How to use string in arithmetic expression in Bash

Question

I have written a simple script that grabs an ip in CDIR format from openstack. The line looks like this:

cidr="10.62.149.62/27"

I used awk to isolate just the IP, and then used awk one last time to store "27" in a variable called POOL_SIZE .

#!/bin/bash
NETWORK=$1
POOL=$( openstack subnet show --insecure $NETWORK -f shell|grep -w "cidr"|awk -F '"' '{print $2}'|awk -F '/' '{print $2}')

Now, I want to be able to subtract 1 from the 27 to get the value 26 and store in into a different variable.

POOL_SIZE=$(( $POOL - 1 ))
echo $POOL_SIZE

However, when I execute this script, I get this output:

- 1 ")syntax error: invalid arithmetic operator (error token is "

If I echo $POOL I get this output:

27

Therefore, I think that there is some character after "27" like a \\r or \\n that is causing the problem.

So, I want to be able to get rid of those extra characters and subtract 1 from 27.

How would I fix this?

Answer 1

Once you have the network, you can strip the address with

prefix_size=${cidr#*/}

and stript any trailing carriage return output by openstack with

prefix_size=${prefix_size%$'\r'}

$'\\r' is a bash extension, so in other more limited POSIX shells you can use tr instead.

prefix_size=$(echo "$prefix_size" | tr -d '\r')

Or, save a carriage return to a variable once to use later:

cr=$(printf '\r')
...
prefix_size=${prefix_size%$cr}