courses-compsci/gen/missing_semester_2020/_subsections/lesson-02.org

#+title: lesson 02 | course overview + the shell
#+HTML_HEAD: <link rel="stylesheet" type="text/css" href="../_share/media/css/missing-semester.css" />
#+HTML_HEAD: <link rel="stylesheet" type="text/css" href="../_share/media/css/org-media-sass/collapsible.css" />
#+HTML_HEAD: <script src="../_share/media/js/collapsible.js"></script>

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

- Curr: https://youtu.be/kgII-YWo3Zw?si=Wm8KLT1ggOGG8W-g&t=1692


*** 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]]
+ [[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][31:26 - finding files]] *current*
+ [[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

** error codes
*** commands to access codes or values
- $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

*** code values
- boolean
  - true returns an error code of 0
  - false returns an error code of 1

** using $@ and $*
- represents all the arguments passed to a script or a function as separate, individual strings. It preserves each argument as a distinct item, even if the argument contains spaces.

*** key points about $@
1. All Arguments: It refers to all the command-line arguments passed to the script.
2. Preserves Spaces: If an argument contains spaces, it is treated as a single argument.
3. Usage: It is commonly used in a loop to iterate over each argument.

*** example
#+begin_src bash
./myscript.sh file1.txt "file with spaces.txt" file3.txt
#+end_src

then $@ will be
=file1.txt "file with spaces.txt" file3.txt=

*** differences between $*
- *$@* treats each argument as a sepearte item
- *$** treats all arguments as a single, combined string


** logical operators
- you can use the error return value in conditionals
*** example using the OR conditional
  #+begin_src bash
  false || echo "Oops fail"
  "Oops fail"
  #+end_src

*** example using the AND conditional
  #+begin_src bash
  true && echo "this will print"
  "this will print"
  #+end_src

** concatenate commands
- you caoncatinate commands using the semicolon: *;*

  #+begin_src bash
  false; echo "this prints always"
  "this prints always"
  #+end_src

** common substitution
- take the output of a command and put it in a variable
 #+begin_src bash
 foo=$(pwd)
 echo $foo
 "/Users/ronny/..."
 #+end_src

- this can also be done by placing that format in a string and it will expand the string
  #+begin_src bash
  echo "the pwd output is: $(pwd)"
  "the pwd output is: /Users/ronny/..."
  #+end_src

** process substitution
*** description
Bash process substitution is a feature that allows you to use the output of a command or a process as if it were a file. It enables you to redirect input or output between processes in a flexible way without needing intermediate temporary files.

*** Syntax
- creates a temporary file descriptor for the output of the command, which can be used as an input file in another command.
  #+begin_src bash
   <(command)
  #+end_src
- creates a temporary file descriptor for writing to the command, which can be used as an output file in another command.
  #+begin_src bash
   >(command)
  #+end_src

*** examples
- this will take b, a, d, c and sort it so the result is 'a, b, c, d'
  #+begin_src bash
  sort <(echo -e "b\na") <(echo -e "d\nc")
  #+end_src

- this will list the files, send the list to *tee* which will split a portion off to the screen and the rest to grep, followed by text_files.txt

  #+begin_src bash
  ls | tee >(grep "txt" > text_files.txt)
  #+end_src

#+begin_src bash
echo "Starting program at $(date)" # Date will be substituted
echo "Running program $0 with $# arguments with pid $$"

for file in "$@"; do
    grep foobar "$file" > /dev/null 2> /dev/null

    # when pattern is not found, grep has exit status 1
    # we redirect STDOUT and STDERR to a null register since we do not care about them

    if [[ "$?" - ne 0 ]]; then
        echo "File $file does not have any foober adding one"
        echo "# foobar" >> "$file"
     fi
done
#+end_src

- *$$* pid given for program
- *$#* number of arguments
- *$@* expands to all the arguments
  - can be used in a for loop
- *2>* refers to STDERR
- *>* refers to STDOUT

** test utility

test is a bash utility that you can use to test the condition of a file.  look at the man page for more info

** curly braces
curly braces are used as a form of program command expansion.

the braces contain a number of arguments seperated by commans that will expand into arguments for the program

*** example
#+begin_src bash
touch foo{,1,2,10}
touch foo foo1 foo2 foo10

cp foo{,.old}
cp foo foo.old
#+end_src

*** ranges
this takes the format {1..20}

#+begin_src bash
touch directory{1..4}/foo{a..z}.txt
#+end_src

** python scripts
*** making them bash executable
- reference the python compiler at the top of the file
  - do it directly:
        #+begin_src bash
         #!/usr/local/bin/python
        #+end_src
 - use the env program to do so
        #+begin_src bash
          #!/usr/bin/env python
        #+end_src

*** args can be found through sys.argv[1:]
  #+begin_src python
import sys
for arg in reversed(sys.argv[1:]):
    print(arg)
  #+end_src

** checking for problems in bash scripts using 'shellcheck'
#+begin_src bash
shellcheck some_bash_script.sh
#+end_src

- will tell you problems
- and warnings

** man command
- works with both os, and installed tools
- 'tldr' is easier to read

** find