compsci-missing_semester_2020/_subsections/lesson-01.org

#+title: Lesson 01 | Course Overview + Shell
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* Links
#+attr_html: :class links
- [[../toc.org][TOC | Missing Semester]]
- [[https://www.youtube.com/playlist?list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J][Playlist: Missing Semester]]

*** timestamps
:PROPERTIES:
:CUSTOM_ID: timestamp
:END:

#+attr_html: :class playlist
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=20s][00:20 why we're doing this class]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=251s][04:11 the shell]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=338s][05:38 install a terminal and a shell]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=364s][06:05 the shell prompt]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=411s][06:51 shell prompt]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=522s][08:42 how does the shell know what these programs are]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=672s][11:12 paths]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=738s][12:18 absolute path]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=759s][12:39 relative paths]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=1034s][17:14 tilde character]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=1336s][22:16 directories]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=1376s][22:56 execute on directories]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=1514s][25:14 cp command]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=1547s][25:47 rm]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=1590s][26:30 create a new directory]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=1912s][31:52 pipe character]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=2044s][34:04 grep]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=2145s][35:45 the root user]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=2239s][37:19 kernel paramters]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=2612s][43:32 scroll lock led]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=2706s][45:06 open a file]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=2829s][47:09 lecture notes]]
- [[https://www.youtube.com/watch?v=Z56Jmr9Z34Q&list=PLyzOVJj3bHQuloKGG59rS43e29ro7I57J&t=2875s][office hours]]

* Notes
** why
- there are many tools that make life easier for programmers
- lecture notes and recordings are available online

** the shell
- programs are meant to be integrates with each other through the shell
- most common shell is Bourne Again Shell (bash)

** shell prompt
- typical shell prompt:
        #+attr_html: :width 300px
        file:../_share/media/img/lecture01/ex01.png

- shells can be customized extensibly
- arguments are seperated by whitespace
- multiple words can be passed as a single argument via:
  - quotes
    #+begin_src bash
    echo "Hello world"
    #+end_src

  - escapes
    #+begin_src bash
    echo Hello\ world
    #+end_src

** how does the shell know where programs are
- via environment variable
  - things that are set whenever you start your shell
    - where is your home directory
    - what is your username
    - what are the PATHs to your programs
- shells, and bash in particular, are programming languages

** paths
*** PATH environment variable

- a list seperated by colons (:)
- when bash is asked to run a program bash will search through the directories in path until it finds the program

*** absolute vs relative path

- pwd: print working directory
- all relative paths are relative to your current working directory
- absolute path starts with '/'

*** special directories
- '.' current directory
- '..' parent directory

*** tilde (~)
- always expands to your home directory

*** dash (-)
- toggles to the previous directory you were at

  #+begin_src bash
  cd /home/ronny/programs/notes/courses
  cd ../../
  cd -
  pwd
  #+end_src

the output will be
=/home/ronny/programs/notes/courses=

** permissions
*** groups of three
- read
- write
- execute

*** directories
- read: are you allowed to see the files
- write: can you rename the files in the directory
- execute: "search" are you allowed to enter this directory
  - to access a file in a directory, you need execute permissions on all directories in the path

** piping
- every program has two streams
  - input stream
  - output stream
- shell allows you to change streams
  - angle brackets
  - left angle < : rewire input
  - right angle > : rewire output

- example
  #+begin_src bash
   echo hello > hello.txt
   cat < hello.txt
  #+end_src