source: dev-team blog

Update #3: We’ve updated redsn0w to include “one-click” support for those of you running the tethered 4.2.1 jailbreak. Using command-line arguments, you can now bypass the screens you’d normally see as you use redsn0w to “Just boot tethered for now”.

The available command line arguments are:

-j to ask redsn0w to “Just boot now tethered for now”
-i <filename> to specify your reference IPSW
-o for old-bootrom iPod touch 2G and iPhone 3GS
-b <filename> to specify your own boot logo png

For example, to get redsn0w for Mac to do a tethered boot of an iPod touch 4G jailbroken at 4.2.1:

open ~/Desktop/redsn0w.app —args -j -i ~/Desktop/iPod4,1_4.2.1_8C148_Restore.ipsw

This assumes both redsn0w and the IPSW are on your OS X desktop, so modify as necessary! Included in the zip is an example script file that you can double click on to launch redsn0w like this (the Windows example assumes everything is in C:\). (Mac users: please remember to change the permissions of your custom *.command files to allow execution.)

This should help ease the pain of the tethered jailbreak until @comex comes up with a 4.2.1 untether (or for those of you with legit access to the 4.2b3 IPSW, until the “Jailbreak Monte” untether is out of beta)!

* PLEASE UPGRADE TO iTunes 10.1 FOR BEST RESULTS
* WINDOWS 7 USERS SHOULD RUN redsn0w IN “XP COMPATIBILITY” MODE
* Make sure you’re using a USB 2.0 port

* OS X
* Windows