Network Interface Name Detection in Conky

When one changes connection type (say, from ethernet to Wi-Fi), the interface name changes (e.g. eth0 → wlan1). To avoid changing Conky config file all the time, here’s a little Lua function for finding the network interface name.

function findInterface()
    local handle = io.popen('ip a | grep "state UP" | cut -d: -f2 | tr -d " "')
    local result = handle:read('*a'):gsub('\n$','')
    handle:close()
    return result
end

1. ip a gives everything about connection info. Each entry looks like

    3: wlp3s0f0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
   

2. Grep the “state UP” and extract the second field, using : as a delimiter. Trim off the spaces around.

3. Use io.popen() to grep the shell output. The use of :read('*a') is necessary. It appends the trailing newline \n character to the output. That’s fine for the function conky_myspeed(upordown), but conky_myspeedgraph(upordown, h, w). Remove that with :gsub('\n$',''). The g in gsub() stands for global. The sub() method requires two arguments limiting the scope of the string substitution.

4. Since the Lua function is to be called from time to time by Conky, it’s a good idea to :close() the stream after having finished.

I tried putting everything in one single line without :close()ing the stream. A number 1 prepending with some white spaces were prepended into the output.

The rest of the Conky Lua script consist mainly of routine usage of conky_parse(). To avoid repetitions like ${lua myupspeed} and ${lua mydownspeed}, I’ve added one more argument in the function conky_myspeed(upordown).

To end this post, I’m going to include a short and simple Perl-compatible regex pattern (PCRE) for matching the IP address in case that

curl http://ipecho.net/plain

returns an HTML document from an ISP that requires web portal login.

curl http://ipecho.net/plain | grep -Po '(\.?[0-9]{2,3}){4}' || echo 'Not found!'

That’s not a proper PCRE for IP extraction, since it matches .0.0.0.0. However, that’s easy to understand and short enough to be put inside the .conkyrc config file, so that git diff’s output won’t be too heavy.