(Ba)sh parameter expansion not consistent in script and interactive shell [duplicate]

Question

I encountered the following issue with parameter expansion while trying to write a script for setting the metric of my VPN's default route.

For testing purposes, I've simplified the code to the following:

vpn_route='default via 10.106.160.1 proto static metric 50'

echo ip route del "$vpn_route"
echo ip route add "${vpn_route/metric +([0-9])/metric 101}"
echo ip route add "${vpn_route/%metric */metric 101}"

Once I remove the echo commands, this should

1. delete the existing default route
2. Create a new route with the metric <number> part replaced by metric 101

For testing, I added a second variant with a less specific pattern.

If I execute this code in an interactive bash shell, it works as expected:

$ vpn_route='default via 10.106.160.1 proto static metric 50'
$ echo ip route del "$vpn_route"
ip route del default via 10.106.160.1 proto static metric 50
$ echo ip route add "${vpn_route/metric +([0-9])/metric 101}"
ip route add default via 10.106.160.1 proto static metric 101
$ echo ip route add "${vpn_route/%metric */metric 101}"
ip route add default via 10.106.160.1 proto static metric 101

However, if I execute the script, I get the following:

$ cat lower-vpn-priority.sh
#!/usr/bin/env bash

vpn_route='default via 10.106.160.1 proto static metric 50'

echo ip route del "$vpn_route"
echo ip route add "${vpn_route/metric +([0-9])/metric 101}"
echo ip route add "${vpn_route/%metric */metric 101}"

$ bash lower-vpn-priority.sh
ip route del default via 10.106.160.1 proto static metric 50
ip route add default via 10.106.160.1 proto static metric 50
ip route add default via 10.106.160.1 proto static metric 101

Can anyone explain to me what's going on here? :)

Answer 1

You need to enable extended globs for +([0-9]) to work. It's probably set in your bashrc, so is enabled in the interactive shell, but not in the script. Add shopt -s extglob to the script to enable it. See Pattern Matching in the manual. Without it, the pattern doesn't match, and ${var/pattern/replacement} leaves the value as-is.