While running GNU/Linux, it is common to find different versions of the same software residing on your hard disk. This is especially true for programming language compilers. For example, Java for Linux comes in different forms. One is the open source version which is popularly known as the Blackdown java and the other is the official release from Sun Microsystems which is the original Java. Then there is the GCJ which is a GNU compiler for Java. Many times users have more than one version of Java installed and it becomes necessary to let the system know which Java executable is favoured by the user in order to avoid ambiguity in command execution. This is just one example. In fact it need not be just related to programming, it can also be related to say, mail transport agents - like suppose you have two mail transport agents sendmail and postfix installed on your machine and you want to easily choose one from the other as the default MTA.
GNU/Linux has built-in a good functionality to sort out this issue to the end users tastes. In an earlier post, I had covered in a concise way how it is accomplished in Red Hat and Fedora. Debian (based) distributions also have a similar tool in 'update-alternatives' which can be used to easily make a program the default one over any other similar programs.
Steven Kroon has writen a very good article which explains in detail by aid of examples, the usage of update-alternatives which makes a very informative read.
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