How to use the shell, terminal and the advanced tools

Introduction

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Why use English instead of Chinese when writing a blog? As time goes by, the more I have learned, the more I have to handle with the English documents or papers. So, I realized it was time for me to adapt to the full English environment to improve my ability.

Though it's not easy for me to write a blog in English, as my mother tongue isn't English, I still write this blog as the first step to determine that I will make full use of my energy to do so.

Just start it from "The Missing Semester of Your CS Education".

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Catalog

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How to use the shell, terminal and the advanced tools

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Permissions

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User

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root

can do anything
# : a root(shell)
$ : not a root(shell)
sudo : run command as a root
sudo su : switch as a root user

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Files and Directories

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ls -l : gives the additional information
- ---------- : 1+3+3+3
- d--------- : directory
  - --------- : permissions
    - 1-3 --- : set for the owner of the file
    - 4-6 --- : set for the group of the file
    - 7-9 --- : set for the others of the file
    - --- : read( r ), write( w ), execute( x )
      - File: allowed to read, write, and execute this file
      - directory :
        
        read: allowed to see which files are inside this directory
        
        write: allowed to rename, create, or remove files within that directory
        
        execute: allowed to enter this directory
      - - : do not have that permission
    - How about 7 ?
      - --- can be present by using the three bits binary digit
      - 7 = 111 : rwx
        
        1 : set the permission
        
        0 : not set the permission

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Command

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date : show the time
echo : display the given arguments that are separated by whitespace
- If the arguments consist of multiple words
  
  Use the "" or ' to quote it.
- You can use \ to escape single characters
which : allows us to know the path of the program that we want to run
tee : write the contents of input to a file and to the standard out
mv : change the location or name of the file
rm : remove a file
- rm -r : remove a directory and all the files within it
rmdir : remove the empty directory
mkdir : create a new directory
man : for manual pages of the program
cat : cat < files > standard output
cd : change the current working directory
- ~ : home directory
- - : previous directory
tail : Print the last 10 lines of each FILE to standard output
pwd : Print the name of the current working directory.
find
xdg-open
We can give a path to ls by offering arguments(flags | options)
- flags and options usually start with a -
- --help can print out a bunch of information about that command
  - Usage
    - ... : means zero or one or more options
    - [] : means optional
  - Funtion description
    - flag: - + single letter
      - No value content also is a flag
    - option: anything that does take a value

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In reality, we don't need to memorize them all, because we can query the usage of the command by adding the --help or use the man for more information about it.

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stream

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Every program by default has two primary streams
- Input stream : keyboard(default)
- output stream : terminal(default)
- rewire these streams
  - < : rewire the input for this program to be the contents of this file
  - > : rewire the output for this program to be the contents of this file
- >> : append the content but not overwrite the original one.
| : left program output right program input

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Path

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When we simply type the echo hello or date, it will work and print the result on the terminal, but how can our shell locate them? The answer is: by paths.

Our shell can locate the program through the environment variable. Paths are a way to name the location of a file on your computer.

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Environment variable: a variable

Things that are set whenever you start your shell
Example: echo $PATH

It shows you all of the paths on my machine that the shell will search for programs
What is paths?
- Linux/macOS: / separate the paths
- Windows: \ separate the paths
- Absolute paths: fully determine the location of a file
- Relative paths: relative to where you currently are
- Command pwd can show the present working directory
Working directory

All the relative paths are relative to the current working directory
- Configure the terminal can show the full path
- . : the current directory
- .. : the parent directory

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Run program

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Methods for running the program anyway
- method1: give the name of the program and let the shell figure out where it is as if you have added the absolute paths to the $PATH
- method2: give the absolute paths of the program
Program always works on the current working directory(without any other arguments)
Command ls can list all the files in the current directory
In fact, you can regard the shell and the Bash(Bourne Again Shell) as a kind of programing language
- You can run a program with arguments
- You can do things like while loops, for loops, conditionals, functions and variables
ctrl+L : clear the terminal and go back to the top

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Exercises

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Already on Linux
mkdir missing && cd missing
man touch
touch semester
echo '#!/bin/sh' > semester && echo 'curl --head --silent https://missing.csail.mit.edu' >> semester
./semester

ls -l

-rw-rw-r-- : no permission to execute the file
sh semester

#(root) run the /bin/sh

$ run the ./semester
man chmod
chmod u+x semester

./semester
./semester | grep 'last-modified' > ~/last-modified.txt
cat /sys/class/power_supply/BAT0/capacity_level

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References

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https://missing.csail.mit.edu/2020/course-shell/

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