This is another post that is primarily for my own benefit for the next time I forget how to do this.

I wanted to read routing information directly from /proc/net/route using bash, because you never know what may or may not be available in the minimal environment of a Docker container (for example, the iproute package is not installed by default in the Fedora Docker images). The contents of /proc/net/route looks something like:

Iface	Destination	Gateway 	Flags	RefCnt	Use	Metric	Mask		MTU	Window	IRTT                                                       
eth0	00000000	0101A8C0	0003	0	0	1024	00000000	0	0	0                                                                          
eth0	37E9BB42	0101A8C0	0007	0	0	20	FFFFFFFF	0	0	0

If I want the address of the default gateway, I can trivially get the hexadecimal form like this:

awk '$2 == "00000000" {print $3}' /proc/net/route

Which gives me:

0101A8C0

This is in little-endian order; that is, the above bytes represent 1 1 168 192, which you may recognize better as 192.168.1.1. So, we need to convert this into a sequence of individual octets, reverse the order, and produce the decimal equivalent of each octet.

The following gives us the octets in the correct order, prefixed by 0x (which we’re going to want in the next step):

awk '$2 == "00000000" {print $3}' /proc/net/route |
  sed 's/../0x& /g' | tr ' ' '\n' | tac

We can put this into a bash array like this:

octets=($(
awk '$2 == "00000000" {print $3}' /proc/net/route |
  sed 's/../0x& /g' | tr ' ' '\n' | tac
))

And we convert those hexadecimal octets into decimal like this:

printf "%d." ${octets[@]} | sed 's/\.$/\n/'

An interesting feature of the Bash printf command – and one that may be surprising to people who are coming from a C background – is that:

The format is re-used as necessary to consume all of the arguments.

That means, that a command like this:

printf "%d." 1 2 3 4

Will yield:

1.2.3.4.

If we put this all together, we might end up with something like:

hexaddr=$(awk '$2 == "00000000" {print $3}' /proc/net/route)
ipaddr=$(printf "%d." $(
  echo $hexaddr | sed 's/../0x& /g' | tr ' ' '\n' | tac
  ) | sed 's/\.$/\n/')