Editing Software Development Platforms (section)

==A Discussion of Directories==

It's generally not a good idea to mess with the directories that "belong" to OS-X.  This includes /usr .  Installing stuff to /usr is generally a very bad idea, since you might clobber system files.

Instead, it is conventional to install unix-style software into /usr/local .  Thus for example, the binaries get installed into /usr/local/bin, the libraries into /usr/local/lib, and so on.  This is acceptable, but has some drawbacks.  The biggest one is that typically it is impossible to install multiple versions of the same sotware simultaneously.  Another is that it can be hard to uninstall particular software, since the files get spread around all the subdirectories of /usr/local .

An alternative to using /usr/local for everything is to install particular packages into their own dedicated directores.  Conventionally, these are subdirectories of /opt .  So for example, we could install VTK 5.6.1 into /opt/vtk-5.6.1 .  Then the binaries would end up in /opt/vtk-5.6.1/bin, and the libraries in /opt/vtk-5.6.1/lib, and so on.  Now we can have multiple versions, and uninstallation is easy - just delete the directory.  One drawback is that, unlike /usr/local/bin and /usr/local/lib, these won't be on your paths by default, so they can't be found.  Generally we need to do something like this:

 export PATH=/opt/vtk-5.6.1/bin:"$PATH"
 export DYLD_LIBRARY_PATH=/opt/vtk-5.6.1/lib:"$DYLD_LIBRARY_PATH"

These lines can go in .bash_profile for a permanent change, or, you can put them into another file, say setvars.sh, and just do this to run it and import the environment variables:

 source setvars.sh

One tip is that you can create links in opt to your favorite versions, like this:

 cd /opt
 ln -s vtk-5.6.1 vtk

Then you don't have to change your .bash_profile or setvars.sh file when you want to switch to a different version.  You simply change the symbolic links in /opt .  The export lines become simply

 export PATH=/opt/vtk/bin:"$PATH"
 export DYLD_LIBRARY_PATH=/opt/vtk/lib:"$DYLD_LIBRARY_PATH"

I will generally assume that this has been done.

There is one additional "standard" directory, and that is /opt/local .  It belongs to macports, so it's best not to mess with it.