What does the 'wildcard 4' refer to in the context of creating custom run levels?

The term 'wildcard 4' in the context of creating custom run levels refers to a run level designation number. Run levels in Linux are predefined states that represent the mode of operation of the operating system. Each run level is associated with a specific set of services and processes that are activated or deactivated when entering that run level.

In traditional SysV init systems, run levels are numbered from 0 to 6, with each number corresponding to a different operational state. For instance, run level 0 is used for shutting down the system, while run levels 1 through 5 are for different multi-user states, and run level 6 is for rebooting the system.

The term 'wildcard' implies that it can be used to target all processes associated with a specific run level. By using a designation number like '4', administrators can effectively customize or manage services tied to that run level for specific purposes, such as establishing development or test environments separate from standard operational modes.

This understanding of run level designation is crucial for effective system administration and configuration in Linux environments.