Which command is used to automatically start designated units at system boot time?

The command used to automatically start designated units at system boot time is "systemctl enable." When this command is executed, it creates the necessary symbolic links in the appropriate directories to ensure that the specified unit, such as a service, is started automatically when the system boots. This is essential for services that need to run consistently without manual intervention every time the system is restarted.

Using "systemctl enable" is fundamental for system administration, as it allows for the management of automatic service startups, making sure everything necessary for the operation of the system is available right from the beginning. It streamlines the boot process and improves system reliability, as critical services can be set to start as part of the boot sequence.

The other commands serve different purposes. "systemctl stop" is used to immediately stop an active service, while "systemctl disable" prevents a unit from starting automatically at boot time. "systemctl mask" completely disables a unit from being started, either manually or automatically, which is more extreme than just disabling it. Therefore, "systemctl enable" is specifically focused on setting units to start at boot, making it the correct choice in this context.