Device number in linux

Linux allocated devices (4.x+ version)В¶

This list is the Linux Device List, the official registry of allocated device numbers and /dev directory nodes for the Linux operating system.

The LaTeX version of this document is no longer maintained, nor is the document that used to reside at lanana.org. This version in the mainline Linux kernel is the master document. Updates shall be sent as patches to the kernel maintainers (see the Documentation/process/submitting-patches.rst document). Specifically explore the sections titled “CHAR and MISC DRIVERS”, and “BLOCK LAYER” in the MAINTAINERS file to find the right maintainers to involve for character and block devices.

This document is included by reference into the Filesystem Hierarchy Standard (FHS). The FHS is available from http://www.pathname.com/fhs/.

Allocations marked (68k/Amiga) apply to Linux/68k on the Amiga platform only. Allocations marked (68k/Atari) apply to Linux/68k on the Atari platform only.

This document is in the public domain. The authors requests, however, that semantically altered versions are not distributed without permission of the authors, assuming the authors can be contacted without an unreasonable effort.

DEVICE DRIVERS AUTHORS PLEASE READ THIS

Linux now has extensive support for dynamic allocation of device numbering and can use sysfs and udev ( systemd ) to handle the naming needs. There are still some exceptions in the serial and boot device area. Before asking for a device number make sure you actually need one.

To have a major number allocated, or a minor number in situations where that applies (e.g. busmice), please submit a patch and send to the authors as indicated above.

Keep the description of the device in the same format as this list. The reason for this is that it is the only way we have found to ensure we have all the requisite information to publish your device and avoid conflicts.

Finally, sometimes we have to play “namespace police.” Please don’t be offended. We often get submissions for /dev names that would be bound to cause conflicts down the road. We are trying to avoid getting in a situation where we would have to suffer an incompatible forward change. Therefore, please consult with us before you make your device names and numbers in any way public, at least to the point where it would be at all difficult to get them changed.

Your cooperation is appreciated.

Additional /dev/ directory entries¶

This section details additional entries that should or may exist in the /dev directory. It is preferred that symbolic links use the same form (absolute or relative) as is indicated here. Links are classified as “hard” or “symbolic” depending on the preferred type of link; if possible, the indicated type of link should be used.

Compulsory links¶

These links should exist on all systems:

/dev/fd	/proc/self/fd	symbolic	File descriptors
/dev/stdin	fd/0	symbolic	stdin file descriptor
/dev/stdout	fd/1	symbolic	stdout file descriptor
/dev/stderr	fd/2	symbolic	stderr file descriptor
/dev/nfsd	socksys	symbolic	Required by iBCS-2
/dev/X0R	null	symbolic	Required by iBCS-2

Recommended links¶

It is recommended that these links exist on all systems:

/dev/core	/proc/kcore	symbolic	Backward compatibility
/dev/ramdisk	ram0	symbolic	Backward compatibility
/dev/ftape	qft0	symbolic	Backward compatibility
/dev/bttv0	video0	symbolic	Backward compatibility
/dev/radio	radio0	symbolic	Backward compatibility
/dev/i2o*	/dev/i2o/*	symbolic	Backward compatibility
/dev/scd?	sr?	hard	Alternate SCSI CD-ROM name

Locally defined links¶

The following links may be established locally to conform to the configuration of the system. This is merely a tabulation of existing practice, and does not constitute a recommendation. However, if they exist, they should have the following uses.

/dev/mouse	mouse port	symbolic	Current mouse device
/dev/tape	tape device	symbolic	Current tape device
/dev/cdrom	CD-ROM device	symbolic	Current CD-ROM device
/dev/cdwriter	CD-writer	symbolic	Current CD-writer device
/dev/scanner	scanner	symbolic	Current scanner device
/dev/modem	modem port	symbolic	Current dialout device
/dev/root	root device	symbolic	Current root filesystem
/dev/swap	swap device	symbolic	Current swap device

/dev/modem should not be used for a modem which supports dialin as well as dialout, as it tends to cause lock file problems. If it exists, /dev/modem should point to the appropriate primary TTY device (the use of the alternate callout devices is deprecated).

For SCSI devices, /dev/tape and /dev/cdrom should point to the cooked devices ( /dev/st* and /dev/sr* , respectively), whereas /dev/cdwriter and /dev/scanner should point to the appropriate generic SCSI devices (/dev/sg*).

/dev/mouse may point to a primary serial TTY device, a hardware mouse device, or a socket for a mouse driver program (e.g. /dev/gpmdata ).

Sockets and pipes¶

Non-transient sockets and named pipes may exist in /dev. Common entries are:

/dev/printer	socket	lpd local socket
/dev/log	socket	syslog local socket
/dev/gpmdata	socket	gpm mouse multiplexer

Mount points¶

The following names are reserved for mounting special filesystems under /dev. These special filesystems provide kernel interfaces that cannot be provided with standard device nodes.

/dev/pts	devpts	PTY slave filesystem
/dev/shm	tmpfs	POSIX shared memory maintenance access

Terminal devices¶

Terminal, or TTY devices are a special class of character devices. A terminal device is any device that could act as a controlling terminal for a session; this includes virtual consoles, serial ports, and pseudoterminals (PTYs).

All terminal devices share a common set of capabilities known as line disciplines; these include the common terminal line discipline as well as SLIP and PPP modes.

All terminal devices are named similarly; this section explains the naming and use of the various types of TTYs. Note that the naming conventions include several historical warts; some of these are Linux-specific, some were inherited from other systems, and some reflect Linux outgrowing a borrowed convention.

A hash mark ( # ) in a device name is used here to indicate a decimal number without leading zeroes.

