bless

Set volume bootability and set the active boot volume.
bless has 5 modes of execution: Folder Mode, Mount Mode, Device Mode, NetBoot Mode, and Info Mode.

Syntax (brief)

Info Mode:
   --info [dir]    Print blessing information for a specific volume, or the
                   currently active boot volume if dir is not specified
   --getBoot       Suppress normal output and print the active boot volume
   --version       Print bless version number
   --plist         For any output type, use a plist representation
   --verbose       Verbose output

File/Folder Mode:
   --file file     Set file as the blessed boot file
   --folder dir    Set dir as the blessed directory
   --bootinfo [file]
                   Use file to create a "BootX" file in the blessed dir
   --bootefi [file]
                   Use file to create a "boot.efi" file in the blessed dir
   --setBoot       Set firmware to boot from this volume
   --openfolder dir Set dir to be the visible Finder directory
   --verbose       Verbose output

Mount Mode:
   --mount dir     Use this mountpoint in conjunction with --setBoot
   --file file     Set firmware to boot from file
   --setBoot       Set firmware to boot from this volume
   --verbose       Verbose output

Device Mode:
   --device dev    Use this block device in conjunction with --setBoot
   --setBoot       Set firmware to boot from this volume
   --verbose       Verbose output

NetBoot Mode:
   --netboot       Set firmware to boot from the network
   --server url    Use BDSP to fetch boot parameters from url
   --verbose       Verbose output

Syntax (Detailed)

     bless --folder directory [--folder9 directory] [--file file]
           [--bootinfo [file]] [--bootefi [file]] [--bootBlockFile file]
           [--save9] [--saveX] [--use9] [--label name | --labelfile file]
           [--setBoot] [--openfolder directory] [--nextonly] [--shortform]
           [--legacy] [--legacydrivehint device] [--options string]
           [--quiet | --verbose]

     bless --mount directory [--file file] [--setBoot] [--nextonly]
           [--shortform] [--legacy] [--legacydrivehint device]
           [--options string] [--quiet | --verbose]

     bless --device device [--label name | --labelfile file]
           [--startupfile file] [--setBoot] [--nextonly] [--shortform]
           [--legacy] [--legacydrivehint device] [--options string]
           [--quiet | --verbose]

     bless --netboot --server url [--nextonly] [--options string]
           [--quiet | --verbose]

     bless --info [directory] [--getBoot] [--plist] [--quiet | --verbose]
           [--version]
     bless -help

EXAMPLES

FOLDER MODE
To bless a volume with only Mac OS 9:

bless --folder9 "/Volumes/Mac OS 9/System Folder" --bootBlockFile "/usr/share/misc/bootblockdata"

To bless a volume with only Mac OS X or Darwin, and create the BootX and boot.efi files as needed:

bless --folder "/Volumes/Mac OS X/System/Library/CoreServices" --bootinfo --bootefi

MOUNT MODE
To set a volume containing either Mac OS 9 and Mac OS X to be the active volume:

bless --mount "/Volumes/Mac OS" --setBoot

NETBOOT MODE
To set the system to NetBoot and broadcast for an available server:

bless --netboot --server bsdp://255.255.255.255

INFO MODE
To gather information about the currently selected volume (as determined by the firmware), suitable for piping to a program capable of parsing Property Lists:

bless --info --plist

For a full description of all the bless options see man bless the options available for bless may vary according to the version of OS X you are running.

“If everything seems under control, then you're not going fast enough” - Mario Andretti

Related:

diskutil - Disk utilities - Format, Verify, Repair
mount - Mount a file system
newfs(8)
nvram(8)