The answer to 'my supply line smells bad' is that a foul odor coming from your plumbing supply lines is usually caused by bacterial growth or trapped debris. This can happen when water sits stagnant in the lines, allowing bacteria to multiply and produce unpleasant smells. Common symptoms include a rotten egg or sewage-like odor near faucets or fixtures.
The good news is that this is often a simple problem to fix. Start by flushing the lines - turn on all faucets and run the water for several minutes to clear out any buildup. You can also try disinfecting the lines by pouring a diluted bleach solution down the drains. If the smell persists, there may be a more serious issue like a plumbing leak or clogged vent pipe that will require a professional plumber to diagnose and repair.