Missing Semesters (2020) 1강 The Shell

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Shell

  • One of the primary ways of interacting with the computer
  • Visual interfaces are limited
    • You can only use the functions that the buttons provide.
  • Most platforms provide some sort of shells
    • Windows - Powershell
    • Linux - Bash shell
    • MacOS - Bash shell

 

Shell prompt

  • We can customise our shell prompts
  • We get to write shell commands on the shell prompts
    • We can execute a program with arugments
    • Examples:
      • date prints today’s date
      • echo prints out the arguments given
        • arguments are something that’s separated by a blank space
          • echo hello.
          • echo "Hello world"
          • echo Hello\ world
  • How does the shell know what these programs are?
    • Your OS comes with some built-in programs, stored on your file system.
    • Environment variable
      • Things that are set when we start the terminal.
        • e.g. Where is the home directory? What is my user name?
      • Path variable
        • echo $Path will show all the directories that the shell will search for programs.
        • When we try to run a program on terminal, bash will search through all the paths stated in the path variables for the file/program that matches with the program we typed onto the terminal.
          • e.g. Bash will search for a file called echo throughout all paths stored in the path variable.
          • If I’m not sure which echo I am running, then I can use which echo command to find out what program I am running.

 

  • UNIX systems use / as the root directory. Windows doesn’t.
  • Use pwd to print my working directory.
  • Use cd to change my current working directory.
    • We can use relative paths to navigate around system
    • cd / - Absolute path for root directory
    • cd ... cd ./home, cd ../../../../../
    • Use cd ~ to go to home directory (i.e. /home/username/).
    • Use cd - to go back to the directory you were at.
  • Use ls to see what files and directories are in the current working directory.
  • Use program_name --help to print out what flags and arguments we can use for the program.
    • [] parameters are optional parameters
    • ... means multiple of them.
  • man program_name will show the program manual
  • mv will move files
  • cp will copy files
  • rm will remove files
    • rm -r will remove file/directory in recursive manner
    • rmdir will remove an empty directory

 

Permissions

  • Use ls -l to view files and directories in long listing format.
    • The first few letters tell us about many things
    • If the first letter is d, it’s a directory. Otherwise it’s a file.
    • The rest of the letters mean the permissions granted for each user groups:
      • The owner of the file
      • The group that owns this file
      • Anyone else
    • r - read permission
    • w - write permission
    • x - execute permission
    • - - Don’t have permission

 

Streams

  • Every program has 2 primary streams by default
    • Input stream
    • Output stream
  • < or > will rewire the streams
    • < rewire the input of the program to be the contents of a file
    • > rewire the output of the program into a file.
    • e.g. echo hello > hello.txt will save ‘hello’ to hello.txt. Nothing will print, but the content of hello.txt will be hello. We can check this by cat hello.txt. cat will print the contents of a file.
    • We can also use cat < hello.txt
    • We can copy contents of a file without using cp, by doing cat < hello.txt > hello2.txt. This will print the contents of ‘hello.txt’ and output it to ‘hello2.txt’.
  • >> will append the output.
    • cat < hello.txt > hello2.txt, then cat < hello.txt >> hello2.txt will print hello\n hello.
  • We can use | to make more sophisticated streams.
    • | will take the output of the left side’s program, to be the input of the right side’s program.
    • e.g. ls -l / | tail -n1. ls -l / will print the files in root directory in long format. tail -n1 will read data and output the last line. Overall, this code will output the last line of the ls -l /.
    • e.g. curl --head --silent google.com | grep --ignore-case content-length | cut --delimiter=' ' -f2. The first part will grab the information of ‘google.com’. The second part uses grep command to extract the information with the header ‘content-length’. The third part then cuts the string by a space, and then outputs the second field (which is the value of the content-length of google.com).

 

Root user mode

  • Use sudo to activate superuser mode.
    • ‘do as superuser’.
    • You don’t want to run as superuse all the time. Often critical failures mas superuse may lead to complete system failure.
  • Use cd /sys
    • These files are not actual files in your system.
    • These are various kernels parameters (i.e. the core of your computer)
    • e.g. cd /sys/backlight/intel_backlight/, cat brightness to show the current brightness of the screen. We can use sudo su to activate root user mode, and then use sudo echo 500 > brightness to modify the brightness.
      • When running as a superuser (i.e. root user), we will see # on the terminal.
        • We use exit to exit the root user mode.
      • When not running as a super user, we see $ on the terminal.
    • Running echo 1000 | sudo tee brightness will work without being a root user. The tee command will write the input data to a file, but also will print the data at the same time.

 

How to open a file

  • Use xdg-open to open a file in Linux. This will use an appropriate program to open a file.
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