You can use various command line options to control how the emulator
starts. Run b2 with --help to get a full list; the (probably) most
useful ones are described here.
You can run b2 from the command line and supply a single disk image
file name. In this case, b2 will load the given disk image into drive
0 as an in-memory image (as if using -0 FILE) and auto-boot (as if
using -b).
If there's an existing copy of b2 running, the disk will be booted using that.
(This feature is mainly here to support double clicking from a file explorer, but it's potentially useful from the command line too.)
Load disc image FILE into drive 0 or drive 1, as if using File >
Drive X > In-memory disc image....
When using -0 or -1, you can also supply --0-direct or
--1-direct respectively, indicating that the disc image should be
accessed directly as if using File > Drive X > Direct disc image....
(These options are named after a previous iteration of the UI, which is why they're not terribly mnemonic.)
Tries to autoboot the disc in drive 0. The emulator just pretends SHIFT is being held while starting up.
Start up with config CONFIG. The name is exactly as seen in the list
of configs shown in File > Change config or Tools >
Configurations.
Reset window positions. The docking/tabbing system can occasionally get itself into a mess, and this is a workaround for that...
Select screen update timing method. The default, --vsync, should
work fine, but try --timer if it feels like the display update rate
is poor even though the emulator claims to be running at ~1x real
speed.
Whichever you choose, the option is sticky, and will be used for subsequent runs even when neither option is supplied.