Example Lua script interfacing with systemd D-BUS service

ldbus_systemd.lua

-- nix-shell -p luajit luajitPackages.ldbus luajitPackages.inspect

local ldbus = require("ldbus")
local inspect = require("inspect")

local conn = assert(ldbus.bus.get("system"), "System bus connection failed")

function do_method_call(conn, destination, path, iface, method_name, args, f)
    local msg = assert(ldbus.message.new_method_call(destination, path, iface, method_name), "Failed to create message")
    local iter = ldbus.message.iter.new()
    msg:iter_init_append(iter)

    -- Pack arguments
    for idx, v in ipairs(args or {}) do
        assert(iter:append_basic(v), "Failed to append argument " .. tostring(idx))
    end

    local reply, err = conn:send_with_reply_and_block(msg)
    if err then
        error("Method call failed: " .. err)
    end

    assert(reply, "Nil method call response")
    assert(reply:iter_init(iter), "Message has no arguments")

    -- XXX: if msg or reply fall out of scope, we'll get segmentation fault sooner or later...
    return f(iter, msg, reply)
end


-- ListUnits(out a(ssssssouso) units);
local ret = do_method_call(conn, "org.freedesktop.systemd1" , "/org/freedesktop/systemd1", "org.freedesktop.systemd1.Manager" , "ListUnits", nil, function(iter, msg, reply)
    local array_iter = iter:recurse()
    local struct_names = {
        "primary_unit_name",
        "description",
        "load_state",
        "active_state",
        "sub_state",
        "follwing_unit",
        "unit_object_path",
        "queued_job_id",
        "job_type",
        "job_object_path",
    }

    local all_units = {}
    repeat
        assert(array_iter:get_arg_type() == ldbus.types.struct, "Expected structure")
        assert(array_iter:get_signature() == "(ssssssouso)", "Expected structure")
        local struct_iter = array_iter:recurse()

        local i = 1
        local obj = {}
        repeat
            local t = struct_iter:get_signature()
            local v = struct_iter:get_basic()

            local name = struct_names[i]
            obj[name] = v
            i = i + 1
        until not struct_iter:next()

        table.insert(all_units, obj)
    until not array_iter:next()

    return all_units
end)

print(inspect(ret))

-- RestartUnit(in s name, in s mode, out o job)
local ret2 = do_method_call(conn, "org.freedesktop.systemd1" , "/org/freedesktop/systemd1", "org.freedesktop.systemd1.Manager" , "RestartUnit", {"chronyd.service", "fail"}, function(iter, msg, reply)
    return "ok"
end)
print(inspect(ret2))

Self-note: os.execute({"systemctl", "restart", "chronyd.service"}) might be a better idea after all...