How To:Linux下如何通過命令檢查網卡是否插上網線

主要工具為ethtool來檢查，主要關注的字段為"Link detected"，注意如下的輸出，其中em4實際物理上並未插上網線，而em1是插上網線的：

# 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，還可以ifconfig 命令：

1.   首先讓我們通過xshell登錄一台linux服務器，查看一下當前這台機器有幾塊網卡：ifconfig -a;我們這台機器有兩塊網卡分別是eth0和eth1
2.  

   然后讓我們模擬一下環境：拔掉eth1的網線，再次執行：ifconfig -a這條命令，對比一下eth0和eth1的區別，發現eht1不再running。沒錯這個字段就是看網卡是否有連線的，或者網線是否有問題的。

   
3.  

   接着讓我們把eth1的網線插上，再執行一遍：ifconfig -a；對比一下eth0和eth1，發現此時eth1已經running了。