There is a command called column which allows you to format the lines produced by a command into symmetrical rows/columns, which can then be easily converted into CSV or JSON.

This will be quite handy for reading and populating files. In this case, I needed to populate /etc/ethers/ from the arp table

More info on column

Note: This is based on the column command in fedora linux, it may be different in other distros

Creating a table

This bit is simple enough, we delete unneeded fields with awk, and then use the command column -t to turn it into a symmetrical table.

ip -4 neighbor | grep --invert-match FAILED 
| awk '{print $1,"\t",$5,"\t",$3}' 
| column -table 


172.17.0.2       02:42:ac:11:ba:32  docker0        
172.17.29.71     52:54:00:85:be:17  docker0        
192.168.0.12     52:54:00:24:fe:41  virbr2         
192.168.1.19     b8:27:eb:15:ed:18  enp3s0         
192.168.1.1      44:d9:e7:9e:de:52  enp3s0         
10.53.2.53       b8:27:eb:15:ad:98  enp3s0         
172.17.65.56     52:54:00:74:be:21  docker0        
192.168.122.229  52:54:00:24:ff:68  virbr0 

Adding identifiers to the fields

column is a powerful command for making tables, lets specify some field names for the table.

We do this by adding the argument --table-columns to `column' and giving a comma seperated list of field names e.g.

column --table --table-columns "IPv4Address,MacAddress,InterfaceName,Comment" --json 

We can even get JSON!

{
  "table": [
    {
      "ipv4address": "172.17.0.2",
      "macaddress": "02:42:ac:11:ba:32",
      "interfacename": "docker0",
      "comment": null
    },
    {
      "ipv4address": "172.17.29.71",
      "macaddress": "52:54:00:85:be:17",
      "interfacename": "docker0",
      "comment": null
    },
    {
      "ipv4address": "192.168.0.12",
      "macaddress": "52:54:00:24:fe:41",
      "interfacename": "virbr2",
      "comment": null
    },
    {
      "ipv4address": "192.168.1.19",
      "macaddress": "b8:27:eb:15:ed:18",
      "interfacename": "enp3s0",
      "comment": null
    },
    {
      "ipv4address": "192.168.1.1",
      "macaddress": "44:d9:e7:9e:de:52",
      "interfacename": "enp3s0",
      "comment": null
    },
    {
      "ipv4address": "10.53.2.53",
      "macaddress": "b8:27:eb:15:ad:98",
      "interfacename": "enp3s0",
      "comment": null
    },
    {
      "ipv4address": "172.17.65.56",
      "macaddress": "52:54:00:74:be:21",
      "interfacename": "docker0",
      "comment": null
    },
    {
      "ipv4address": "192.168.122.229",
      "macaddress": "52:54:00:24:ff:68",
      "interfacename": "virbr0",
      "comment": null
    }
  ]
}