How to detect whether a physical cable is connected to network card slot on Linux - LinuxConfig.org

Various tools can be used to detected a physical cable carrier state. However, the easiest accomplish this task is by using basic native tools like cat or grep thus to avoid any need to for additional software installation.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-detect-whether-a-physical-cable-is-connected-to-network-card-slot-on-linux