Dev means in linux

What does /dev/sda in Linux mean?

stand for? What does it mean? I have both Fedora and Ubuntu installed and if I explore them using Ext2explore from Windows, I see these names:

3 Answers 3

TL;DR: It has to do with the way Linux (and other Unixes as well) name their drives, much in the way that Windows uses C:, D:, etc. (NOTE: This is what we call a metaphor. In other words, a blatant lie that helps people understand without being even remotely accurate. Read on for a more detailed explanation. )

• /dev/ is the part in the Unix directory tree that contains all «device» files — Unix traditionally treats just about everything you can access as a file to read from or write to.
• sd originally identified a SCSI device, but since the proliferation of USB (and other removable) data carriers, it became a catch-all for any block device (another Unix term; in this context, anything capable of carrying data) that wasn’t already accessible via IDE. When SATA came around, the developers figured it’d be much easier and much more convenient for everyone to add it into the existing framework rather than write a whole new framework.
• The letter immediately after sd signifies the order in which it was first found — a,b,c. z, Aa. Az. etc. (Not that there are many situations in the real world where more than 26 discrete block devices are on the same bus. )
• Finally, the number after that signifies the partition on the device. Note that because of the rather haphazard way PCs handle partitioning, there are only four «primary» partitions, so the numbering will be slightly off from the actual count. This isn’t a terrible problem as the main purpose for the naming scheme is to have a unique and recognizable identifier for each partition found in this manner.

To answer your specific question: /dev/sda9 means the ninth partition on the first drive.

A metaphor is usually related in some sort of tangential way that is more closely related to the addressee’s experience than the concept or thing one wishes to explain. So one might explain the workings of a processor in terms of tractors and muddy boots to a farmer, for example. The hope is the addressee will be able to utilise the symbolic link(s) in order to mentally construct a working model of whatever the addresser is describing, without first-hand knowledge. So it’s only really a lie in the same way a block-chain is a lie about a pass-phrase. In fact, a blockchain is a sort of metaphor.

/dev is your filesystem representation of devices your system understands — providing a mechanism for applications to access data on the device without needing to know specifically what the device is.

sd is for (originally) scsi disk devices, however it seems to now refer to removable devices in general and SATA devices

and the letter is just the number of the device, starting at a, with the number indicating the partition.

sd originates from the driver sd-mod . It literally stands for scsi disk.

The reason (S)ATA disks are also listed as SCSI disks is, SCSI commands pretty much provides a superset of features that can be provided by ATA commands, therefore modern systems (including Windows, AFAIK) will have an implementation of SCSI-ATA Translation Layer (SATL) in the system (in Linux it is provided by the libata driver) to talk to the (S)ATA disks, while the upper layer of the system can be generalized.

As you may not aware of, USB drives «speaks» SCSI (i.e. takes and responds to SCSI commands), no matter if it supports the USB Attached SCSI Protocol or not. Also, most of the USB HDDs/SSDs are SATA disks bridged to USB. For those the bridge provides the SATL, but not the operating system.

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Understanding the /dev Directory in Linux

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1. Introduction

Linux and Unix-like operating systems follow a Filesystem Hierarchy Standard (FHS). The FHS defines the structure of the file system. In other words, it defines the main directories and their contents. One of the most important directories on the system is the /dev/ directory. In this tutorial, we’ll get a deeper understanding of the /dev/ directory and its functions.

2. What Is the Function of the /dev Directory?

The /dev/ directory consists of files that represent devices that are attached to the local system. However, these are not regular files that a user can read and write to; these files are called devices files or special files:

[[email protected] dev]# ls -l total 0 crw-r--r--. 1 root root 10, 235 Sep 29 12:08 autofs drwxr-xr-x. 3 root root 60 Sep 29 12:07 bus lrwxrwxrwx. 1 root root 3 Sep 29 12:10 cdrom -> sr0 drwxr-xr-x. 2 root root 3100 Sep 29 12:10 char crw-------. 1 root root 5, 1 Sep 29 12:08 console lrwxrwxrwx. 1 root root 11 Sep 29 12:07 core -> /proc/kcore drwxr-xr-x. 3 root root 60 Sep 29 12:07 cpu crw-------. 1 root root 10, 62 Sep 29 12:08 cpu_dma_latency drwxr-xr-x. 7 root root 140 Sep 29 12:08 disk brw-rw----. 1 root disk 253, 0 Sep 29 12:08 dm-0 . 

Device files are abstractions of standard devices that applications interact with via I/O system calls. The device files that correspond to hardware devices fall into two main categories. Mainly character special files and block special files.

