man `8 UDEVADM`

UDEVADM(8) udevadm UDEVADM(8)

NAME
udevadm - udev management tool

SYNOPSIS
udevadm [--debug] [--version] [--help]

udevadm info [options] [devpath]

udevadm trigger [options] [devpath]

udevadm settle [options]

udevadm control option

udevadm monitor [options]

udevadm test [options] devpath

udevadm test-builtin [options] command devpath

udevadm wait [options] device|syspath

udevadm lock [options] command

DESCRIPTION
udevadm expects a command and command specific options. It controls the
runtime behavior of systemd-udevd, requests kernel events, manages the
event queue, and provides simple debugging mechanisms.

OPTIONS
-d, --debug
Print debug messages to standard error. This option is implied in
udevadm test and udevadm test-builtin commands.

-h, --help
Print a short help text and exit.

udevadm info [options] [devpath|file|unit...]
Query the udev database for device information.

Positional arguments should be used to specify one or more devices.
Each one may be a device name (in which case it must start with /dev/),
a sys path (in which case it must start with /sys/), or a systemd
device unit name (in which case it must end with ".device", see
systemd.device(5)).

-q, --query=TYPE
Query the database for the specified type of device data. Valid
TYPEs are: name, symlink, path, property, all.

--property=NAME
When showing device properties using the --query=property option,
limit display to properties specified in the argument. The argument
should be a comma-separated list of property names. If not
specified, all known properties are shown.

--value
When showing device properties using the --query=property option,
print only their values, and skip the property name and "=".

Cannot be used together with -x/--export or -P/--export-prefix.

-p, --path=DEVPATH
The /sys/ path of the device to query, e.g.
[/sys/]/class/block/sda. This option is an alternative to the
positional argument with a /sys/ prefix. udevadm info
--path=/class/block/sda is equivalent to udevadm info
/sys/class/block/sda.

-n, --name=FILE
The name of the device node or a symlink to query, e.g.
[/dev/]/sda. This option is an alternative to the positional
argument with a /dev/ prefix. udevadm info --name=sda is
equivalent to udevadm info /dev/sda.

-r, --root
Print absolute paths in name or symlink query.

-a, --attribute-walk
Print all sysfs properties of the specified device that can be used
in udev rules to match the specified device. It prints all devices
along the chain, up to the root of sysfs that can be used in udev
rules.

-t, --tree
Display a sysfs tree. This recursively iterates through the sysfs
hierarchy and displays it in a tree structure. If a path is
specified only the subtree below and its parent directories are
shown. This will show both device and subsystem items.

-x, --export
Print output as key/value pairs. Values are enclosed in single
quotes. This takes effects only when --query=property or
--device-id-of-file=FILE is specified.

-P, --export-prefix=NAME
Add a prefix to the key name of exported values. This implies
--export.

-d, --device-id-of-file=FILE
Print major/minor numbers of the underlying device, where the file
lives on. If this is specified, all positional arguments are
ignored.

-e, --export-db
Export the content of the udev database.

-c, --cleanup-db
Cleanup the udev database.

-w[SECONDS], --wait-for-initialization[=SECONDS]
Wait for device to be initialized. If argument SECONDS is not
specified, the default is to wait forever.

-h, --help
Print a short help text and exit.

--no-pager
Do not pipe output into a pager.

The generated output shows the current device database entry in a terse
format. Each line shown is prefixed with one of the following
characters:

Table 1. udevadm info output prefixes
+-------+----------------------------+
|Prefix | Meaning |
+-------+----------------------------+
|"P:" | Device path in /sys/ |
+-------+----------------------------+
|"M:" | Device name in /sys/ (i.e. |
| | the last component of |
| | "P:") |
+-------+----------------------------+
|"R:" | Device number in /sys/ |
| | (i.e. the numeric suffix |
| | of the last component of |
| | "P:") |
+-------+----------------------------+
|"U:" | Kernel subsystem |
+-------+----------------------------+
|"T:" | Kernel device type within |
| | subsystem |
+-------+----------------------------+
|"D:" | Kernel device node |
| | major/minor |
+-------+----------------------------+
|"I:" | Network interface index |
+-------+----------------------------+
|"N:" | Kernel device node name |
+-------+----------------------------+
|"L:" | Device node symlink |
| | priority |
+-------+----------------------------+
|"S:" | Device node symlink |
+-------+----------------------------+
|"Q:" | Block device sequence |
| | number (DISKSEQ) |
+-------+----------------------------+
|"V:" | Attached driver |
+-------+----------------------------+
|"E:" | Device property |
+-------+----------------------------+

udevadm trigger [options] [devpath|file|unit]
Request device events from the kernel. Primarily used to replay events
at system coldplug time.

