Systemctl: find out which commands are applicable for a service

How do I find out which commands are applicable for a service with systemctl?

# systemctl reload nagios.service
failed to reload nagios.service: Job type reload is not applicable for unit nagios.service.

With init.d you could run the script without command and see the usage info for applicable commands:

# /etc/init.d/nagios 
Usage: /etc/init.d/nagios {start|stop|status|restart|condrestart|try-restart|reload|force-reload|configtest}

Solution 1:

systemctl is not like the good old init-scripts. According to its man page, it supports the following commands that are like the init-script commands you are searching for:

• start Start (activate) one or more units specified on the command line.
• stop Stop (deactivate) one or more units specified on the command line.
• reload Asks all units listed on the command line to reload their configuration.
• restart Restart one or more units specified on the command line. If the units are not running yet, they will be started.
• try-restart Restart one or more units specified on the command line if the units are running. This does nothing if units are not running. Note that, for compatibility with Red Hat init scripts, condrestart is equivalent to this command.
• reload-or-restart Reload one or more units if they support it. If not, restart them instead. If the units are not running yet, they will be started.
• reload-or-try-restart Reload one or more units if they support it. If not, restart them instead. This does nothing if the units are not running. Note that, for compatibility with SysV init scripts, force-reload is equivalent to this command.

So the group of these commands essentially boils down to start, stop and reload. Since start and stop are necessary for most service types (and you normally know if a service does not support one of them), the only thing you could want to know about is if a given service supports reload or not.

Most times, you want it to either reload or restart: use systemctl reload-or-restart nagios then. To really find out what command (if any) is executed for reloading, you can look into its service file. This can usually be found in /lib/systemd/system/ or /usr/lib/systemd/system and is named $SERVICENAME.service (in your case nagios.service). This file contains commands for starting, stopping and maybe reloading the service, namely ExecStart, ExecStop and ExecReload. If you're interested in that, you could do a grep ExecReload /lib/systemd/system/nagios.service to find out if a service supports reloading or not.

TL;DR

systemctl essentially supports start, stop and reload. To find out if a Service supports reloading, just try it. If you need to reload multiple services in a script or something, use reload-or-restart, which reloads the service if it has an ExecReload-command stored, otherwise it restarts it.