[talks/lbs] Added variables and data types slides for shells

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@@ -658,6 +658,133 @@ In summary, it installs bash, coreutils and shadow "packages" (from [http://s.mi
 
 ## 3.1. Shells
 
+&&lineb2
+
+### Variables and data types
+
+Before talking about the different features newer shells support, we'll roughly cover variables and data types (mostly [bash](https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Shell-Parameters) specifics).
+
+A variable, also called parameter, is created with the syntax (NO spaces between the =):
+
+```
+name=value
+```
+
+Where `name` is either a combination of letters (upper and lower case), numbers and the underscore character, which always begins with a letter, or one of select few **positional** and **special** parameters.
+
+&&/slide
+
+
+
+&&slide &&font35
+
+#### Positional and special parameters
+
+[Positional parameters](https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Positional-Parameters) are numbers from 1 to N, where each one corresponds to a shell (or function, or script) argument.
+
+Some of the [special parameters](https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Special-Parameters) are:
+
+- `0`, correspoinding to the shell's or script's name,
+- `@` is all positional parameters, separated by spaces
+- `#` is the number of positional parameters
+- `?` is the last executed (foreground) process
+- `$` is process ID of the current shell.
+
+`value`, without any special surounding characters (more on that later), is treated as a string (but it mustn't have spaces!), however it may also be omitted, in which case it is the empty string `""`.
+
+&&/font &&/slide
+
+
+
+&&slide &&font35
+
+#### Data types
+
+In most shells, there are 3 data types: strings, integers and (one-dimentional) indexed arrays.
+[bash has more](https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#index-declare), like references and associative arrays, but those aren't universal.
+
+A value is a **string** when surrounded by single `'` or double `"` quotes,  
+an **integer** when it isn't surrounded by anything and composed only of digits (if there are letters, it is a string; it's [quite tricky](https://tldp.org/LDP/abs/html/untyped.html) when a value is interpreted as a string or integer) and  
+an **array** when surrounded by braces `()`, where inside elements are strings or integers, separated by spaces.
+An indexed array can also be created by specifying the value at any index:
+
+```
+name[index]=value
+```
+
+&&/font &&/slide
+
+
+
+&&slide
+
+#### Mixing types and operations
+
+```
+name=value
+```
+
+| Type  |Value with|           string operation            |           integer operation           |               array operation               |
+|-------|----------|---------------------------------------|---------------------------------------|---------------------------------------------|
+|string |          |value                                  |0                                      |value when index is 0, empty string otherwise|
+|integer|          |empty string, but sometimes value      |value                                  |value when index is 0, empty string otherwise|
+|array  |          |operation is done on element at index 0|operation is done on element at index 0|value                                        |
+
+&&/slide
+
+
+
+&&slide
+
+#### Parameter expansion
+
+With [parameter expansion](https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Shell-Parameter-Expansion), actions are done with the general variable (entity) or on strings and arrays values.
+Operations on integers are done with [arithmetic expansion](https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Arithmetic-Expansion). 
+
+Parameter expansion is started with the character `$` and either a name or curly braces.
+Retrieving a value is done with the forms `$name`, `${name}` and `${name[index]}` for arrays.
+
+Some parameter expansions in bash:
+
+- `${#parameter}` length of characters
+- `${parameter:offset}` and `${parameter:offset:length}` return substring from index offset
+- `${parameter/pattern/string}` return a string, where every match on pattern is replaced by string
+- `${!name[@]}` a (space separated) sequence of indecies from 0 to length (for defined values) of array name
+
+&&/slide
+
+
+
+&&slide
+
+#### Arithmetic expansion
+
+Arithmetic expansions allow evaluation of [arithmetic expressions](https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Shell-Arithmetic).
+They're in the form `$(( expression ))`.
+
+Some useful expressions are:
+
+- `++name`, `name++`, `--name`, `name++` postfix and prefix incrementation and decrementation
+- `+`, `-`, `*`, `/`, `%` normal arithmetic operators, `**` exponentiation
+- `~`, `<<`, `>>`, `&`, `^`, `|` bitwise operators
+- `==`, `!=`, `>`, `>=`, `<`, `<=` logical comparison
+- `!`, `&&`, `||` logical not, and, or
+- `expr ? exprTrue : exprFalse` conditional
+- `=`, `+=`, ..., `<<=`, ... assignment operators
+- `expr1 , expr2` consequtive expressions and `(expr)` bracing
+
+Expressions work with both variable names and integers.
+
+&&/slide
+
+
+
+&&clide
+
+### Bourne shell
+
+Most modern shells support a lot of the more defying features of the [Bourne Shell](https://en.wikipedia.org/wiki/Bourne_shell), which include:
+
 - [Job control](https://en.wikipedia.org/wiki/Job_control_%28Unix%29): Management of currently running groups of processes  
   + `&`: A process can be ran in the background with an ampersand (&) at the end of the command: