Question

writing a small script to perform house keeping tasks on my servers.

I want to grab just the X in ethX,

echo `grep -m1 -oE "eth[0-9]*" /etc/network/interfaces | grep -oE [0-9]*`

this works, but I was wondering if someone knew a one time command that can do it, ie a regular experession that can take a word for instance, room456 and only spit out the number.

Answer 1

Use positive lookbehind to match any digit string that is preceded by eth:

$ cat file
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500

$ grep -Po '(?<=eth)[0-9]+' file
0
1