Isolate the last octet from an IP address and put it in a variable

Question

The following script:

IP=`ifconfig en0 inet | grep inet | sed 's/.*inet *//; s/ .*//'`

isolates the IP address from ipconfig command and puts it into the variable $IP. How can I now isolate the last octet from the said IP address and put it in a second variable - $CN

for instance:

$IP = 129.66.128.72 $CN = 72 or $IP = 129.66.128.133 $CN = 133...

Its better to not use old and outdated back-tics, but instead use parentheses like this `IP=$(ifconfig.....)` — Jotne, Aug 04 '14 at 12:43

score 14 · Answer 1 · edited Dec 12 '16 at 20:07

14

Use "cut" command with . delimiter:

IP=129.66.128.72
CN=`echo $IP | cut -d . -f 4`

CN now contains the last octet.

edited Dec 12 '16 at 20:07

jacknad

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answered Aug 04 '14 at 12:40

Danilo Raspa

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You can shorten it some to:`CN=$(cut -d. -f4 <<<$IP)` – Jotne Aug 04 '14 at 12:44
`<<<` is non-standard and not supported by all shells. – chepner Aug 04 '14 at 13:23

score 9 · Answer 2 · answered Aug 04 '14 at 12:31

9

In BASH you can use:

ip='129.66.128.72'
cn="${ip##*.}"
echo $cn
72

Or using sed for non BASH:

cn=`echo "$ip" | sed 's/^.*\.\([^.]*\)$/\1/'`
echo $cn
72

Using awk

cn=`echo "$ip" | awk -F '\\.' '{print $NF}'`

answered Aug 04 '14 at 12:31

anubhava

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Shorter using awk `cn=$(awk -F. '$0=$4' <<< $ip)` – Aug 04 '14 at 12:57
@Jidder: I believe only BASH supports `<<<` that's why I used pipes in non BASH solutions. – anubhava Aug 04 '14 at 12:59
Oh okay,didn't know it was specific to bash, is `$()` bash specific as well ? – Aug 04 '14 at 13:05
2

No, [`$(...)`](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_03) is in the POSIX standard. – chepner Aug 04 '14 at 13:21
Ah that is correct, thank for correcting @chepner me. Always helpful. – anubhava Aug 04 '14 at 13:24

score 2 · Accepted Answer · answered Aug 04 '14 at 12:50

2

Shortcut:

read IP CN < <(exec ifconfig en0 | awk '/inet / { t = $2; sub(/.*[.]/, "", t); print $2, t }')

answered Aug 04 '14 at 12:50

konsolebox

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`<( ... )` is non-standard and not supported by all shells. – chepner Aug 04 '14 at 13:23

score 1 · Answer 4 · answered Aug 04 '14 at 12:36

I would not use ifconfig to get the IP, rather use this solution, since it gets the IP needed to get to internet regardless of interface.

IP=$(ip route get 8.8.8.8 | awk '{print $NF;exit}')

and to get last octet:

last=$(awk -F. '{print $NF}' <<< $IP)

or get the last octet directly:

last=$(ip route get 8.8.8.8 | awk -F. '{print $NF;exit}')

Isolate the last octet from an IP address and put it in a variable

4 Answers4