Missing-L1-Shell

Lecture1: The shell

​ 所有不加exercise的盲目啃理论都是耍流氓！

Topic1: The Shell

What is the shell?

现在随着计算机的高度发展，user interface的种类各式各样，graphical interface, voice interfaces, and even AR/VR.

shell也是interface的一种，是一种textual interface；是用户和计算机之间通信(communicate)连接的桥梁。

常见的shell搭载平台： terminal

shell prompt: execute/ type in arguments;

$User:\rightleftharpoons:PC. $ (中间就是搭载shell)这个桥梁

Using the shell

~$: date

~$: echo hello

我们所输入的这些命令(cmd)，某些有参数(arguments)，某些有flag(–version / -t)

空格在shell中的影响很大， 如果我们输入的一个参数它是由空格组成的话，我们可以’’或’将我们的参数引用起来。我觉得那种斜杠或者反斜杠的方式很容易记混淆

How does shell know how to find the dateor echo programs?

shell is a programming environment, just like Python or Ruby, and so it has variables, conditionals, loops, functions.

当你在shell 中运行你所输入的cmd的是欧，你实际是write a small bit of code that your shell interprets

如果你所输入的一个命令，即你ask the shell to execute the cmd that doesn’t match any of its programming keywords, 翻译来说，如果你所输入的cmd，不是shell 的built-in cmd,此时shell就会consult an environment variable called $PATH

$PATH 这个环境变量，path the way (in which directories) the shell should search for programs when it is given a cmd;

这个很重要，在L2的lecture notes中也有所体现，c.f.shebang的意义。

~$: echo $PATH
~$: which python
/usr/bin/python
~$: which echo
/usr/bin/echo

which: find out which file is executed for a given program.

by pass $PATH by giving the path to the file we want to execute;

~$: /bin/echo $PATH

Navigating in the shell

In general, when we run a program, it will operate in the current directory unless we tell it otherwise.

drwxr-xr-x

当某些程序加载出问题的时候，debug的思路可以转变为看看有没有运行权限。

** When you get permission denied errors, it is usually because you need to do something as root. **

Connecting programs

c.f. L4 data wranggling

Normally, a program’s input and output are both your terminal. That is, your keyboard as input and your screen as output.

The simplest form of redirection is < file and > file. These let you rewire the input and output streams of a program to a file respectively:

You can also use >> to append to a file. Where this kind of input/output redirection really shines is in the use of pipes.

The | operator lets you “chain” programs such that the output of one is the input of another

A versatile and powerful tool

Operations like |, >, and < are done by the shell, not by the individual program. echo and friends do not “know” about |.

|and >are set up by the shell

比如我想更改sys/backlight/brightness这个文件，

我可能在命令行中输入：

echo 500 > brightness

get permission denied;

然后我可能就会尝试：

sudo echo 500 > brightness

still get permission denied;

出现这个still get permission denied的原因就在于，

我输入sudo echo 500 > brightness是为了执行echo这个程序使用sudo权限，然后500 作为echo程序的arguments，然后把echo程序的输出重定向为brightness这个文件的输入。

既然想brightness这个文件的输入，shell 就要打开这个文件，但是open the brightness file也需要sudo的权限。

解决方式：

1. 在命令之前加入#prompt;

   当prompt 从$到#改变的时候，意味着su的权限；

2. 同样也有不直接进入root用户的解决方法，因为进入root用户有的时候可能随机更改了一些问题。

   echo 1600 | sudo tee brightness

echo 1600 send it to sudo tee cmd, tee cmd take the 1600(input) send to a standard out put(这里是一个file)

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One thing you need to be root in order to do is writing to the sysfs file system mounted under /sys.

sysfs exposes a number of kernel parameters as files, so that you can easily reconfigure the kernel on the fly without specialized tools.

Note that sysfs does not exist on Windows or macOS.

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Exercises and Review

1. | v.s >

2. 第六题中，Run the command by explicitly starting the sh interpreter, and giving it the file semester as the first argument, i.e. sh semester. Why does this work, while ./semester didn’t?

   是跟根目录有关吗？

   不是呀，是跟 执行权限有关系，昨天才把chmod命令反应过来今天就忘了真的是…

   -rwxr--r-- 1 chixinning staff 61 3 5 14:12 semster

   其实这个就是一个9+2位的permission bits,前3位是文件所有者，中3位是同组，后3位是其他用户。剩下2位设置组标识位和用户标示位。

   chmod(“filename”,0644)就是对应的是permission bits.

   6的2进制表示是110即对应rw-(1可以0不可以的逻辑嘛)

   这样的话我可以用chmod命令更改一下执行权限，这也是昨天在exercise中遇到的permission bits!

3. 视频真心常看常新！

4. 复习的工作总是辛苦的！

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谢谢你请我吃糖果❤️

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