Method 1<\/p>\n
Using dmesg<\/p>\n
Using dmesg is one of the 1st things to do for inquiring current state of system:<\/p>\n
Example:<\/p>\n
dmesg | sed ‘\/eth.*Link is\/h;$;d’<\/p>\n
[1667676.292871] e1000e: eth0 NIC Link is Up 100 Mbps Full Duplex, Flow Control: Rx\/Tx<\/p>\n
Method 2<\/p>\n
\/sys\/class\/net\/<\/p>\n
cat \/sys\/class\/net\/eth0\/carrier<\/p>\n
1<\/p>\n
The number 1 in the above output means that the network cable is connection physically
\nto your’s network card slot.<\/p>\n
Or<\/p>\n
cat \/sys\/class\/net\/eth0\/operstate<\/p>\n
up<\/p>\n
Method 3<\/p>\n
Using ethtool command<\/p>\n
Syantax : ethtool interface_name | grep Link d<\/p>\n
Example:<\/p>\n
ethtool eth0 | grep Link d<\/p>\n
Link detected: yes<\/p>\n
we can use bash for loop again to check all network interfaces it once:<\/p>\n
for i in $( ls \/sys\/class\/net ); do echo -n $i; ethtool $i | grep Link d; done<\/p>\n
Sample output:<\/p>\n
eth0 Link detected: yes
\neth1 Link detected: no
\nlo Link detected: yes
\nwlan0 Link detected: no<\/p>\n
NOTE:<\/p>\n
The only problem with the above ethtool output is that it will not detect connected<\/p>\n
cable if your network interface is down. Consider a following example:<\/p>\n
# ethtool eth0 | grep Link d
\nLink detected: yes
\n# ifconfig eth0 down
\n# ethtool eth0 | grep Link d
\nLink detected: no
\n# ifconfig eth0 up
\n# ethtool eth0 | grep Link d
\nLink detected: yes<\/p>\n