All roads lead to Rome, but PulseAudio is not far behind! In fact, how the PulseAudio client library determines how to try to connect to the PulseAudio server has no less than 13 different steps. Here they are, in priority order:
2) If the PULSE_SERVER environment variable is set, that’s the server string used, and nothing else.
3) Next, it goes to X to check if there is an x11 property named PULSE_SERVER. If there is, that’s the server string, nothing else. (There is also a PulseAudio module called module-x11-publish that sets this property. It is loaded by the start-pulseaudio-x11 script.)
4) It also checks client.conf, if such a file is found, for the default-server key. If that’s present, that’s the server string.
So, if none of the four methods above gives any result, several items will be merged and tried in order.
First up is trying to connect to a user-level PulseAudio, which means finding the right path where the UNIX socket exists. That in turn has several steps, in priority order:
5) If the PULSE_RUNTIME_PATH environment variable is set, that’s the path.
6) Otherwise, if the XDG_RUNTIME_DIR environment variable is set, the path is the “pulse” subdirectory below the directory specified in XDG_RUNTIME_DIR.
7) If not, and the “.pulse” directory exists in the current user’s home directory, that’s the path. (This is for historical reasons – a few years ago PulseAudio switched from “.pulse” to using XDG compliant directories, but ignoring “.pulse” would throw away some settings on upgrade.)
8) Failing that, if XDG_CONFIG_HOME environment variable is set, the path is the “pulse” subdirectory to the directory specified in XDG_CONFIG_HOME.
9) Still no path? Then fall back to using the “.config/pulse” subdirectory below the current user’s home directory.
Okay, so maybe we can connect to the UNIX socket inside that user-level PulseAudio path. But if it does not work, there are still a few more things to try:
10) Using a path of a system-level PulseAudio server. This directory is /var/run/pulse on Ubuntu (and probably most other distributions), or /usr/local/var/run/pulse in case you compiled PulseAudio from source yourself.
11) By checking client.conf for the key “auto-connect-localhost”. If so, also try connecting to tcp4:127.0.0.1…
12) …and tcp6:[::1], too. Of course we cannot leave IPv6-only systems behind.
13) As the last straw of hope, the library checks client.conf for the key “auto-connect-display”. If it’s set, it checks the DISPLAY environment variable, and if it finds a hostname (i e, something before the “:”), then that host will be tried too.
To summarise, first the client library checks for a server string in step 1-4, if there is none, it makes a server string – out of one item from steps 5-9, and then up to four more items from steps 10-13.
And that’s all. If you ever want to customize how you connect to a PulseAudio server, you have a smorgasbord of options to choose from!