File system 'overlays'

Question

Concrete use case: In the Eclipse IDE, new 'plugins' can be added by copying a plugin's file(s) into the $ECLIPSE_HOME/plugins directory. However, I want to keep my original Eclipse installation 'clean' without additional plugins because I want to run this basic installation on its own at times.

What is a way of avoiding having to copy the files (and hence therefore not being able to run a clean version) and instead logically 'overlaying' the contents of another directory so that it appears to be in the directory at runtime?

e.g. something like:

gravelld@gravelld-laptop:~$ ls $ECLIPSE_HOME/plugins/
org.junit_3.8.2.v200706111738
org.junit4_4.3.1
org.junit.source_3.8.2.v200706111738
gravelld@gravelld-laptop:~$ ls myplugins/
org.dangravell.myplugin.jar
gravelld@gravelld-laptop:~$ overlay myplugins/ $ECLIPSE_HOME/plugins
gravelld@gravelld-laptop:~$ ls $ECLIPSE_HOME/plugins/
org.dangravell.myplugin.jar
org.junit_3.8.2.v200706111738
org.junit4_4.3.1
org.junit.source_3.8.2.v200706111738

Another use case may be around patching and so on...

Can something be done with symbolic links or mnt for this?

Thanks!

Answer 1

Have a look at Manage your eclipse environment article, especially the Method 3

Creating a links folder to manage product extensions

If you have product extensions sitting on your file system, like the one we made in Method 1, you can create a few simple files in your Eclipse program directory to notify Eclipse that it needs to check these directories for plug-ins.

First, create a directory inside your Eclipse installation folder (for example, /opt/eclipse) called links. Within this folder, you can create *.link files (for example, emfPlugins.link). Each link file points to a product extension location. Eclipse will scan this links folder on startup and find the plug-ins in each product extension pointed to by a link file.

This is still supported in eclipse3.4 even though the new p2 provisioning system is quite different.

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Now that the "'links' directory mechanism" is known, it means the difference between a vanilla eclipse and an eclipse with custom common plugins is just the presence of that 'links' directory.

So, why not have a 'vanilla eclipse distribution' with a symbolic link inside, 'links', pointing to ../links ?

Any user getting that vanilla eclipse would have at first no 'links' directory alongside it, so it will run as a vanilla distribution. But as soon the user creates a links directory or make another symbolic link to a common remote 'links' directory, that same distribution will pick up the common plugins remote directory...

/path/links -> /remote/links/commonPlugins
/eclipse/links -> ../links

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Finally, if you create the "/remote/links/commonPlugins" with a given group "aGroup", and protect it with a '750' mask, you have yourself one eclipse setup which will be:

• vanilla eclipse for any user whom 'id -a' does not include 'aGroup'
• eclipse with plugins for any user part of "aGroup"

Answer 2

You could use an overlay filesystem for this. The three overlay filesystems that I know of in Linux are unionfs, aufs, and minifo.

Unionfs is included in recent Ubuntu kernels.