hotplug - Online Manual Page Of Unix/Linux

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NAME
hotplug - Linux hotplugging support scripts

SYNOPSIS
hotplug [ help | --help | NAME ]

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

Information about an event is passed in the form of environment variables. Most agents
accept a "ACTION" variable that describes the type of the event. It is set to "add" or
"remove" to signify that some device has just been connected to or disconnected from the
system.
The location of the device is provided in the variable "DEVPATH". With kernel version 2.5
and later it is set to the corresponding sysfs device path without the mountpoint but a
leading slash.

hotplug agents aren't necessarily called only by the kernel though.

Hotplug agents should be used to dynamically adapt to system configuration changes. It
helps to think of them as just delivering events from different layers of the Linux soft-
ware. So low level bus framework code will deliver events when new physical devices are
plugged in, and bus agents make sure the device's driver is loaded, configured, and acti-
vated. (They use depmod(8) style device descriptions, used among other things to publish
MODULE_DEVICE_TABLE entries found in the source to kernel mode device drivers.) Then
other layers in the Linux stack will take over, reporting higher level events like "there
is a new network interface". Agents at those levels tend to have more complex tasks.

When used during system initialization, hotplugging may be called "coldplugging" to high-
light certain problems. For example, program files and data needed to run hotplug agents
might be on filesystems that aren't yet available.

FILES
/lib/modules/*/modules.*map depmod output
/proc/sys/kernel/hotplug specifies hotplug program path
/sbin/hotplug hotplug program (default path name)
/etc/hotplug/* hotplug files
/etc/hotplug/NAME.agent hotplug subsystem-specific agents
/etc/hotplug/NAME* subsystem-specific files, for agents
/etc/hotplug/NAME/DRIVER driver setup scripts, invoked by agents
/etc/hotplug/usb/DRIVER.usermap depmod data for user-mode drivers