This chapter describes the way to extract information about the devices and understand a few rudimentary operations.

Device files

When you list files in /dev,

$ ls -l /dev
crw-rw-rw-   1 root   tty       5,     2 Oct  6 19:28 ptmx
drwxr-xr-x   2 root   root             0 Oct  6 18:07 pts
crw-rw-rw-   1 root   root      1,     8 Oct  6 18:07 random
crw-rw-r--+  1 root   netdev   10,    62 Oct  6 18:07 rfkill
lrwxrwxrwx   1 root   root             4 Oct  6 18:07 rtc -> rtc0
crw-------   1 root   root    249,     0 Oct  6 18:07 rtc0
brw-rw----   1 root   disk      8,     0 Oct  6 18:07 sda
brw-rw----   1 root   disk      8,     1 Oct  6 18:07 sda1
brw-rw----   1 root   disk      8,     2 Oct  6 18:07 sda2
brw-rw----   1 root   disk      8,     3 Oct  6 18:07 sda3
...

You can find device files. These device files are sometimes called "device nodes".

The first characther in each row represents a type of a device:

• b: block device
• c: character device
• p: pipe device
• s: socket device

One or two numbers before the date are the major and minor device numbers that help the kernel identify the device.

Block device

Block devices typically refer to disk devices. Since the total size of the devices are fixed, they are easily indexed and processes can randomly access to any block.

Character device

Character devices work with data streams.
You can only read characters from or write characters to character devices.
Character devices do not have a size.

Pipe device

skip

Socket device

Sockets are special-purpose interfaces that are frequently used for interprocess communication.

Remark

Not all devices have device files because the block and character device I/O interfaces are not appropriate in all cases (e.g., network devices)

The issues of /dev device files?

/dev directory is a convenient way for user processes to handle devices, however, there are several issues for kernel:

• Simple names do not tell much about the info of device
• Device names are assigned in the order in which they are found (one physical device can get different name on reboot)

To overcome these, Linux kernel offers the sysfs interface through files and directories (/sys/devices). For example, a hard disk at /dev/sda might have the following path in sysfs:

/sys/devices/pci0000:00/0000:00:1f.2/host0/target0:0:0/0:0:0:0/block/sda

Note that these two paths have different purposes. The /dev file is for user processes to use the device, whereas the /sys/devices path is for viewing information and manage the device**.

Device Name summary

Hard disks: /dev/sd*

The kernel makes separate device files for the partitions on a disk.

Terminals: /dev/tty*, /dev/pts/*, and /dev/tty

Terminals are devices for moving characters between a user process and an I/O device, usually for text output to a terminal screen.

Column: Display Modes and VIrtual Consoles

Linux has two display modes: text-mode and X Window System server (graphics mode).
Linux supports virtual consoles to multiplex the display. Each virtual console can run either in text or graphics mode. When you are in text-mode, you can switch between virtual consoles by pressing ALT-F* (function key like F1, F2). When you are in X11 session, you can do that by CTRL-ALT-F*.

Contents above are entirely based on How Linux Works, 2nd Edition: What Every Superuser Should Know