'sed' : How to add new line after string match + 2 lines -

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i add new line after string match + 2 lines.

here file :

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

i want add 'gateway 192.168.1.1' after found 'iface eth1' + 2 lines.

example: need after execute sed command

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

i know how find , move 2 lines after, append line after specific string, etc. not combine 2 operation. steph

this seems work:

sed '/^iface eth1/{n;n;s/$/\ngateway 192.168.1.1/}' input.txt

add -i option sed save result input.txt.


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