existing, known working instance of Qt. The advantage of installing Qt in a custom directory such as /opt/qt-&qt4-version; or /opt/qt-&qt5-version; or is that it keeps all the package files consolidated in a dedicated directory hierarchy. By using this method, an update can be made without overwriting a previous installation and users can easily revert to a previous version by changing one symbolic link or merely changing the PATH variable. It also allows a developer to maintain multiple versions of Qt4 or Qt5 for testing.