Autosave: 2023-02-03 14:51:52

Programming_Languages/Shell/Expansions_and_substitutions.md

@@ -14,12 +14,12 @@ Below are the main forms of expansion and substitution:
 | Representation | Name                 |
 | -------------- | -------------------- |
 | `~` .          | Tilde expansion      |
-| `{...}`        | Brace expansion      |
+| `{..}`         | Brace expansion      |
 | `${...}`       | Parameter expansion  |
 | `$(...)`       | Command substitution |
 | `$((...))`     | Arithmetic expansion |
 
-## Brace expansion
+## Brace expansion: `{..}`
 
 Brace expansion is for changing a smaller part of a greater whole programmatically. This is best understood by looking at examples:
 
@@ -44,7 +44,7 @@ echo {a...c}
 a b c
 ```
 
-We can also set sequences. If we wanted to count to twenty in intervals of two:
+We can also set sequences. If we wanted to count to twenty in intervals of two
 
 ```
 echo {1..20..2}
@@ -53,9 +53,9 @@ echo {1..20..2}
 
 > Note that we type _two_ dots **not** an elipsis
 
-## Example use case
+### Example use case
 
-We might use brace expansion to generate sequential file names, eg.
+We might use brace expansion to generate sequential file names using a pre-defined naming scheme, eg.
 
 ```
 touch file_{01..12}{a..d}
@@ -69,6 +69,28 @@ file_02a
 file_02b
 ...
 file_12d
-
-
 ```
+
+The syntax here basically means: for each of the elements in the first list, run the second list against them.
+
+## Parameter expansion: `${...}`
+
+We use most frequently for returning the value of stored [variables](/Programming_Languages/Shell/Variables_and_data_types.md). Techically we do not have to use the braces, we can retrieve with just `$var` however it's better to use them to minimise interpretation fuck-ups which happen a lot.
+
+When the braces are used, this allows us to transform the values before they are returned such as only returning from the 6th character: `${var:6}`.
+
+## Command substition: `$(...)`
+
+Command substitution (circle-brackets) allows us to put the output of one command inside another. Bash runs the bracketed command in a [sub-shell](/Programming_Languages/Shell/Shell_sessions.md) and then returns it to the main user shell.
+
+For example:
+
+```bash
+echo "The current directory is $(pwd)."
+```
+
+## Arithemtic expansion: `$((...))`
+
+We use arithmetic expansion when we want to calculate numerical values
+
+See [Working with numbers in Bash](/Programming_Languages/Shell/Working_with_numbers_in_Bash.md) for more.

Programming_Languages/Shell/Quote_marks.md

@@ -0,0 +1,31 @@
+---
+categories:
+  - Programming Languages
+tags:
+  - shell
+---
+
+# Quote marks
+
+## Single-quotes (aka _strong_ quotes)
+
+Bash will interpret everything in the string as a literal:
+
+```bash
+echo 'The directory is $(pwd)'
+# The directory is $(pwd)
+```
+
+## Double-quotes
+
+Bash will interpret strings as strings but will interpret expansions and substitutions as executable processes:
+
+```bash
+$pointlessVar='directory'
+
+echo "The ${pointlessVar}"
+
+# The directory is /home/thomas
+```
+
+It is therefore generally best to use double quotes whenever we wish to return mixed values.

Programming_Languages/Shell/Test.md

@@ -0,0 +1,12 @@
+---
+categories:
+  - Programming Languages
+tags:
+  - shell
+---
+
+# Test in bash
+
+`test` is a built-in command that is used to compare values or determine whether something is the case.
+
+When we run a test the result we get back is a return status of a `0` or a `1`

Programming_Languages/Shell/Variables_and_data_types.md

@@ -38,22 +38,6 @@ echo ${PRIM_VAR_BOOL} # true
 - there is no compunction to use capitals for variables but it can be helpful to distinguish custom variables from program variables (see below)
 - quotation marks at declaration are also not strictly necessary however they can help avoid bugs. Also serves as a reminder that every type is basically a string at the end of the day
 
-### Variables that hold references to programs
-
-We can store a reference to a bash program with slightly different syntax:
-
-```bash
-user="$(whoami)"
-```
-
-When we want to invoke a program variable we don't need to use brackets:
-
-```bash
-echo $user # thomasbishop
-```
-
-> Note that when we declare anything in bash (any time `=` is used) we **do not use spaces!** If you do, the variable will not be set.
-
 ## Declarations
 
 You can achieve a sort of typing through the `declare` keyword, although bear in mind this is not enforced and you do not have to use it.

Programming_Languages/Shell/Working_with_numbers_in_Bash.md

@@ -0,0 +1,75 @@
+---
+categories:
+  - Programming Languages
+tags:
+  - shell
+---
+
+# Working with numbers in Bash
+
+We distinguish:
+
+- **arithmetic expansion** `$(( ))`
+  - returns the result of literal numbers that can then be stored in a variable
+- **artihmetic evaluation** `(( ))`
+  - perform calculations on _existing_ variables
+
+An example of expansion:
+
+```bash
+a=3
+((a += 3))
+echo $a
+# 6
+((a++))
+# 7
+```
+
+> Note: we do not use a dollar-sign when referring to variables within arithmetic evaluation, there is no need. If we do, we get an error. This is because we are using an [expansion](/Programming_Languages/Shell/Expansions_and_substitutions.md), therefore the variables are already being interpreted as variables.
+
+## Declaring variables as integers
+
+It is good practice to safeguard against Bash treating numbers as strings to declare them as integers in addition to using arithmetic evaluation, e.g:
+
+```bash
+declare -i b=3
+```
+
+Whilst this isn't a strict type, it means we can do this:
+
+```bash
+b=$b+4
+echo b
+# 7
+```
+
+Without getting `3+4` in return
+
+## No decimals in bash
+
+Bash does not support decimal calculations natively. This is what you'd get for example:
+
+```bash
+echo $(( 1/3 ))
+# 0 (not 0.33)
+```
+
+So work with decimals you should use `awk` or `bc` ("basic calculations").
+
+Example of using `bc`:
+
+```bash
+declare -i c=1
+declare -i d=3
+e=$(echo "scale=3; $c/$d" | bc)
+# 0.333
+```
+
+## Random numbers
+
+Generate a pseudo-random number between 1 and 20:
+
+```bash
+echo $(( 1 + RANDOM % 10))
+# 18
+```