The Raspberry Pi is a small, affordable single-board computer that you will use to design and develop fun and practical IoT devices while learning programming and computer hardware. In addition, you will learn how to set up up the Raspberry Pi environment, get a Linux operating system running, and write and execute some basic Python code on the Raspberry Pi. You will also learn how to use Python-based IDE (integrated development environments) for the Raspberry Pi and how to trace and debug Python code on the device. Please note that this course does not include discussion forums.
Lecture 3.1 - Permissions
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树莓派（Raspberry Pi）平台及其 Python 程序设计
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从本节课中
Module 2
The Raspberry Pi is typically installed with a Linux-based operating system, so we present the basics of Linux and its use. We describe some of the main features including navigating the file system and managing processes. We describe the text-based user interface through the shell and we overview the graphic user interface which is the default with the Raspian Linux distribution.
与讲师见面
Ian HarrisProfessor
Department of Computer Science
[MUSIC]
So files have permission.
Files have owners first, a file has an owner,
an owner is the person who created the file.
The person being the user who created the file, so
the username of the person who created the file.
And files can also have permissions, access permissions.
Now the permissions generally come in three categories.
Read permission, write permission, and execute permission.
Read permission, means you can read the file,
write permission means you can write data to the file.
Execute means you can run the file, if it's a program you can run it.
So read, write, execute.
And the initials, the letters you use for those are R, W, and X for
each one of those permissions.
Now, different permissions can be assigned to different users
according to the type of user.
So there are three categories of permission, three grouping of people, or
groupings of users, for every file when you're talking about permissions.
So you can talk about permissions for the user.
The user is a file, is always referring to the file owner.
So the user permissions are the permissions for the file owner.
Note that the user can be prevented.
The user might prevent himself from writing to it for safety sake or
something like that.
So, you can have user permissions for the owner.
You can have group permissions that are permissions for a group of users.
So we haven't talked about this, but you can define a group of users.
For instance, say I've got two users, me and Joe,Ian and Joe.
I can make a group that's called my group and it just contains Ian and Joe.
Then that group can have permissions for access in this file.
So you can user permissions, group permissions, and
then you can other permissions, which is permissions for everybody else.
These permissions, we're not gonna talk about how to change them right now.
Let's just talk about how to see them.
The way you view file permissions generally is to type ls, get a,
ls is a directory, right?
ls -l, use the -l option,
it gives you the long form, and you see all the data about each file.
So here I'm doing ls -l, I got there's actually five files in here,
four of which are directory.
So desktop, indiecity, python_games, and testdir,
those are all directories, test1 is a file.
They color them differently but they all have permissions.
So the permissions you can see in the left-most column [COUGH].
So there's a d for the directory.
D designates directory and dash doesn't but after that you see nine characters.
Nine characters in a row and some of those characters are dashes.
So let's look at that first file, desktop, for a second, right?
If you look in its permission fields there's a d, telling me it's a directory,
but then there's rwx.
Then there's r-x, then there's r-x.
So, the r, w, and x stand for read, write, and execute.
The first three characters are the user permissions.
The next three characters are the group permissions.
The next three characters are the other permissions, for everybody else.
So for desktop, what that says is, the user can read or write or
execute the file or the directory.
In this case, a directory, but then, the group can only, if there were a group,
which there is not, but if there were a group, it could read and
execute, could not write to the directory.
And the other, which is everybody else in the world, can read from the directory,
and execute things in the directory, but it cannot write to the directory.
There's a dash there, instead of a W,
there's a dash, so they indicate they don't have write permission.
Now, if you look at that file, test1, right?
Test 1 a few rows down, the user has read permission and write permission but
no execute permission, which is fine cuz it's not an executable program.
Then the group and the other have only read permission, but no write permission.
So that's how you read the file permissions and you can change those to,
we'll talk about those another time.
Now, [COUGH] in addition to the user account that we're using,
we're using the pi account, that's the default account.
There's another account called the Root Account.
Linux machines all have a root account.
Root Account has the highest permission level.
So the Root Account has access permission for everything.
Key files and directories are only accessible by root.
So, if you look at [COUGH] a Linux operating system,
the operating system itself is composed of many files, okay?
There are many files and executables and things that make up the operating system.
And so some of these files and directories,
some subset of these are tagged only for root.
Only by root and only root has permission, read and write access permission, or
at least write access.
This is for safety's sake because if you start going around and
messing with some of these files you're actually altering the operating system
itself, altering features of the operating system.
If you don't know what you're doing you can ruin the operating system.
So some of these files have only root permission and
some of these directories only root can access them.
Now, sometimes you need root permission.
You don't wanna have root permission in general,
because it's a little bit dangerous, it's not safe.
If you do something in root, it's done.
If I accidentally delete a file and
it's some critical file, if you're not root it won't let you.
But if you're root, it will let you.
So root is dangerous to be in all the time.
But, there are certain cases where you need root permission,
say to install a program.
A lot of times installing programs you need to be root, because installing
a program requires putting files in all kinds of directories, and certain
directories are only accessible by root, and maybe you need to modify a registry,
I'm talking Windows, you need to modify some file that only root has access to.
So maybe installing programs a lot of time,
you need to be rude, not always, but sometimes.
Change in the operating systems, so
if you wanna modify the operating system, update the operating system,
installs drivers into the operating system, you often need to be root.
So there are a set of things that you need to do with root.
The way you generally do that is you use a command called Sudo, S-U-D-O.
SU is short for Switch User, that's a command, switch user DO.
So you prefix your command with Sudo.
So, let's say I wanna do LS, I wanna get a directory.
I type LS and that's fine, right?
But if I want to get it directly that only root has permission to look at,
I wanna look at that directory.
I can't just type ls, I have to type sudo ls, sudo ls and
it will do the ls command as if I were root.
So it just applies root permission to a single command which is a safer way
to do it.
You don't wanna generally become root unless you really know what you're doing.
Thank you.
[MUSIC]