3. What Is the Difference Between Character Special Files and Block Special Files?

Character special files are simple interfaces to character devices. Likewise, block special files are simple interfaces to block devices. The difference between these devices depends on how the operating system reads data off of them. A driver communicates with a character device by sending single characters as data such as bytes. In addition, character devices don’t require buffering when communicating with a driver. On the other hand, a driver accesses data from block devices through a cache. Moreover, a driver communicates with a block device by sending an entire block of data. For example, character devices are sound cards or serial ports, whereas block devices are hard disks or USBs. We identify block and character devices by the letter that appears in front of the permissions. The letter ‘b’ that is displayed in the first column denotes a block device. On the other hand, the letter ‘c’ shown in the first column symbolizes a character device:

[[email protected] dev]# ls -l total 0 crw-r--r--. 1 root root 10, 235 Sep 16 11:52 autofs drwxr-xr-x. 2 root root 280 Sep 16 11:52 block drwxr-xr-x. 2 root root 60 Sep 16 11:52 bsg drwxr-xr-x. 3 root root 60 Sep 16 11:52 bus lrwxrwxrwx. 1 root root 3 Sep 16 13:05 cdrom -> sr0 drwxr-xr-x. 2 root root 3100 Sep 16 13:05 char crw-------. 1 root root 5, 1 Sep 16 11:52 console lrwxrwxrwx. 1 root root 11 Sep 16 11:52 core -> /proc/kcore drwxr-xr-x. 3 root root 60 Sep 16 11:52 cpu crw-------. 1 root root 10, 62 Sep 16 11:52 cpu_dma_latency drwxr-xr-x. 7 root root 140 Sep 16 11:52 disk brw-rw----. 1 root disk 253, 0 Sep 16 11:52 dm-0 brw-rw----. 1 root disk 253, 1 Sep 16 11:52 dm-1 brw-rw----. 1 root disk 253, 2 Sep 16 11:52 dm-2

The majority of the devices we see are character or block devices. However, other types of devices exist, such as socket and pipe devices.

4. What Are Pseudo-Devices?

Pseudo-devices don’t correspond to hardware devices. That is to say, they provide several useful functions. For example /dev/random is a function that generates random data. Pseudo-devices are character devices as well. We can tell the difference between pseudo-devices and other character devices using the major number of a device. Furthermore, device drivers associate with a device through a unique value called a major number. A major and minor number appear when running the ls -l command. A comma separates the two numbers. First, there is the major number then comes the minor number. In our case, the driver associated with pseudo-devices is driver 1 since our major number is 1:

[[email protected] dev]# ls -l |grep random crw-rw-rw-. 1 root root 1, 8 Sep 29 12:08 random

We can also view the major and minor numbers by running stat. These numbers are displayed under device type:

[[email protected] dev]# stat /dev/random File: /dev/random Size: 0 Blocks: 0 IO Block: 4096 character special file Device: 6h/6d Inode: 10216 Links: 1 Device type: 1,8 Access: (0666/crw-rw-rw-) Uid: ( 0/ root) Gid: ( 0/ root) Context: system_u:object_r:random_device_t:s0 Access: 2021-09-16 11:52:35.644848848 -0400 Modify: 2021-09-16 11:52:35.644848848 -0400 Change: 2021-09-16 11:52:35.644848848 -0400 Birth: -

5. Why Are There So Many Symbolic Links?

Some device files in the /dev/ directory don’t appear as block or character devices. Instead, symbolic links are listed:

[[email protected] dev]# ls -lhaF| grep ^l lrwxrwxrwx. 1 root root 3 Sep 17 09:07 cdrom -> sr0 lrwxrwxrwx. 1 root root 11 Sep 17 09:06 core -> /proc/kcore lrwxrwxrwx. 1 root root 13 Sep 17 09:06 fd -> /proc/self/fd/ lrwxrwxrwx. 1 root root 12 Sep 17 09:06 initctl -> /run/initctl| lrwxrwxrwx. 1 root root 28 Sep 17 09:06 log -> /run/systemd/journal/dev-log= lrwxrwxrwx. 1 root root 4 Sep 17 09:06 rtc -> rtc0 lrwxrwxrwx. 1 root root 15 Sep 17 09:06 stderr -> /proc/self/fd/2 lrwxrwxrwx. 1 root root 15 Sep 17 09:06 stdin -> /proc/self/fd/0 lrwxrwxrwx. 1 root root 15 Sep 17 09:06 stdout -> /proc/self/fd/1|

Symbolic links under the /dev directory are there for many reasons. For example, when looking at /dev/stdin, we see that it points to /proc/self/fd/0. This is because the/proc/self directory sits on the /proc filesystem. When a process reads /proc/self, it gets information about itself using its process id. So the/proc/self directory is a symbolic link as well:

[[email protected] dev]# ls -l /proc/self/fd/0 lrwx------. 1 root root 64 Sep 17 12:02 /proc/self/fd/0 -> /dev/pts/0

The subdirectory ../fd signifies a file descriptor. A file descriptor is a unique reference to a file or input/output (IO) resource. It assists in identifying the file or input/output resource that a process has to work on. For example, the/dev/pts/0 directory exists in memory. The contents under the directory are pseudo-terminals that allow applications to receive input and display output:

[[email protected] pts]# ls -l /dev/pts total 0 crw--w----. 1 root tty 136, 0 Sep 17 12:33 0 c---------. 1 root root 5, 2 Sep 17 09:06 ptmx

Additionally, another reason for symbolic links under the /dev directory is a name change. Some applications may still reference older names of files hence the use of symbolic links:

[[email protected] dev]# ls -l total 0 lrwxrwxrwx. 1 root root 3 Sep 17 09:07 cdrom -> sr0 

6. Conclusion

In this article, we looked at the /dev directory and the contents that fall under it. We got to understand the different special files and their respective functions. Furthermore, we discussed why some files are implemented as symbolic links. To sum it up, the /dev directory is a vital part of *nix systems. This is because it highlights the architecture of *nix systems, which is “everything is a file”.

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