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Cover image for How to manage versions using update-alternatives
webduvet
webduvet

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How to manage versions using update-alternatives

Background

Recently I needed to run the end to end tests in the older chrome browser as the tool the chrome driver available to me was only working with the older version of chrome.
The solution instead of downgrading my chrome installation was to install multiple versions of chrome-browser. More on this in my recent article how and where to get older versions of google-chrome.
Long story short, I would like to be able to hotswap the chrome version. Any python, node or java developer most likely uses some type or form of version manager. In java world people might be using sdkman [www.sdkman.org] or node js world us used nvm www.getnvm.org.

What is update-alternatives

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

It is a tool provided by Debian based distributions which is designed to help to manage multiple versions or different implementations of a command or program, read more on Debian Alternatives. Update-alternatives in Debian based system like Ubuntu is used as default option by apt, however, it offers the command-line interface to interact with the settings and to add and remove entries manually.

It can also provide functionality to update something what is called slave links to cater for associated software or manual pages which also need to change when the main target changes.
The analogy in the nodejs world would be corresponding npm installation or in python world it would be corresponding pip.

How to install another version with update-alternatives

if you plan to use update-alternavives to e.g. install the multiple node versions you will download the desired version and unpack it in the directory of the choice In my case I need older version of google chrome so it would be /opt/google/chromium/1027016/chrome-linux/.

This will contain binary /opt/google/chromium/1027016/chrome-linux/chrome

if you already have chrome installed and you did that from the Debian package by

sudo apt install google-chrome-stable.deb
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your already have used update-alternatives and running

update-alternatives --display google-chrome
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will show output:

google-chrome - auto mode
link best version is /usr/bin/google-chrome-stable
link currently points to /usr/bin/google-chrome-stable
link google-chrome is /usr/bin/google-chrome
/usr/bin/google-chrome-stable - priority 200
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This indicates that there is only one version of google-chrome and it shows the location and priority for auto mode (the higher version takes precedence).
I don't want to go into too much detail here as it could be found in man pages or online debian alternatives.

Running the following lists the existing links stored by update-alternatives.

update-alternatives --list google-chrome

?> /usr/bin/google-chrome-stable
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We can see also here that there is only one. If we follow the symbolic links and investigate further we get to:

/usr/bin/google-chrome --> /etc/alternatives/google-chrome
/etc/alternatives/google-chrome --> /usr/bin/google-chrome-stable
/usr/bin/google-chrome-stable --> /opt/google/chrome/google-chrome
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The last one is hard link to executable file and paint a pretty clear picture how the update-alternatives work. /usr/bin/google-chrome-stable is an executable and /usr/bin/google-chrome is a symbolic link to the executable so we can just type google-chrome and it starts the browser. Update-alternatives sticks in an extra layer of symbolic links in the middle of which it has the full control.

To add another version to it execute the following

sudo update-alternatives --install /usr/bin/google-chrome google-chrome /opt/google/chromium/1027016/chrome-linux/chrome
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if you want to use this as default link in auto mode you can add the priority number at the end, higher number takes precedence. For manual intervention enter the following command:

sudo update-alternatives --config google-chrome
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output (depending on given priority nubmer this might differ):

There are 2 choices for the alternative google-chrome (providing /usr/bin/google-chrome).

Selection    Path                                              Priority   Status
------------------------------------------------------------
  0            /opt/google/chromium/1027016/chrome-linux/chrome   300       auto mode
  1            /opt/google/chromium/1027016/chrome-linux/chrome   300       manual mode
* 2            /usr/bin/google-chrome-stable                      200       manual mode

Press <enter> to keep the current choice[*], or type selection number:
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if you choose one option the link will no longer be in auto mode, but you can return anytime to automode by running

sudo update-alternative --auto google-chrome
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slave links

As mentioned before, these are associated links which should change when master link changes. The examples are node and npm or python and pip. It also works with manual pages as some packages install also manual pages. This is also the case when installing google-chrome as deb package as above.
Version managers not based on update-alternatives usually add/edit the entry in manpath pointing to the correct man page or edit (add) entry in $PATH environment variable.
update-alternatives does this via mentioned slave links.

In case of google-chrome, even though it uses update-alternatives by default, it does feature the manual entry, but it does not use slave links and the installer simply copies the man page in the man location. Which prevents update-alternatives to apply sim link here. However, even though we can't do anything with the hard link we can add another manual entry just for the specific version. I'm install chromium so I would be interested to have chromium manual entry

manual entry for google chrome can be found here:

man -w google-chrome
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We could manually move man entry for google-chrome-stable elsewhere and let update-alternatives utilize slave links, but we could also add another man entry via slave link to demonstrate flexibility. Let's do it.

sudo update-alternatives --install /usr/bin/google-chrome google-chrome /opt/google/chromium/chrome-linux/chrome \
--slave /usr/share/man/man1/chromium.1.gz chromium.1.gz /opt/google/chromium/chromium.1.gz
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The result of this will be following. If we use the google-chrome-stable the manual entry for chromium via man chromium will not be available, but swapping the version for chromium will make the manual entry for chromium available along the original manual entry which is/was set as hard link (actual file).

Summary

This short articles demonstrate the basic capabilities of the tool update-alternatives. More detail can be found on manual pages or on Debian website.

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