transmission/docs/Environment-Variables.md at bfd7797c5e8299893168da538f65072f47a8d777

Users can set environmental variables to override Transmission's default behavior and for debugging.

Transmission-Specific Variables

If TRANSMISSION_HOME is set, Transmission will look there for its settings instead of in the default location.
If TRANSMISSION_WEB_HOME is set, Transmission will look there for the Web Interface files, such as the javascript, html, and graphics files.
If TR_CURL_VERBOSE is set, debugging information for libcurl will be enabled. More information about libcurl's debugging mode is available here.
If TR_DEBUG_FD is set to an integer, that integer is treated as a file descriptor and very verbose debugging information is written to it. For example, here is how to turn on debugging and save it to a file named "runlog" when running Transmission from a bash shell:
```
$ export TR_DEBUG_FD=2
$ transmission 2>runlog
```
If TR_DHT_VERBOSE is set, then Transmission will log all of the DHT's activities in excrutiating detail to standard error.

If TRANSMISSION_WEB_HOME is not set, non-Mac platforms will look for the Web Interface files in XDG_DATA_HOME and in XDG_DATA_DIRS as described in the XDG Base Directory Specification. XDG_DATA_HOME has a default value of $HOME/.local/share/.
If TRANSMISSION_HOME is not set, Unix-based versions of Transmission will look for their settings in $XDG_CONFIG_HOME/transmission/. XDG_CONFIG_HOME has a default value of $HOME/.config/.
If HOME is set, it's used in three ways:
1. by the XDG variables, as described above
2. If TRANSMISSION_HOME is not set, Mac-based versions of Transmission will look for their settings in $HOME/Library/Application Support/Transmission
3. $HOME/Downloads is the default download directory.

Transmission uses the libcurl library for http- and https-based tracker announces and scrapes. Transmission doesn't support proxies, but libcurl itself honors a handful of environment variables to customize its proxy behavior.