If your Mac doesn't have a keyboard to start up in safe mode or if you can't use the Shift key
If your Mac doesn't have a keyboard available to start in safe mode, but you have remote access to your Mac, you can configure the Mac to start up in safe mode using the command line.

Access the command line by opening Terminal remotely, or logging into the computer using SSH.
Use the following Terminal command:
sudo nvram boot-args="-x"

If you want to start in verbose mode as well, use this instead:
sudo nvram boot-args="-x -v"

After using safe mode, use this Terminal command to return to a normal startup:
sudo nvram boot-args=""