Takes device specifications as positional arguments. See the
description of info above.

-v, --verbose
Print the list of devices which will be triggered.

-n, --dry-run
Do not actually trigger the event.

-q, --quiet
Suppress error logging in triggering events.

-t, --type=TYPE
Trigger a specific type of devices. Valid types are "all",
"devices", and "subsystems". The default value is "devices".

-c, --action=ACTION
Type of event to be triggered. Possible actions are "add",
"remove", "change", "move", "online", "offline", "bind", and
"unbind". Also, the special value "help" can be used to list the
possible actions. The default value is "change".

--prioritized-subsystem=SUBSYSTEM[,SUBSYSTEM...]
Takes a comma separated list of subsystems. When triggering events
for devices, the devices from the specified subsystems and their
parents are triggered first. For example, if
--prioritized-subsystem=block,net, then firstly all block devices
and their parents are triggered, in the next all network devices
and their parents are triggered, and lastly the other devices are
triggered. This option can be specified multiple times, and in that
case the lists of the subsystems will be merged. That is,
--prioritized-subsystem=block --prioritized-subsystem=net is
equivalent to --prioritized-subsystem=block,net.

-s, --subsystem-match=SUBSYSTEM
Trigger events for devices which belong to a matching subsystem.
This option supports shell style pattern matching. When this option
is specified more than once, then each matching result is ORed,
that is, all the devices in each subsystem are triggered.

-S, --subsystem-nomatch=SUBSYSTEM
Do not trigger events for devices which belong to a matching
subsystem. This option supports shell style pattern matching. When
this option is specified more than once, then each matching result
is ANDed, that is, devices which do not match all specified
subsystems are triggered.

-a, --attr-match=ATTRIBUTE=VALUE
Trigger events for devices with a matching sysfs attribute. If a
value is specified along with the attribute name, the content of
the attribute is matched against the given value using shell style
pattern matching. If no value is specified, the existence of the
sysfs attribute is checked. When this option is specified multiple
times, then each matching result is ANDed, that is, only devices
which have all specified attributes are triggered.

-A, --attr-nomatch=ATTRIBUTE=VALUE
Do not trigger events for devices with a matching sysfs attribute.
If a value is specified along with the attribute name, the content
of the attribute is matched against the given value using shell
style pattern matching. If no value is specified, the existence of
the sysfs attribute is checked. When this option is specified
multiple times, then each matching result is ANDed, that is, only
devices which have none of the specified attributes are triggered.

-p, --property-match=PROPERTY=VALUE
Trigger events for devices with a matching property value. This
option supports shell style pattern matching. When this option is
specified more than once, then each matching result is ORed, that
is, devices which have one of the specified properties are
triggered.

-g, --tag-match=TAG
Trigger events for devices with a matching tag. When this option is
specified multiple times, then each matching result is ANDed, that
is, devices which have all specified tags are triggered.

-y, --sysname-match=NAME
Trigger events for devices for which the last component (i.e. the
filename) of the /sys/ path matches the specified PATH. This option
supports shell style pattern matching. When this option is
specified more than once, then each matching result is ORed, that
is, all devices which have any of the specified NAME are triggered.

--name-match=NAME
Trigger events for devices with a matching device path. When this
option is specified more than once, then each matching result is
ORed, that is, all specified devices are triggered.

-b, --parent-match=SYSPATH
Trigger events for all children of a given device. When this option
is specified more than once, then each matching result is ORed,
that is, all children of each specified device are triggered.

--initialized-match, --initialized-nomatch
When --initialized-match is specified, trigger events for devices
that are already initialized by systemd-udevd, and skip devices
that are not initialized yet.

When --initialized-nomatch is specified, trigger events for devices
that are not initialized by systemd-udevd yet, and skip devices
that are already initialized.

Typically, it is essential that applications which intend to use
such a match, make sure a suitable udev rule is installed that sets
at least one property on devices that shall be matched. See also
Initialized Devices section below for more details.

WARNING: --initialized-nomatch can potentially save a significant
amount of time compared to re-triggering all devices in the system
and e.g. can be used to optimize boot time. However, this is not
safe to be used in a boot sequence in general. Especially, when
udev rules for a device depend on its parent devices (e.g. "ATTRS"
or "IMPORT{parent}" keys, see udev(7) for more details), the final
state of the device becomes easily unstable with this option.

-w, --settle
Apart from triggering events, also waits for those events to
finish. Note that this is different from calling udevadm settle.
udevadm settle waits for all events to finish. This option only
waits for events triggered by the same command to finish.