Virtual consoles and the console device¶

Virtual consoles are full-screen terminal displays on the system video monitor. Virtual consoles are named /dev/tty# , with numbering starting at /dev/tty1 ; /dev/tty0 is the current virtual console. /dev/tty0 is the device that should be used to access the system video card on those architectures for which the frame buffer devices ( /dev/fb* ) are not applicable. Do not use /dev/console for this purpose.

The console device, /dev/console , is the device to which system messages should be sent, and on which logins should be permitted in single-user mode. Starting with Linux 2.1.71, /dev/console is managed by the kernel; for previous versions it should be a symbolic link to either /dev/tty0 , a specific virtual console such as /dev/tty1 , or to a serial port primary ( tty* , not cu* ) device, depending on the configuration of the system.

Serial ports¶

Serial ports are RS-232 serial ports and any device which simulates one, either in hardware (such as internal modems) or in software (such as the ISDN driver.) Under Linux, each serial ports has two device names, the primary or callin device and the alternate or callout one. Each kind of device is indicated by a different letter. For any letter X, the names of the devices are /dev/ttyX# and /dev/cux# , respectively; for historical reasons, /dev/ttyS# and /dev/ttyC# correspond to /dev/cua# and /dev/cub# . In the future, it should be expected that multiple letters will be used; all letters will be upper case for the “tty” device (e.g. /dev/ttyDP# ) and lower case for the “cu” device (e.g. /dev/cudp# ).

The names /dev/ttyQ# and /dev/cuq# are reserved for local use.

The alternate devices provide for kernel-based exclusion and somewhat different defaults than the primary devices. Their main purpose is to allow the use of serial ports with programs with no inherent or broken support for serial ports. Their use is deprecated, and they may be removed from a future version of Linux.

Arbitration of serial ports is provided by the use of lock files with the names /var/lock/LCK..ttyX# . The contents of the lock file should be the PID of the locking process as an ASCII number.

It is common practice to install links such as /dev/modem which point to serial ports. In order to ensure proper locking in the presence of these links, it is recommended that software chase symlinks and lock all possible names; additionally, it is recommended that a lock file be installed with the corresponding alternate device. In order to avoid deadlocks, it is recommended that the locks are acquired in the following order, and released in the reverse:

1. The symbolic link name, if any ( /var/lock/LCK..modem )
2. The “tty” name ( /var/lock/LCK..ttyS2 )
3. The alternate device name ( /var/lock/LCK..cua2 )

In the case of nested symbolic links, the lock files should be installed in the order the symlinks are resolved.

Under no circumstances should an application hold a lock while waiting for another to be released. In addition, applications which attempt to create lock files for the corresponding alternate device names should take into account the possibility of being used on a non-serial port TTY, for which no alternate device would exist.

Pseudoterminals (PTYs)В¶

Pseudoterminals, or PTYs, are used to create login sessions or provide other capabilities requiring a TTY line discipline (including SLIP or PPP capability) to arbitrary data-generation processes. Each PTY has a master side, named /dev/pty[p-za-e][0-9a-f] , and a slave side, named /dev/tty[p-za-e][0-9a-f] . The kernel arbitrates the use of PTYs by allowing each master side to be opened only once.

Once the master side has been opened, the corresponding slave device can be used in the same manner as any TTY device. The master and slave devices are connected by the kernel, generating the equivalent of a bidirectional pipe with TTY capabilities.

Recent versions of the Linux kernels and GNU libc contain support for the System V/Unix98 naming scheme for PTYs, which assigns a common device, /dev/ptmx , to all the masters (opening it will automatically give you a previously unassigned PTY) and a subdirectory, /dev/pts , for the slaves; the slaves are named with decimal integers ( /dev/pts/# in our notation). This removes the problem of exhausting the namespace and enables the kernel to automatically create the device nodes for the slaves on demand using the “devpts” filesystem.

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4 Useful Way to Know Plugged USB Device Name in Linux

As a newbie, one of the many things you should master in Linux is identification of devices attached to your system. It may be your computer’s hard disk, an external hard drive or removable media such USB drive or SD Memory card.

Using USB drives for file transfer is so common today, and for those (new Linux users) who prefer to use the command line, learning the different ways to identify a USB device name is very important, when you need to format it.

Once you attach a device to your system such as a USB, especially on a desktop, it is automatically mounted to a given directory, normally under /media/username/device-label and you can then access the files in it from that directory. However, this is not the case with a server where you have to manually mount a device and specify its mount point.

Linux identifies devices using special device files stored in /dev directory. Some of the files you will find in this directory include /dev/sda or /dev/hda which represents your first master drive, each partition will be represented by a number such as /dev/sda1 or /dev/hda1 for the first partition and so on.

List All Linux Device Names

Now let’s find out device names using some different command-line tools as shown:

Find Out Plugged USB Device Name Using df Command

To view each device attached to your system as well as its mount point, you can use the df command (checks Linux disk space utilization) as shown in the image below:

Find USB Device Name Using df Command

Use lsblk Command to Find USB Device Name

You can also use the lsblk command (list block devices) which lists all block devices attached to your system like so:

List Linux Block Devices

Identify USB Device Name with fdisk Utility

fdisk is a powerful utility which prints out the partition table on all your block devices, a USB drive inclusive, you can run it will root privileges as follows:

List Partition Table of Block Devices

Determine USB Device Name with dmesg Command

dmesg is an important command that prints or controls the kernel ring buffer, a data structure which stores information about the kernel’s operations.

Run the command below to view kernel operation messages which will as well print information about your USB device:

dmesg – Prints USB Device Name

That is all for now, in this article, we have covered different approaches of how to find out a USB device name from the command line. You can also share with us any other methods for the same purpose or perhaps offer us your thoughts about the article via the response section below.

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