hotplug − Linux hotplugging support scripts
hotplug [ help | --help | NAME ]
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.).
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
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. 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.
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 software. 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 activated. (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 highlight certain problems. For example, program files and data needed to run hotplug agents might be on filesystems that aren’t yet available.
/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
/etc/init.d/hotplug hotplug system service script
The http://linux-hotplug.sourceforge.net/ web site.
hotplug is developed by Linux Hotplugging Project http://linux-hotplug.sourceforge.net/
This manual page was originally written by Fumitoshi UKAI <ukai [AT] debian.jp>, for the Debian GNU/Linux system (but may be used by others), based on original Japanese version of manpage.