[Problem]
The --schemadir option of gsettings command has no effect on list-schemas, list-relocatable-schemas, or etc.

"gsettings help list-schemas" says that the --schemadir option may be given. It seems, however, that an additional schema directory is not respected. g_settings_list_schemas() internally initialize the schema sources. So, it returns only the schemas installed on the system.

gio/gsettings-tool.c:
151 static void
152 gsettings_list_schemas (GSettings   *settings,
153                         const gchar *key,
154                         const gchar *value)
155 {   
156   output_list (g_settings_list_schemas ());
157 } 


On the other hand, an additional directory is respected with list-keys, get, set, etc..


[How to Reproduce]
$ pwd
/tmp
$ echo $XDG_DATA_DIRS
/usr/share/gnome:/usr/local/share/:/usr/share/
$ cat org.example.additional.gschema.xml
<?xml version="1.0" encoding="UTF-8"?>
<schemalist>
  <schema path="/org/example/additional/" id="org.example.additional">
    <key type="b" name="dummy-key">
      <default>true</default>
      <summary>Dummy key</summary>
      <description>Dummy key in additional schema</description>
    </key>
  </schema>
</schemalist>
$ glib-compile-schemas .
$ gsettings list-keys org.example.additional
No such schema 'org.example.additional'
$ gsettings --schemadir . list-keys org.example.additional
dummy-key
$ gsettings --schemadir . list-schemas | grep org.example.additional
$ echo $?
1
$ exit