For example: whereas System V has the runlevel 0, systemd defines the poweroff target. They are similar and different at the same time: a target associates a name to a set of services (that is: systemd units) that should be activated/deactivated when that target is selected. Systemd uses the concept of targets instead of runlevels. Now, what are the runlevels for Debian? Good question: runlevels are a System V concept, while Debian has switched from System V to systemd years ago. Read the wikipedia article linked above if you want to know more. Note: there is also a runlevel “S”, but we don’t talk about that here. 5: Start the system normally with appropriate display manager (with GUI).As you can imagine, not only they are used differently across different flavours of UNIX: they are used differently even across different distributions of Linux! But at least in the Linux case there is a standard: the Linux Standard Base defines the following specification for runlevels: Runlevels 2 to 5 are available for different modes of operations that are usually defined by those who designed the system. Runlevels 0, 1 and 6 are pretty standard: the system enters runlevel 0 when it’s being halted, 6 when it’s rebooted, and 1 when it goes into single-user mode (known also as “rescue mode” in Debian). Runlevels are then different states of a UNIX system, and only one is active at any given time. Roughly, a runlevel associates a number (what we humans call “the runlevel”) to the services that should be activated and deactivated when the system enters that state. Luckily, in UNIX systems there is the concept of runlevels. The system booted well, but after I typed the root password the boot process went a bit too far, loaded the infamous leftovers of the driver and here we go again, with a blank screen. The first attempt to get out of the mud was to force a reboot of the system and in rescue mode. But unfortunately the only way to log in was from the console, so I was “de facto” locked out. The fix is easy, if you can enter the system in some other way: log in and remove anything related to the Nvidia driver. The system booted fine and was functional, but those leftovers where enough to make the screen go blank. I had rebooted the laptop and there were some leftovers around from an attempted installation of the proprietary Nvidia driver. Recently, while testing a configuration of Linux on a Lenovo laptop, I messed up.
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