Ny documentation for udev builtins

documentationreferenceudev

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?

Best Answer

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?

Related Solutions

Udev “urbnum”

As per struct usb_device:

urbnum

    number of URBs submitted for the whole device

where URB stands for USB Request Block. An URB consists of all relevant information to execute any USB transaction and deliver the data and status back.
See also:
Kernel Documentation
USB Urbs

Udev rule to match any usb storage device

A storage device is in the block subsystem, so you'll want SUBSYSTEM=="block" in your rule, like this:

ACTION=="add", KERNEL=="sd?", SUBSYSTEM=="block", ENV{ID_BUS}=="usb", \
    RUN+="/path/to/script"

If you're using systemd, you could run a systemd unit each time a USB storage device is added. Create the unit file, e.g. /etc/systemd/system/my-usb-rule.service:

[Service]
Type=oneshot
ExecStart=/path/to/script

and the rule, e.g. /etc/udev/rules.d/85-my-usb-rule.rules:

ACTION=="add", KERNEL=="sd?", SUBSYSTEM=="block", ENV{ID_BUS}=="usb", \
    ENV{SYSTEMD_WANTS}="my-usb-rule.service"

Now udev will trigger my-usb-rule.service (which in turn will execute your script) on any usb storage device add event.

Don't forget to reload the configuration after you edit the rules/units:

udevadm control --reload
systemctl daemon-reload

Best Answer

Related Solutions

Udev “urbnum”

Udev rule to match any usb storage device

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