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I'm trying to interact with the Notification service through DBus in getting it to send a notification by manually sending in messages to its destination. I will make a call to the Notify method of the interface org.freedesktop.Notifications which has the following signature:

.Notify                             method    susssasa{sv}i u            -

I'm stuck on the part where I have to provide array of type string and variant. But according to the manpage of dbus-send

           <contents>   ::= <item> | <container> [ <item> | <container>...]
           <item>       ::= <type>:<value>
           <container>  ::= <array> | <dict> | <variant>
           <array>      ::= array:<type>:<value>[,<value>...]
           <dict>       ::= dict:<type>:<type>:<key>,<value>[,<key>,<value>...]
           <variant>    ::= variant:<type>:<value>
           <type>       ::= string | int16 | uint16 | int32 | uint32 | int64 | uint64 | double | byte | boolean | objpath

<array> accepts a <type> and not a <container> to which <variant> belongs. So I get an error.

$ dbus-send --session --print-reply --dest=org.freedesktop.Notifications /org/freedesktop/Notifications org.freedesktop.Notifications.Notify \
string:'me' \
uint32:0 \
string:"hello world" \
string:"" \
string:"" \
dict:string:variant:'urgency',byte:1 

dbus-send: Unknown type "variant"

What is the syntax for signature of a{sv}?

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The syntax is indeed dict:string:variant:'urgency',byte:1, but it is only implemented in 'dbus' 1.15.0 or later, which is currently the "development branch" of 'dbus' and not yet found in any distribution.

I would instead recommend either busctl (systemd) or gdbus (GLib2):

busctl call --user org.freedesktop.Notifications \
                   /org/freedesktop/Notifications \
                   org.freedesktop.Notifications Notify \
                   susssasa{sv}i \
                   "me" \
                   0 \
                   "hello world" \
                   "" \
                   "" \
                   0 \
                   1 "urgency" y 1 \
                   0

In busctl the signature is specified upfront (like it would be in the actual D-Bus message on the wire) with all parameters following, e.g. ssu "hello" "world" 123; only variant parameters are immediately preceded by their actual type. Since there are no separators, an array or a dict is specified with its length first, followed by its values or key/value pairs.

gdbus call -e -d org.freedesktop.Notifications \
              -o /org/freedesktop/Notifications \
              -m org.freedesktop.Notifications.Notify \
              "me" \
              0 \
              "hello world" \
              "" \
              "" \
              "[]" \
              "{'urgency': <byte 1>}" \
              0

gdbus generally expects GNOME's GVariant text format syntax, although as a shortcut it will auto-detect the method signature and will accept bare strings – but only if they're a stand-alone parameter, while in other cases strings need to be quoted inside the argument, e.g. "'hello'" for a string parameter (or even as "@s 'hello'" if you want to be really explicit) or "<'hello'>" for a variant(string). Within a dict it would look like "{'key': <'value'>}".

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