Final answer:
Run-level 8 is not valid in the context of Unix and Linux operating systems. Standard run-levels are usually from 0 to 6, including S or s for single-user mode, each defining a different state in which the system operates.
Step-by-step explanation:
The run level that is not valid among the given options is 8. In the context of Unix and Linux operating systems, run-levels are defined states that the system's software operates in. Here's a quick rundown of standard run-levels:
- S or 's' - Single-user mode, used for maintenance or emergency repairs.
- 0 - Halt the system.
Run-level 8 is not a standard run-level in Unix or Linux systems. Standard run-levels are typically from 0 to 6, including S or S for single-user mode. Some systems might have additional run-levels, but they are usually within the range of 0 to 6, with 7 not typically being used. Each run-level represents a particular state of the machine that defines which processes and services are active.