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HAL:

It provides information service to dynamically manage devices (Removal devices like CD-ROM discs or USB card readers) using specialized callout programs when attached. For example, callout hal-system-storage-mount mounts a device whose entries specified in HAL device information files that set policies for storage management.

  • Impacts /etc/fstab file used to manage file systems
  • hal-storage-mount: Mount a device
  • hal-storage-eject: Remove device
  • Uses a sysfs file system to manage removable device files as needed.
  • It provides information about device files and manages the configuration for removable devices like file systems for USB, readers, cameras and CD-ROMs. 
  • /etc/hal: Configuration directory
  • /etc/hal/fdi: The HAL system administrator's device information files
  • /usr/share/hal/fdi: The device information for configuring HAL information support and policies

Hides Device Implementation From Applications

To abstract the process of desktops and applications accessing devices managed as objects. The applications do not know anything about the device, even its symbolic name. It requests a device, and then HAL -daemon, ‘hald' runs as a ‘haldaemon‘ process. It provides the same using the D-Bus structure. The HAL Daemon maintains a dynamic database of connected hardware devices.

HAL Device Manager (hal-device-manager)

Accessed via System -> Administration -> Hardware menu entry.
A part of hal-genome package and displays the information provided by the HAL daemon for devices as an expandable tree for all devices arranged by category in the left panel. The right-panel displays information about the selected device.