Script works locally but not via Policy

When you say locally, do you mean the current logged in user runs the commands?

If it is, then the policy is running the command as root, and the locally is running the command as the current user. Try setting the last line to sudo $3 in the front of it, to run the line as the logged in user...

sudo -u $3 /usr/local/bin/mysides add Movies file:///$HOMEDIR/Movies

Although mysides is not guaranteed to work any longer you should try to use this:

#!/bin/zsh

loggedInUser=$( echo "show State:/Users/ConsoleUser" | /usr/sbin/scutil | /usr/bin/awk '/Name :/ && ! /loginwindow/ { print $3 }' )
loggedInUserHome=$(/usr/bin/dscl . -read "/users/$loggedInUser" NFSHomeDirectory | cut -d " " -f 2)

if [[ -n "$loggedInUser" ]]; then
    if [[ -n "$loggedInUserHome" ]]; then
        echo "Adding Movies to $loggedInUser's Finder Sidebar..."
        /usr/local/bin/mysides add Movies "file:///$loggedInUserHome/Movies"
    else
        echo "Error: Could not determine $loggedInUser's home folder."
        exit 1
    fi
else
    echo "Nobody is logged in; skipping."
fi

exit 0

@PaulHazelden sorry I forgot to mention that I've tried that iteration already to no avail.

@ryan.ball no luck with that script.

@user-VrNMiDTLrF I don't think it is the script syntax is causing you the issue. I think that mysides is not supported with later macOS versions like I mentioned initially. Check out the issues created on the mysides github.

@ryan.ball Yeah, with that in mind I'm going to try deploying a LaunchAgent instead. Thanks for your help.