--uuid
Trigger the synthetic device events, and associate a randomized
UUID with each. These UUIDs are printed to standard output, one
line for each event. These UUIDs are included in the uevent
environment block (in the "SYNTH_UUID=" property) and may be used
to track delivery of the generated events.

--wait-daemon[=SECONDS]
Before triggering uevents, wait for systemd-udevd daemon to be
initialized. Optionally takes timeout value. Default timeout is 5
seconds. This is equivalent to invoke invoking udevadm control
--ping before udevadm trigger.

-h, --help
Print a short help text and exit.

In addition, optional positional arguments can be used to specify
device names or sys paths. They must start with /dev/ or /sys/
respectively.

udevadm settle [options]
Watches the udev event queue, and exits if all current events are
handled.

-t, --timeout=SECONDS
Maximum number of seconds to wait for the event queue to become
empty. The default value is 120 seconds. A value of 0 will check if
the queue is empty and always return immediately. A non-zero value
will return an exit code of 0 if queue became empty before timeout
was reached, non-zero otherwise.

-E, --exit-if-exists=FILE
Stop waiting if file exists.

-h, --help
Print a short help text and exit.

See systemd-udev-settle.service(8) for more information.

udevadm control option
Modify the internal state of the running udev daemon.

-e, --exit
Signal and wait for systemd-udevd to exit. No option except for
--timeout can be specified after this option. Note that
systemd-udevd.service contains Restart=always and so as a result,
this option restarts systemd-udevd. If you want to stop
systemd-udevd.service, please use the following:

systemctl stop systemd-udevd-control.socket systemd-udevd-kernel.socket systemd-udevd.service

-l, --log-level=value
Set the internal log level of systemd-udevd. Valid values are the
numerical syslog priorities or their textual representations:
emerg, alert, crit, err, warning, notice, info, and debug.

-s, --stop-exec-queue
Signal systemd-udevd to stop executing new events. Incoming events
will be queued.

-S, --start-exec-queue
Signal systemd-udevd to enable the execution of events.

-R, --reload
Signal systemd-udevd to reload the rules files and other databases
like the kernel module index. Reloading rules and databases does
not apply any changes to already existing devices; the new
configuration will only be applied to new events.

-p, --property=KEY=value
Set a global property for all events.

-m, --children-max=value
Set the maximum number of events, systemd-udevd will handle at the
same time.

--ping
Send a ping message to systemd-udevd and wait for the reply. This
may be useful to check that systemd-udevd daemon is running.

-t, --timeout=seconds
The maximum number of seconds to wait for a reply from
systemd-udevd.

-h, --help
Print a short help text and exit.

udevadm monitor [options]
Listens to the kernel uevents and events sent out by a udev rule and
prints the devpath of the event to the console. It can be used to
analyze the event timing, by comparing the timestamps of the kernel
uevent and the udev event.

-k, --kernel
Print the kernel uevents.

-u, --udev
Print the udev event after the rule processing.

-p, --property
Also print the properties of the event.

-s, --subsystem-match=string[/string]
Filter kernel uevents and udev events by subsystem[/devtype]. Only
events with a matching subsystem value will pass. When this option
is specified more than once, then each matching result is ORed,
that is, all devices in the specified subsystems are monitored.

-t, --tag-match=string
Filter udev events by tag. Only udev events with a given tag
attached will pass. When this option is specified more than once,
then each matching result is ORed, that is, devices which have one
of the specified tags are monitored.

-h, --help
Print a short help text and exit.

udevadm test [options] [devpath|file|unit]
Simulate a udev event run for the given device, and print debug output.

-a, --action=ACTION
Type of event to be simulated. Possible actions are "add",
"remove", "change", "move", "online", "offline", "bind", and
"unbind". Also, the special value "help" can be used to list the
possible actions. The default value is "add".

-N, --resolve-names=early|late|never
Specify when udevadm should resolve names of users and groups. When
set to early (the default), names will be resolved when the rules
are parsed. When set to late, names will be resolved for every
event. When set to never, names will never be resolved and all
devices will be owned by root.

-h, --help
Print a short help text and exit.

udevadm test-builtin [options] [command] [devpath|file|unit]
Run a built-in command COMMAND for device DEVPATH, and print debug
output.

-h, --help
Print a short help text and exit.

udevadm wait [options] [device|syspath] ...
Wait for devices or device symlinks being created and initialized by
systemd-udevd. Each device path must start with "/dev/" or "/sys/",
e.g. "/dev/sda", "/dev/disk/by-path/pci-0000:3c:00.0-nvme-1-part1",
"/sys/devices/pci0000:00/0000:00:1f.6/net/eth0", or
"/sys/class/net/eth0". This can take multiple devices. This may be
useful for waiting for devices being processed by systemd-udevd after
e.g. partitioning or formatting the devices.

