Returning the output from Perl Regex

Question

I've been trying to just return the inet values from UP adapters in ip addr,

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: enp0s7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 00:19:21:f5:04:42 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.100/24 brd 192.168.1.255 scope global enp0s7
       valid_lft forever preferred_lft forever
    inet6 fe80::4672:94c8:d31b:a04a/64 scope link
       valid_lft forever preferred_lft forever
3: virbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default qlen 1000
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
    inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0
       valid_lft forever preferred_lft forever
4: virbr0-nic: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
    link/ether 52:54:00:29:38:4d brd ff:ff:ff:ff:ff:ff

I have this regex to find that, but it returns the whole line, as is expected with grep:

([0-9]+:.{1,}state UP.{1,}(\n.{1,}?){1,}inet )\K([0-9]{1,3}\.?){4}

returns this:

inet 192.168.1.100/24 brd 192.168.1.255 scope global enp0s7

What I want it to do is just return 192.168.1.100 (the IP after inet). Can grep do this somehow, and if not, what is the correct way of going about this?

EDIT: OK, here's the relevant part of the script:

NUMOFNETWORKADAPTERS="$(ip addr | grep -Pc '[0-9]{1}: ')"
IPOFACTIVEADAPTER="$(ip addr | grep -Pc '(?<=state UP(.+\n){2,$NUMOFNETWORKADAPTER}?    inet )[^/]{1,15}')"

echo "LOCAL IP: $IPOFACTIVEADAPTER"

But this gives a lookbehind assertion is not fixed length error.

Answer 1

grep may not be the best option here, though with a PCRE enabled grep and -o one can grab the IP addresses with a zero-width lookbehind match to find inet and then match the IP following that.

ip addr | grep -Po '(?<=inet )[^/]+'

For the multi-line problem of "get inet addr of devices that are up", something like

ip link | awk -F: '/state UP/{print $2}' \
| while read updev; do \
  ip addr show $updev | grep -Po '(?<=inet )[^/]+'; done

may be more suitable.

Answer 2

Actually, the regexp

([0-9]+:.{1,}state UP.{1,}(\n.{1,}?){1,}inet )\K([0-9]{1,3}\.?){4}

which you first presented, works, with an appropriate environment and Perl:

$ perl -l -0 -ne 'print $& if /([0-9]+:.{1,}state UP.{1,}(\n.{1,}?){1,}inet )\K([0-9]{1,3}\.?){4}/'  < output_of_your_ip_addr
192.168.1.100

-0 reads the file in full (instead of line-by-line), and $& contains the matching part (what is left after \K drops the left part) .

---

Though if Perl is usable, I would just write

ip addr | perl -l -0ne  'print $1 while /state UP.*?inet ([0-9.]+)/sg'

to print all matches. (Change while to if to only take the first match. $1 contains what the first () matched.)