Directories and Device Nodes

Question 1

The term file refers to…?

Answer 1

regular files, directories, symbolic links, device nodes, and others.

Question 2

All files have common attributes..?

Answer 2

User owner, group owner, permissions, and timing information.

Question 3

File meta-information is contained in…?

Answer 3

a data structure called inodes.

Question 4

File names are contained in a data structure called…?

Answer 4

directory entries (dentries)

Question 5

File meta-information can be examined with the…?

Answer 5

ls -l and stat commands.

Question 6

Dentries…?

Answer 6

associate filenames with inodes.

Question 7

Inodes which associate all of a file’s…?

Answer 7

attributes with its content.

Question 8

every directory is referenced at least twice…?

Answer 8

once by itself (as the directory “.”), and once by its parent (with an actual directory name, such as report).

See Diagram 3.2 Dentry Tables for the report, report/html, and report/html/figures directories.

https://academy.redhat.com/courses/rha030-6.1/rha030_fsmngt_dirdev.html#rha030_fsmngt_dirdev_discussion

Question 9

directories are simply collections of…?

Answer 9

dentries for the files the directory is said to contain, which map filenames to inodes.

Question 10

Every directory contains at least…?

Answer 10

2 links, one from its own directory entry “.”, and one from its parent’s entry with the directory’s conventional name. Directories are referenced by additional link for every subdirectory, which refer to the directory as “..”.

Question 11

Device nodes exist in the filesystem, but do not…?

Answer 11

contain data in the same way that regular files, or even directories and symbolic links, contain data. Instead the job of a device node is to act as a conduit to a particular device driver within the kernel.

Question 12

When a user writes to a device node, the device node transfers the information to…?

Answer 12

the appropriate driver in the kernel.

Question 13

When a user would like to collect information from a particular device, they read from…?

Answer 13

that device’s associated device node, just as reading from a file.

Question 14

By convention, device nodes live within a dedicated directory called…?

Answer 14

/dev

(As there are over 7000 entries in the /dev directory, the output has been truncated to only the first several files. Focusing on the first character of each line, most of the files within /dev are not regular files or directories, but instead either character device nodes (“c”), or block device nodes (“b”). The two types of device nodes reflect the fact that device drivers in Linux fall into one of two major classes, character devices and block devices.)

Question 15

Block devices are devices that …?

Answer 15

read and write information a chunk (“block”) at a time.

Question 16

Block devices customarily allow random access, meaning that a block of data could be …?

Answer 16

read from anywhere on the device, in an order. Examples of block devices include hard drives, floppy drives, and CD/ROM drives.

Question 17

Character devices are often devices that…?

Answer 17

read and write information as streams of bytes (“characters”).

Question 18

There is a natural concept of what it means to read or write the…?

Answer 18

“next” character.

Question 19

Examples of character devices include …?

Answer 19

keyboards, mice, sound cards, and printers. Some character devices drivers support memory buffers as well.

Question 20

The real distinction between character and block device drivers relates to how the …?

Answer 20

devices interacts with the Linux kernel.

Block devices (“disks”) interact with the unified I/O cache, while character devices bypass the cache and interact with processes directly.

Question 21

When a user logs in, they take ownership of…?

Answer 21

the device node that controls their terminal.

Processes that they run are then able to read input or write output to the terminal. In general, the permissions on device nodes do not allow standard users to access devices directly. Two categories of exceptions occur.

Question 22

Because users need to be able to communicate with the system, they (or, more exactly, the processes that they run) must be able to read from and write to…?

Answer 22

the terminal they are using. Usually, part of the process of logging into a system involves transferring ownership of the terminal’s device node to the user.

Question 23

In Red Hat Enterprise Linux, users can be granted special permissions not because of who they are, but because of where…?

Answer 23

they logged in from.

Namely, when a user logs into a virtual console, or the graphical X server, they are considered a “console user”.

Question 24

Console users are granted access to hardware devices associated with the console, such as …?

Answer 24

the floppy drive and sound card.

Question 25

When the console user logs out of the system, ownerships for these devices are restored to…?

Answer 25

system defaults.

(None of this happens if a user logs in over the network, for example. (If the user is not sitting at the machine, would it be reasonable for them to use the floppy drive?)

Question 26

Linux (and Unix) uses device nodes to allow users access to devices…?

Answer 26

on the system.

Question 27

The managing of devices on a Linux system can be…?

Answer 27

A large and complicated topic.

Question 28

When writing programs, programmers do not need to deal with …?

Answer 28

device details. They can treat all input and output as if they were simply reading or writing to a file.

Question 29

Access to devices can be controlled through the same techniques of…?

Answer 29

file ownerships permissions that are used for regular files.