How to find out the connected interface using linux command

Method 1

To find out the connected state of a network cable in Linux, just run:

$ cat /sys/class/net/enp5s0/carrier

Sample output:

1

If you got output as “1” (Number one), It means that the network cable is connected with the network card. Also, you can do this with the following command too:

$ cat /sys/class/net/enp5s0/operstate

Sample output:

up

Method 2:

In case the above commands doesn’t help, there is another tool called “ethtool” to help you. ethtool is used to query and control network device driver and hardware settings, particularly for wired Ethernet devices.

Run the following command to find out the connected state of a network cable:

$ sudo ethtool enp5s0

Sample output:

Settings for enp5s0:
Supported ports: [ TP MII ]
Supported link modes: 10baseT/Half 10baseT/Full 
100baseT/Half 100baseT/Full 
Supported pause frame use: No
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full 
100baseT/Half 100baseT/Full 
Advertised pause frame use: Symmetric Receive-only
Advertised auto-negotiation: Yes
Link partner advertised link modes: 10baseT/Half 10baseT/Full 
100baseT/Half 100baseT/Full 
Link partner advertised pause frame use: Transmit-only
Link partner advertised auto-negotiation: Yes
Speed: 100Mb/s
Duplex: Full
Port: MII
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000033 (51)
drv probe ifdown ifup
Link detected: yes

To find out the state of wireless network card:

$ sudo ethtool wlp9s0

Sample output:

Settings for wlp9s0:
Link detected: yes

Let us disconnect the cable, and see what happens.

$ sudo ethtool enp5s0

Sample output:
Settings for enp5s0:
Supported ports: [ TP MII ]
Supported link modes: 10baseT/Half 10baseT/Full 
100baseT/Half 100baseT/Full 
Supported pause frame use: No
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full 
100baseT/Half 100baseT/Full 
Advertised pause frame use: Symmetric Receive-only
Advertised auto-negotiation: Yes
Link partner advertised link modes: 10baseT/Half 10baseT/Full 
100baseT/Half 100baseT/Full 
Link partner advertised pause frame use: Transmit-only
Link partner advertised auto-negotiation: Yes
Speed: 100Mb/s
Duplex: Full
Port: MII
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000033 (51)
drv probe ifdown ifup
Link detected: no

Src: https://ostechnix.com/how-to-find-out-the-connected-state-of-a-network-cable-in-linux/