sudo transmission-daemon --config-dir /folder/I/want
doesn't alter any permanent configuration. It runs
transmission-daemon and only this instance uses the directory you specified. Note the program places itself in the background, so it seems excessive to run
sudo service transmission-daemon start just after.
Although the documentation says that
$HOME/.config/transmission-daemon is the default location for daemon settings, it also says:
Some Linux distributions' start script for
transmission-daemon use different location.
/etc/default/transmission-daemon is the right place to modify
--config-dir permanently. Relevant lines tailored to your request are:
Note: the syntax is simple and straightforward; I expect it to fail if the path contains spaces. To use any valid path (with spaces etc.) one may fix it somehow (obligatorily with
/etc/init.d/transmission-daemon or so) but this would be against the KISS principle. You can simply perform a trick I present down below.
An alternative permanent approach is to make your default config path a symlink to the desired real location:
sudo mv /etc/transmission-daemon/ /etc/transmission-daemon.old/
sudo ln -s "/folder/I/want" /etc/transmission-daemon
I tend to think that modifying config files is the right approach in general, the way it's meant to be, something by design; while a symlink may go as a cheap trick. This trick, however, can be very powerful and foolproof. As you can see it can compensate for some limitations of beautifully simple config syntax; or it can save you from learning some awfully complex config syntax, if you ever encounter one.