compsci-missing_semester_2020/_subsections/lesson-02.org

#+title: lesson 02 | course overview + the shell
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#+HTML_HEAD: <link rel="stylesheet" type="text/css" href="../_share/media/css/org-media-sass/collapsible.css" />
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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=kgII-YWo3Zw&t=54s][00:54 - control flow functions]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=227s][03:47 - sequential execution]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=425s][07:05 - standard input]] *current*
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=444s][07:24 - error code]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=517s][08:37 - logical operators]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=603s][10:03 - concatenate commands]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=645s][10:45 - common substitution]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=675s][11:15 - process substitution]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=935s][15:35 - comparison operator]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=1173s][19:33 - curly braces]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=1359s][22:39 - python script]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=1707s][28:27 - man command]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=2175s][36:15 - finding files]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=2190s][36:30 - grep]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=2573s][42:53 - fuzzy finder]]
+ [[https://www.youtube.com/watch?v=kgII-YWo3Zw&t=2649s][44:09 - history substring search]]

* Notes

** bash
*** spaces are critical with bash
  - this works:
  #+begin_src bash
  foo=bar
  echo $foo
  #+end_src

  - this doesn't:
  #+begin_src bash
  foo = bar
  echo $foo
  #+end_src

    - the output of that will be
     =zsh: command not found: foo=

    - what happens in the above example is that we are effectively calling the "foo" program with the arguments: "=" and "bar"

*** quotes
- you can use double or single quotes to print a value
    #+begin_src bash
    echo "Hello"
    #+end_src
    #+begin_src bash
    echo 'Hello'
    #+end_src

- double quotes can interpolate variables
  #+begin_src bash
  echo "Value is $foo"
  #+end_src

  will return: =Value is bar=

- single quotes can NOT interpoloate variables
  #+begin_src bash
  echo 'Value is $foo'
  #+end_src

  will return: =Value is $foo=

** sequential execution

*** bash functions
- mcd.sh
#+begin_src bash
mcd() {
        mkdir -p "$1"
        cd "$1"
        }
#+end_src bash

- creates a function that can be executed after loading
  - $1 is a global variable referring to the first parameter

*** source
- using source mcd.sh
  =source mcd.sh=

- then carry out the function
  =mcd testdir=

- this will create a new directory and cd into it as per the definied functioned

*** variables
- $0 -- name of the script we are currently in
  - if you run this in the shell it will display 'bash'
- $1->$9 -- the first through ninth argument given to a script
- $? -- the last error message
- $_ --- last argument of the previous call

- !! -- recreates the last call with it's arguments
  - use with sudo if you found your call needed a sudo and you don't feel like retyping it again

 #+begin_src bash
 ls -lah
 sudo !!
 #+end_src