Back to the posts

How to correctly manage different versions in Ubuntu

For many programs, it's necessary to install multi versions such as gcc4.8, gcc6, and gcc7 because it's possible that some projects need a different version of the compiler. Even JVM languages, I sometimes need to use Java 8 or Java 9 depending on the project.

What is /etc/alternatives/ ?

So, what is a correct way of managing these different versioned programs?

Thankfully, Ubuntu makes it easy to manage different versions using /etc/alternatives.

ls -al /etc/alternatives/ | head
total 184
drwxr-xr-x   2 root root 24576 Mar 13 11:40 .
drwxr-xr-x 177 root root 12288 Mar 11 23:47 ..
lrwxrwxrwx   1 root root    45 Apr 26  2017 ABORT.7.gz -> /usr/share/postgresql/9.5/man/man7/ABORT.7.gz
lrwxrwxrwx   1 root root    21 Mar 14  2017 aclocal -> /usr/bin/aclocal-1.15
lrwxrwxrwx   1 root root    37 Mar 14  2017 aclocal.1.gz -> /usr/share/man/man1/aclocal-1.15.1.gz
lrwxrwxrwx   1 root root    55 Apr 26  2017 ALTER_AGGREGATE.7.gz -> /usr/share/postgresql/9.5/man/man7/ALTER_AGGREGATE.7.gz
lrwxrwxrwx   1 root root    55 Apr 26  2017 ALTER_COLLATION.7.gz -> /usr/share/postgresql/9.5/man/man7/ALTER_COLLATION.7.gz
lrwxrwxrwx   1 root root    56 Apr 26  2017 ALTER_CONVERSION.7.gz -> /usr/share/postgresql/9.5/man/man7/ALTER_CONVERSION.7.gz
lrwxrwxrwx   1 root root    54 Apr 26  2017 ALTER_DATABASE.7.gz -> /usr/share/postgresql/9.5/man/man7/ALTER_DATABASE.7.gz

As you can see /etc/alternatives/ is a directory that contains symbolic links to a specific version of the program.

For example,

ls -al /etc/alternatives/clang
lrwxrwxrwx 1 root root 16 Mar 13 11:22 /etc/alternatives/clang -> /usr/bin/clang-7

You can see that my clang is pointing clang-7.0. So, you just have to create a symbolic link inside alternatives directory, and you will always use the right version.

How to update /etc/alternatives/?

Ubuntu also comes with update-alternatives which is a helper utility to manage alternatives symlinks.

How to set a different version?

update-alternatives --config XXX is the basic syntax. For example, if I want to use a different version of gcc, I will do

update-alternatives --config gcc
There are 4 choices for the alternative gcc (providing /usr/bin/gcc).

  Selection    Path              Priority   Status
------------------------------------------------------------
* 0            /usr/bin/gcc-7     50        auto mode
  1            /usr/bin/gcc-4.8   10        manual mode
  2            /usr/bin/gcc-5     20        manual mode
  3            /usr/bin/gcc-6     30        manual mode
  4            /usr/bin/gcc-7     50        manual mode

Press <enter> to keep the current choice[*], or type selection number:

How to add a different version?

If you install with apt-get, it's likely you don't need to do anything. However, if you compile from source codes, this has to be done manually.

It can be done easily by typing update-alternatives --install LINK NAME PATH PRIORITY.

  • LINK: means the default bin path. For example, /usr/bin/java, /usr/bin/gcc
  • NAME: refers to the name of program. It's usually the name of a bin file. e.g., gcc, clang, java.
  • PATH: means a path to the actual version of the program you wish to register. This will be usually a bin file with version name. For example, /usr/bin/gcc-6, /usr/bin/gcc-4.8.
  • PRIORITY: is an integer indicating how to choose a version when there are multiple versions and users did not set a specific version. The highest number will be chosen.

So, it will look like this

sudo update-alternatives --install /usr/bin/java java /usr/local/java/jre1.7.0_09/bin/java 1

In summary,

Choose a different version

sudo update-alternatives --config NAME

Install a new version

sudo update-alternatives --install LINK NAME PATH PRIORITY

© 2021 Mo Kweon. All rights reserved.