How to get netmask from bash?

I should clarify that the code here works for Linux, (note comments and post about other Unices). OP asked for a Linux solution, but it would be good to change the question to "how to get netmask" in general, and have the answer combine the best way for more Unix flavors.

#!/bin/sh
ifconfig "$1" | sed -rn '2s/ .*:(.*)$/\1/p'

./script eth0

About ifconfig in Linux

Along with other traditional networking commands like netstat, arp, rarp and route, ifconfig is part of the net-tools package. Net-tools hasn't been actively developed from a really long time and there are efforts to deprecate it in favor of the newer iproute2 package. For the sake of brevity, if you want more details on this fundamental transition, here are some relevant links:

Case example: Debian Linux

Case example: Arch Linux

Man page for net-tools ifconfig (see BUGS section)

Analysis and comparison of the frameworks from a users perspective

/sbin/ifconfig eth0 | awk '/Mask:/{ print $4;} '

?

I wonder if you really only want the mask. Maybe you really want the network (or IP) with mask, to use in config files, with nmap, or whatever.

In that case, on Linux, you can also parse the output of the ip ... command.

To list all interfaces with their address and mask in CIDR notation:

ip -o -f inet addr show | awk '/scope global/ {print $2, $4}'

Or to restrict it to the default interface:

default_if=$(ip route list | awk '/^default/ {print $5}')
ip -o -f inet addr show $default_if | awk '{print $4}'

which gives me "192.168.1.61/24" on my machine.