I want to send DBus messages to some applications (Psi) by calling command-line commands on the following moments:

• desktop lock (by hand or by idle time),
• entering stand-by mode (suspend to ram),
• entering hibernate mode (suspend to disk),
• waking up from suspend.

How to create such triggers?

PowerDevil appears to have some DBus signals which may be related.

qdbus org.kde.powerdevil /modules/powerdevil.

Note that wake up from suspend is likely to be unavailable in any case, since Plasma would update its clock on resume if this were possible.

bcooksley wrote:PowerDevil appears to have some DBus signals which may be related.

Does console version of qdbus work with signals? Or I can only call methods?

And... PowerDevil works only with laptops. The signal stateChange concerns to plugging notebook to electric socket or leaving it on battery.

But I need signals emitted on screen lock and suspending.

Unfortunate. I suggest you ask the PowerDevil maintainer to add the signals you need, or a capability to run scripts in the user environment before performing the task. This should be possible for all but resuming from suspend though.

The console version of QDBus cannot work with signals unfortunately.