How to use the command 'bless' (with examples)

The bless command is a powerful yet underutilized tool used in macOS systems to manage startup settings for both Mac OS X and Darwin-based environments. This command allows users to set the boot volume, configure startup disk options, and more. With bless, administrators can control how and where the system boots, offering flexibility for various startup configurations. As system management tasks, especially those involving boot configurations, require precision, the bless command is an essential utility in any macOS administrator’s toolkit.

Use case 1: Bless a volume with only Mac OS X or Darwin, and create the BootX and boot.efi files as needed

Code:

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

Motivation:

This use case is critical when setting up a bootable macOS volume containing necessary boot files for a successful startup. By creating the BootX and boot.efi files, the command ensures the system has what it needs to boot into Mac OS X or Darwin environments. This might be needed after a manual installation or upgrade process, where these critical files are either missing or outdated.

Explanation:

• --folder /Volumes/Mac OS X/System/Library/CoreServices: This argument specifies the directory where the bootable system resides. By pointing to the CoreServices folder within a macOS volume, you are identifying where the boot-related files are located.
• --bootinfo: This option is used to update the BootX, a crucial file used by older Mac systems to locate and load the operating system.
• --bootefi: This flag ensures the creation or updating of the boot.efi file, which is essential for Intel-based Mac systems during the boot process.

Example Output:

There is no direct output from this command in the terminal. However, upon successful execution, the specified volume will be properly configured with the necessary boot files.

Use case 2: Set a volume containing either Mac OS 9 and Mac OS X to be the active volume

Code:

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

Motivation:

In a dual-boot environment where both Mac OS 9 and Mac OS X are present, this usage is vital for specifying which OS should be the default active volume. It’s beneficial in scenarios where users need to ensure that a specific volume consistently serves as the primary boot volume each time the system starts.

Explanation:

• --mount /Volumes/Mac OS: This argument defines the volume you want to set as the active boot volume. It is the path where the OS to be booted is located.
• --setBoot: This flag instructs the system to make the specified volume the active boot volume, ensuring that whenever the Mac starts, it uses this volume by default.

Example Output:

The successful application of this command typically results in the system booting from the specified volume on the next startup without terminal feedback.

Use case 3: Set the system to NetBoot and broadcast for an available server

Code:

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

Motivation:

NetBooting is a technique used in network environments where client machines boot from a network server instead of a local drive. This is useful in large computer labs or enterprise settings where managing individual installations is cumbersome. By broadcasting to seek available NetBoot servers, systems using this setup can ensure seamless, centralized boot management.

Explanation:

• --netboot: The flag triggers the NetBoot process, configuring the system to start over the network rather than from a local drive.
• --server bsdp://255.255.255.255: This option designates the server address for the NetBoot. By using 255.255.255.255, a broadcast is made to discover available servers within the network capable of providing NetBoot services.

Example Output:

Success is often silent in terminal feedback but results in the system attempting to boot from a network server, contingent on network configuration and available NetBoot servers.

Use case 4: Gather information about the currently selected volume, suitable for piping to a program capable of parsing Property Lists

Code:

bless --info --plist

Motivation:

This command is incredibly useful for diagnostics and system information gathering. It especially aids administrators in determining the identity and configuration of the current boot volume. By outputting this information in a Property List format, it is highly amenable for further automated processing using scripts or applications that can parse such structured data.

Explanation:

• --info: This flag retrieves metadata about the currently active volume, particularly how the system firmware regards it as a bootable entity.
• --plist: This argument formats the output as a Property List, a type of XML file commonly used in macOS to store serialized data, ideal for automation and further data processing.

Example Output:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>BootVolumePath</key>
    <string>/Volumes/Mac OS X</string>
    <!-- Additional keys and values describing the boot volume -->
</dict>
</plist>

Conclusion:

The bless command is a versatile tool for managing Mac systems’ boot setups, offering different functionalities to manipulate, configure, and inquire about boot volumes. Whether setting up dual-boot environments, preparing systems for network booting, or collecting boot information, these examples highlight its uses in administrative operations essential for precise system management and deployment.