title: 'sed' : How to add new line after string match + 2 lines
body: <p>I would like add a new line after string match + 2 lines.</p>
  
  <p>Here is my file :</p>
  
  <pre><code>allow-hotplug eth0
  auto eth0
  iface eth0 inet static
  address 172.16.2.245
  netmask 255.255.254.0
  gateway 192.168.1.1
  
  allow-hotplug eth1
  #auto eth1
  iface eth1 inet static
  address 192.168.0.240
  netmask 255.255.255.0
  
  iface eth2 inet static
  address 192.168.2.240
  netmask 255.255.255.0
  </code></pre>
  
  <p>I want to add 'gateway 192.168.1.1' after found 'iface eth1' + 2 lines.</p>
  
  <p>example: what I need to get after execute sed command</p>
  
  <pre><code>allow-hotplug eth0
  auto eth0
  iface eth0 inet static
  address 172.16.2.245
  netmask 255.255.254.0
  gateway 172.16.2.254
  
  allow-hotplug eth1
  #auto eth1
  iface eth1 inet static
  address 192.168.0.240
  netmask 255.255.255.0
  gateway 192.168.1.1
  
  iface eth2 inet static
  address 192.168.2.240
  netmask 255.255.255.0
  </code></pre>
  
  <p>I know how to find and move 2 lines after, append line after specific string, etc. but not combine this 2 operation.
  Steph</p>