-t, --timeout=SECONDS
Maximum number of seconds to wait for the specified devices or
device symlinks being created, initialized, or removed. The default
value is "infinity".

--initialized=BOOL
Check if systemd-udevd initialized devices. Defaults to true. When
false, the command only checks if the specified devices exist. Set
false to this setting if there is no udev rules for the specified
devices, as the devices will never be considered as initialized in
that case. See Initialized Devices section below for more details.

--removed
When specified, the command wait for devices being removed instead
of created or initialized. If this is specified, --initialized=
will be ignored.

--settle
When specified, also watches the udev event queue, and wait for all
queued events being processed by systemd-udevd.

-h, --help
Print a short help text and exit.

udevadm lock [options] [command] ...
udevadm lock takes an (advisory) exclusive lock on a block device (or
all specified devices), as per Locking Block Device Access[1] and
invokes a program with the locks taken. When the invoked program exits
the locks are automatically released and its return value is propagated
as exit code of udevadm lock.

This tool is in particular useful to ensure that systemd-
udevd.service(8) does not probe a block device while changes are made
to it, for example partitions created or file systems formatted. Note
that many tools that interface with block devices natively support
taking relevant locks, see for example sfdisk(8)'s --lock switch.

The command expects at least one block device specified via --device=
or --backing=, and a command line to execute as arguments.

--device=DEVICE, -d DEVICE
Takes a path to a device node of the device to lock. This switch
may be used multiple times (and in combination with --backing=) in
order to lock multiple devices. If a partition block device node is
specified the containing "whole" block device is automatically
determined and used for the lock, as per the specification. If
multiple devices are specified, they are deduplicated, sorted by
the major/minor of their device nodes and then locked in order.

This switch must be used at least once, to specify at least one
device to lock. (Alternatively, use --backing=, see below.)

--backing=PATH, -b PATH
If a path to a device node is specified, identical to --device=.
However, this switch alternatively accepts a path to a regular file
or directory, in which case the block device of the file system the
file/directory resides on is automatically determined and used as
if it was specified with --device=.

--timeout=SECS, -t SECS
Specifies how long to wait at most until all locks can be taken.
Takes a value in seconds, or in the usual supported time units, see
systemd.time(7). If specified as zero the lock is attempted and if
not successful the invocation will immediately fail. If passed as
"infinity" (the default) the invocation will wait indefinitely
until the lock can be acquired. If the lock cannot be taken in the
specified time the specified command will not be executed and the
invocation will fail.

--print, -p
Instead of locking the specified devices and executing a command,
just print the device paths that would be locked, and execute no
command. This command is useful to determine the "whole" block
device in case a partition block device is specified. The devices
will be sorted by their device node major number as primary
ordering key and the minor number as secondary ordering key (i.e.
they are shown in the order they'd be locked). Note that the number
of lines printed here can be less than the the number of --device=
and --backing= switches specified in case these resolve to the same
"whole" devices.

-h, --help
Print a short help text and exit.

INITIALIZED DEVICES
Initialized devices are those for which at least one udev rule already
completed execution - for any action but "remove" -- that set a
property or other device setting (and thus has an entry in the udev
device database). Devices are no longer considered initialized if a
"remove" action is seen for them (which removes their entry in the udev
device database). Note that devices that have no udev rules are never
considered initialized, but might still be announced via the sd-device
API (or similar).

EXAMPLE
Example 1. Format a File System

Take a lock on the backing block device while creating a file system,
to ensure that systemd-udevd doesn't probe or announce the new
superblock before it is comprehensively written:

# udevadm lock --device=/dev/sda1 mkfs.ext4 /dev/sda1

Example 2. Format a RAID File System

Similar, but take locks on multiple devices at once:

# udevadm lock --device=/dev/sda1 --device=/dev/sdb1 mkfs.btrfs /dev/sda1 /dev/sdb1

Example 3. Copy in a File System

Take a lock on the backing block device while copying in a prepared
file system image, to ensure that systemd-udevd doesn't probe or
announce the new superblock before it is fully written:

# udevadm lock -d /dev/sda1 dd if=fs.raw of=/dev/sda1

SEE ALSO
udev(7), systemd-udevd.service(8)

NOTES
1. Locking Block Device Access
https://systemd.io/BLOCK_DEVICE_LOCKING

systemd 252 UDEVADM(8)

man 8 UDEVADM

man `8 UDEVADM`