Join John Soldo for an in-depth discussion in this video What are start-up commands?, part of Up and Running with Mac OS X Troubleshooting.
There are times when you might encounter an issue even before you login. Maybe your computer's battery isn't charging. The screen resolution seems off, or your computer just isn't turning on. There are many explanations for these issues. But quite often, it's as simple as a start-up command that can quickly get you back up and running. In Mac OS ten, these are easily executed by holding various keyboard combinations when you turn on the machine. One thing to keep in mind about start-up commands is that they are quick fixes. Now what I mean by this is that they serve a simple and specific purpose.
As you'll see start-up commands are usually associated with high-level macro type issues. Something like battery life could likely be aided by start-up command where as if you are having a problem with a specific document, it probably wouldn't be as helpful, but then again they are also quick and basically harmless so if you feel it might be relevant feel free just to give it a shot. The first two that I am going to discuss the SMC and the NVRAM resets are ones that I like to perform as part of general maintenance at least once a week.