uc-grep

#!/bin/bash
PLUGIN=$1
VERSION=$2
UC=$3
if [ -z "$PLUGIN" ]
then
    echo 'Usage: uc-grep short-name [core-version [https://jenkins-updates.cloudbees.com/update-center/envelope-core-mm/]]'
    exit 1
fi
if [ -z "$VERSION" ]
then
    VERSION=2.375.2
fi
if [ -z "$UC" ]
then
    UC=https://updates.jenkins-ci.org/
fi
curl -sL "${UC}update-center.json?version=$VERSION" | head -n-1 | tail -n+2 | jq -r '.plugins | .["'$PLUGIN'"] | .version'

Negative indices to head aren't supported on macOS (BSD head), so I used update-center.actual.json instead which is JSON instead of JSONP and then just deleted | head -n-1 | tail -n+2, although I'm not sure if there are differences between the files that make that a bad idea.

update-center.actual.json is indeed preferable. I have kept the JSONP variant since the CloudBees CI update center does not yet support the JSON URL pattern.