I install qbittorrent using apt-get on my Raspberry Pi running Raspbian stretch. Because the version is old (v3.x). I followed the guide at https://github.com/qbittorrent/qBitt...ian-and-Ubuntu to install v4.x. One thing I have to do is to update LD_LIBRARY_PATH in order for qbittorrent to find libtorrent which I don't have to do when I used apt-get. How does the apt version find the libtorrent it need?

If you install something using apt you are actually installing a deb file. The deb file was probably created using a tool like dpkg-buildpackage or builddeb. The tools make sure files and other stuff are installed in a "Debian standard location" and in theory they should take care of all the details of ensuring the package works after being installed.

I haven't looked at the qbittorrent code but often code compiled from source will install stuff to a location like /usr/local or some other location that is not a debian standard location and you may have problems finding the shared libraries and other files and so on after installation.

I know from packaging deb files for Bodhi linux that even if one compiles code with the option: --prefix=/usr this does not guarantee all the files will end up in the same location as the same package installed via compiling. Enlightenment any version as well as Moksha (e17 fork) is a good example of this. Compiling these on top of a version installed from a deb file BREAKS things as a result of this "file confusion."

One more thing you may need to run the command sudo ldconfig if you install by source. Deb files will do this automatically for you if needed.

Thanks. I did run ldconfig as part of the build process.
I found qbittorrent installed with apt-get under /usr/bin but I can't find libtorrent under /usr/lib . Is /usr/lib the default or the default lib directory is somewhere else?

Debian often places stuff like that in

/usr/lib/i386-linux-gnu/
/usr/lib/x86_64-linux-gnu/
/usr/lib/aarch64-linux-gnu/
/usr/lib/arm-linux-gnueabihf/

Depending on your system

Thanks. Found it under /usr/lib/arm-linux-gnueabihf/

i've run raspbian after having run debian sid for some time.

it is based on debian. but it is quite different.

you should probably also state which pi it is.

I thought Raspbian Stretch runs on every version of Raspberry Pi incoulding Pi Zero, no?

"Question about differences between apt and compile my own"
i feel that one important point has not been addressed yet:
if you install any software by compiling it yourself & using the install command, you are circumventing the system's package management.
this nedd not be a problem, but you should consider carefully whether you want (need) to do that. you should also keep in mind (or on a list) the applications you installed that way, because it might help to fix weird errors later on.

the package management is the single biggest piece of magic on any gnu/linux system, and on debian it is 'apt'.

For the sake of discussion, what option do I have if I want qbittorrent 4.x but the package maintainer does not update it for stretch? qbittorrent 4 has been built for Debian Buster but not available on Stretch.
https://packages.debian.org/search?k...ll&section=all

https://wiki.debian.org/DontBreakDeb...Stuff_Syndrome
so, for the sake of discussion, the question is:
do you have any real reason to need that newer version?
debian stable is safe as it is, there's no "newer is better" here.