diff command in custom recipe causes deployment process to exit

[stsonline]

• Deployer version: 6.8
• Deployment OS: to cent OS 7

My deploy.php file references a custom recipe that I've built taking inspiration from one of the older NPM recipes. It's to run a composer install much like the npm install but only if the composer.json file is different from the previous release to prevent having to always run it, resulting in faster deployments.

My custom recipe is running the task, but the diff command is causing an exit code 1 which is causing the whole deployment process to exit despite this being the correct error code for the diff command, the exit code 1 implies that the files are different, which they are, so why is Deployer failing on this instead of running my install?

My [my_project]/deploy/recipe/sts-composer.php

<?php
/*
 * Custom composer recipe for us.
 */

namespace Deployer;

set('bin/composer', function () {
    return run('which composer');
});

desc('Install composer packages');
task('composer:install', function () {
    if (has('previous_release')) {
        if (test('[ -d {{previous_release}}/vendor ]')) {
            run('cp -R {{previous_release}}/vendor {{release_path}}');

            // If composer.json is unmodified, then skip running `composer install`
            if (!run('diff {{previous_release}}/composer.json {{release_path}}/composer.json')) {
                return;
            }
        }
    }
    run("cd {{release_path}} && {{bin/composer}} install");
});

This is then loaded into my [my_project]/deploy.php file

<?php
namespace Deployer;

require 'deploy/recipe/sts-composer.php';
...

Thoughts on how to resolve as a workaround for Deployer 6.8 please.