what is update-alternatives command in linux and what is the use of it?

while installation of java in Linux there is some usage of update-alternatives command since i am new to Linux environment i want to know what this command does and what is the use of it

>>sudo update-alternatives --install /usr/bin/java java /usr/lib/java/JDk.../bin/java

2 Answers

Basically it says to your machine to use this alternative of Jave JDK instead of the default one, which, in Linux systems, is OpenJDK.

A brief extract from the man page is better than any answer I could write:

update-alternatives creates, removes, maintains and displays information about the symbolic links comprising the Debian alternatives system.

It is possible for several programs fulfilling the same or similar functions to be installed on a single system at the same time. For example, many systems have several text editors installed at once. This gives choice to the users of a system, allowing each to use a different editor, if desired, but makes it difficult for a program to make a good choice for an editor to invoke if the user has not specified a particular preference.

Debian's alternatives system aims to solve this problem. A generic name in the filesystem is shared by all files providing interchangeable functionality. The alternatives system and the system administrator together determine which actual file is referenced by this generic name. For example, if the text editors ed(1) and nvi(1) are both installed on the system, the alternatives system will cause the generic name /usr/bin/editor to refer to /usr/bin/nvi by default. The system administrator can override this and cause it to refer to /usr/bin/ed instead, and the alternatives system will not alter this setting until explicitly requested to do so.

With --install you specified a link, "/usr/bin/java" a name "java" and a path "/usr/lib/java/JDK...." and you add a group of alternatives to the system. link is the generic name for the master link, name is the name of its symlink in the alternatives directory, and path is the alternative being introduced for the master link.

I hope to be clear enough, Here there is a post regarding java alternatives.

For the complete usage list I suggest to look at the same manual page, typing man update-alternatives on your OS shell;

i want to explain it for you from scratch ... if you have 2 or more versions of a program that starts with a same command in linux you can use update-alternatives command for determining the default version of that program and you can also change the default version of that command easily .. assume that you have 2 version of python on your system. python 2 & python 3. by default python command on linux starts the python2 interpreter on the shell . you think that python 2 is old and you want to use python 3 instead of python2 . one of the solution is that you enter python3 command on the shell and start the python interpreter that its version is more than 3 ... but you want to enter python command and shell recognize that you want to run python3 interpreter . here,update-alternatives command do it's work and determine the default version and versions of a program you want to use for a specific command .

i told you that python command start python 2 interpreter by default . you want to change it . follow the example :

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 1.

when you execute this command on linux you are telling to shell that after this time two links of two different programs are linked to python command on linux .. infact you install the python3 program on python command on linux and two programs exist at 1 command that you can change the defaults of these programs and run what of these programs you want by changing priority in command or configure it manually to set the default program ..

hope this is useful for you !