What command is used to start designated units?

The command used to start designated units is "systemctl start." This command is part of the systemd suite, which is the initialization system used in many Linux distributions to manage services and other system resources. When you issue the "start" command, systemd will activate the specified service unit, allowing it to run.

This command does not affect the units themselves in terms of enabling them to start at boot time; it simply starts them immediately while the system is running. For instance, if a web server service is currently stopped and you wish to start it, using "systemctl start [service_name]" will begin its execution.

The context of the other choices inform their functions: “systemctl restart” would not just start a unit, but also stop it if it was running before starting it again; “systemctl stop” is used to halt an active unit; and “systemctl enable” is intended to modify the unit's settings to start automatically at boot time, rather than starting it on command.