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The man page for udev mentions in several places that certain rules options can be used to invoke 'builtin' commands, which are apparently built in to the udev program itself. However, I haven't been able to find any reference documentation that clearly explains what udev builtins are available; what they do and how they are used.

I have searched the web without much success. Does anyone know if there is a reference anywhere that provides details about these builtin commands?

Unfortunately, this information is missing on manpages and even knowing how to read them(see below) you will find trouble on trying to find that info.

However, the beauty of the opensource relies on having the power to read the sources. If you take a look at the udev-builtin.c source file inside systemd/udev repository and have basic C language knowledge, you will find the following snippet of code: A structure that maps all existing builtin types.

static const struct udev_builtin *builtins[_UDEV_BUILTIN_MAX] = {

#if HAVE_BLKID

        [UDEV_BUILTIN_BLKID] = &udev_builtin_blkid,

#endif

        [UDEV_BUILTIN_BTRFS] = &udev_builtin_btrfs,

        [UDEV_BUILTIN_HWDB] = &udev_builtin_hwdb,

        [UDEV_BUILTIN_INPUT_ID] = &udev_builtin_input_id,

        [UDEV_BUILTIN_KEYBOARD] = &udev_builtin_keyboard,

#if HAVE_KMOD

        [UDEV_BUILTIN_KMOD] = &udev_builtin_kmod,

#endif

        [UDEV_BUILTIN_NET_ID] = &udev_builtin_net_id,

        [UDEV_BUILTIN_NET_LINK] = &udev_builtin_net_setup_link,

        [UDEV_BUILTIN_PATH_ID] = &udev_builtin_path_id,

        [UDEV_BUILTIN_USB_ID] = &udev_builtin_usb_id,

#if HAVE_ACL

        [UDEV_BUILTIN_UACCESS] = &udev_builtin_uaccess,

#endif

};

This struct holds all built-in types, and they map source files depending on what type it is. Example:

• 
  udev-builtin-kmod.c - A Kernel Module loader.


• 
  udev-builtin-keyboard.c - A keyboard handler.


• 
  udev-builtin-usb_id.c - A USB handler that will set the usb type and initialize the device.

Related:

• How do I use man pages to learn how to use commands?

If you just run udevadm test-builtin without arguments it'll list the builtin commands along with a short description for each of them:

udevadm test-builtin

calling: test-builtin

command missing

udevadm test-builtin [OPTIONS] COMMAND DEVPATH



Test a built-in command.



  -h --help     Print this message

  -V --version  Print version of the program



Commands:

  blkid           Filesystem and partition probing

  btrfs           btrfs volume management

  hwdb            Hardware database

  input_id        Input device properties

  keyboard        Keyboard scan code to key mapping

  kmod            Kernel module loader

  net_id          Network device properties

  net_setup_link  Configure network link

  path_id         Compose persistent device path

  usb_id          USB device properties

  uaccess         Manage device node user ACL

Unfortunately, as you've noticed, builtins usage is only explained briefly in the manual.

A practical example can be found in the file 50-udev-default.rules available on your system (under /lib/udev/rules.d/) which contains stuff like:

SUBSYSTEM=="usb", ENV{DEVTYPE}=="usb_device", IMPORT{builtin}="usb_id", IMPORT{builtin}="hwdb --subsystem=usb"

SUBSYSTEM=="input", ENV{ID_INPUT}=="", IMPORT{builtin}="input_id"

ENV{MODALIAS}!="", IMPORT{builtin}="hwdb --subsystem=$env{SUBSYSTEM}"

Not sure, if you don't find anything better, maybe try to explore which builtins are available directly in the source: look for files starting by "udev-builtin"

https://github.com/systemd/systemd/tree/master/src/udev