DESCRIPTION
       hotplug  is  a  program which is used by the kernel to notify user mode
       software when some significant (usually hardware-related)  events  take
       place.   An  example  is  when  a  USB  or Cardbus device has just been
       plugged in.  This is useful for automatically loading  and  setting  up
       drivers, packaged either as kernel modules or as user mode programs.

       The  hotplug  program  path name is registered in /proc/sys/kernel/hot-
       plug, and normally holds the value /sbin/hotplug.  Except for  special-
       ized configurations such as initrd(4) configurations, that identifies a
       shell script which delegates to  specific  "policy  agents",  /etc/hot-
       plug/NAME.agent.   The  name of the agent is given by the kernel as the
       first command-line parameter.  The name corresponds to a specific  ker-
       nel  subsystem  whose  events  it  processes  (e.g. "usb", "pci", "net"
       etc.).

       Information about an event is usually passed to agents in the  form  of
       environment  variables.   Most  agents  accept a "ACTION" variable that
       describes the type of the event.  It can  be,  for  example,  "add"  or
       "remove" to signify that some device has just been connected to or dis-
       connected from the system respectively.  In such requests,  agents  are
       also  given  variables  describing  at the device (and its location) so
       that they can choose the appropriate drivers to set up.

       hotplug agents aren't necessarily called only  by  the  kernel  though.
       While  the  OS  is  initializing, the "cold-plugging" case, the hotplug
       script in /etc/init.d/hotplug is invoked to make sure that  all  device
       connected  at  boot are properly configured, applying the same rules as
       if they were connected later.