Cfrgtkky

According to this answer and many other tutorials throughout the web, ifconfig will list lots of fascinating and useful information. And, that seems the standard way to find out stuff like my netmask.

But, for scripting, I need only the netmask itself for eth0; and man ifconfig was unhelpful with this question.

I am using a VPS, so I don't have control of my netmask; I need to find out what it has been set to.

(Note: While I need eth0, some machines might have another, say eth1 or ens3, which would also be useful.)

• I do not need the entire line the netmask is in:

inet 111.222.333.444 netmask 255.255.255.0 broadcast 111.222.555.666

• I want to avoid a "hackdoor" method, like getting netmask 255.255.255.0 then running sed "s/netmask //" because stuff can change—unless an expert says so
  

Probably this is what you need (note this will not work on Ubuntu 16.04):

#!/bin/bash

IFACE='eth0'

ifconfig | grep -A 7 "$IFACE" | sed -nr 's/^.*netmasks([0-9.]+)ssbroadcast.*$/1/p'

In the first line the name of the network interface is assigned as value of the variable $IFACE - this is useful for scripting, otherwise you can use grep -A 7 'eth0'.

On the second line the output of the command ifconfig will be piped to the grep command, where the option -A 7 will output the following 7 lines after the line with the matched string/regexp. The output of that command will be piped to sed.

Within the sed command:

• the regular expression ^.*netmasks(.*)ssbroadcast.*$ will match to the whole line, from the beginning ^ to the end $, that contains some characters .* and the "keywords" netmaskspace, [0-9.]+ and ssbroadcast in that exact order;




• that line will be substituted (s/old/new/) with the content of the first capture group [(.*)->1], where the regexp [0-9.]+ will match to the strings that are consisted of digits end dots;




• the option -r (or -E) will enable the extended regular expressions, which, in this case, will allow us to use the round brackets freely;




• the option -n with combination of the flag p with output only the matched line and will preserve the rest output of sed.

Here is an extended example that will parse the names of all network interfaces and will do similar as the above command for each of them:

for IFACE in $(ifconfig | sed -nr 's/(^[a-z0-9]+):.*/1/p'); do 

    echo -en "${IFACE}:t"; ifconfig | 

    grep -A 7 "$IFACE" | 

    sed 's/  broadcast.*$//' | 

    sed -rn 's/^.*netmask (.*)$/1/p'; 

done

Sample output of the above command executed on a virtual machine with Ubuntu 18.04:

$ for IFACE in $(ifconfig | sed -nr 's/(^[a-z0-9]+):.*/1/p'); do echo -en "${IFACE}:t"; ifconfig | grep -A 7 "$IFACE" | sed 's/..broadcast.*$//' | sed -rn 's/^.*netmask (.*)$/1/p'; done

ens33:  255.255.255.0

lo:     255.0.0.0