How To:Linux下如何通过命令检查网卡是否插上网线

主要工具为ethtool来检查，主要关注的字段为"Link detected"，注意如下的输出，其中em4实际物理上并未插上网线，而em1是插上网线的：

# ethtool em4

Settings for em4:

        Supported ports: [ TP ]

        Supported link modes:   10baseT/Half 10baseT/Full

                                100baseT/Half 100baseT/Full

                                1000baseT/Half 1000baseT/Full

        Supported pause frame use: No

        Supports auto-negotiation: Yes

        Advertised link modes:  10baseT/Half 10baseT/Full

                                100baseT/Half 100baseT/Full

                                1000baseT/Half 1000baseT/Full

        Advertised pause frame use: Symmetric

        Advertised auto-negotiation: Yes

        Speed: Unknown!

        Duplex: Unknown! (255)

        Port: Twisted Pair

        PHYAD: 2

        Transceiver: internal

        Auto-negotiation: on

        MDI-X: Unknown

        Supports Wake-on: g

        Wake-on: d

        Current message level: 0x000000ff (255)

                               drv probe link timer ifdown ifup rx_err tx_err

        Link detected: no

# ethtool em1

Settings for em1:

        Supported ports: [ TP ]

        Supported link modes:   10baseT/Half 10baseT/Full

                                100baseT/Half 100baseT/Full

                                1000baseT/Half 1000baseT/Full

        Supported pause frame use: No

        Supports auto-negotiation: Yes

        Advertised link modes:  10baseT/Half 10baseT/Full

                                100baseT/Half 100baseT/Full

                                1000baseT/Half 1000baseT/Full

        Advertised pause frame use: Symmetric

        Advertised auto-negotiation: Yes

        Link partner advertised link modes:  100baseT/Full

                                             1000baseT/Full

        Link partner advertised pause frame use: Transmit-only

        Link partner advertised auto-negotiation: Yes

        Speed: 1000Mb/s

        Duplex: Full

        Port: Twisted Pair

        PHYAD: 1

        Transceiver: internal

        Auto-negotiation: on

        MDI-X: on

        Supports Wake-on: g

        Wake-on: d

        Current message level: 0x000000ff (255)

                               drv probe link timer ifdown ifup rx_err tx_err

        Link detected: yes

此外，还可以ifconfig 命令：

首先让我们通过xshell登录一台linux服务器，查看一下当前这台机器有几块网卡：ifconfig -a;我们这台机器有两块网卡分别是eth0和eth1
然后让我们模拟一下环境：拔掉eth1的网线，再次执行：ifconfig -a这条命令，对比一下eth0和eth1的区别，发现eht1不再running。没错这个字段就是看网卡是否有连线的，或者网线是否有问题的。
接着让我们把eth1的网线插上，再执行一遍：ifconfig -a；对比一下eth0和eth1，发现此时eth1已经running了。