Progress on bash and linux notes

content/notes/ready/bash.md

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+---
+title: Shell Scripting [BASH]
+description: 
+summary: 
+draft: true
+tags: 
+author: TrudeEH
+showToc: true
+---
+
+## Bash Language
+
+### Strings
+
+- `""` Defines a string which supports substitutions (`$` and `\`, for example).
+- `''` Defines a string, but preserves its actual value (substitutions are treated as regular characters).
+- [ANSI Escape Sequences](../../ready/c-language.md#ANSI%20Escape%20Sequences) apply when using `""`.
+
+### Comments
+
+```bash
+# comment
+```
+
+### Commands
+
+A shell command consists of the command itself, followed by its arguments.
+
+```bash
+command "arg1" "arg2"
+```
+
+If the first word of a command is a reserved word, bash handles the command, otherwise, it searches for an executable on the system's `$PATH`, a list of directories where a binary could be located.
+
+#### Reserved Words
+
+|   |   |   |   |   |   |
+|---|---|---|---|---|---|
+|`if`|`then`|`elif`|`else`|`fi`|`time`|
+|`for`|`in`|`until`|`while`|`do`|`done`|
+|`case`|`esac`|`coproc`|`select`|`function`|
+|`{`|`}`|`[[`|`]]`|`!`|
+
+### List of Commands
+
+- `command1 ; command2` Execute command2 after command1, sequentially.
+- `command1 &` Execute command1 asynchronously in a subshell.
+- `command1 && command2` *AND*: Only execute command2 if command1 returns 0 (success).
+- `command1 || command2` *OR*: Only execute command2 if command1 returns a non-zero exit value (failure).
+
+### Loops
+
+#### `until`
+
+```bash
+until test-commands; do 
+	...
+done
+```
+
+Execute the code in `...` for as long as `test-commands` return non-zero.
+
+#### `while`
+
+```bash
+while test-commands; do 
+	...
+done
+```
+
+Execute `...` for as long as `test-commands` return 0.
+
+#### `for`
+
+Expand `words` and execute `...` for each member in the resultant list, with `name` bound to the current member.
+
+##### Iterate through List
+
+```bash
+for item in list; do
+	echo $item
+done
+```
+
+##### C-like Loop
+
+```bash
+for (( i=1; i<=10; i++ )); do  
+ echo "Loop number:" $i  
+done
+```
+
+##### Infinite Loop
+
+```bash
+for (( ; ; )); do
+ echo "Press Ctrl+C to stop..."
+done
+```
+
+### Conditional Constructs
+
+#### `if`
+
+```bash
+if test-commands; then
+  ...
+elif more-test-commands; then
+  ...
+else
+	...
+fi
+```
+
+Execute the first `...` if `test-commands` returns 0, and evaluate the next condition otherwise. This process repeats until `else` is found, or one of the `tests` evaluates to a 0.  
+Once any `...` executes, the remaining `if` construct is skipped.
+
+#### `case`
+
+```bash
+case word in
+  p1 | p2 | p3) ... ;;
+  p4 | p5 )
+	  ... 
+	;;
+  *) ... ;;
+esac
+```
+
+Execute the `...` corresponding to the first pattern (`pX`) that matches the `word`.  
+The `|` operator separates multiple patterns, and each clause can end with `;;`, `;&` or `;;&`. It's common to use `*` as the default case, since the pattern will always match.
+
+Using `;&` instead of `;;` would cause the next `...` to be executed as well, and `;;&` would test the next clause, instead of immediately exiting.
+
+#### `select`
+
+```bash
+PS3="Enter a number: "
+
+select option in entry1 entry2 entry3 entry4 entry5
+do
+    echo "Selected character: $option"
+    echo "Selected number: $REPLY"
+done
+```
+
+The `select` command generates a menu, displaying each `entryX` in a list. The user is then prompted to select an option (in this case, a number from 1-5), and the resultant `$option` and `$REPLY` are then provided as variables.
+
+Output:
+
+```
+1) entry1
+2) entry2
+3) entry3
+4) entry4
+5) entry5
+Enter a number:
+```
+
+#### `((...))`
+
+The arithmetic expression is evaluated according to the rules described below (see [Shell Arithmetic]() TODO link to shell arithmetic).
+
+#### `[...](...)`
+
+Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in [Bash Conditional Expressions](https://www.gnu.org/software/bash/manual/bash.html#Bash-Conditional-Expressions).
+
+#### Combine Expressions
+
+- `( expression )` Returns the value of expression. (Can be used to override precedence).
+- `! expression` *NOT* an expression. (`true` if expression is `false`).
+- `exp1 && exp2` *AND* - `true` if both expressions are `true`.
+- `exp1 || exp2` *OR* - `true` if either expressions are `true`.
+
+#### Grouping Commands
+
+Bash allows for commands to be grouped as a single unit. That way, if the group is redirected, the output of every command in the list is passed to a single stream.
+
+- `( list )` Create a subshell (variables created inside it can't be accessed outside).
+- `{ list; }` No subshell is created.
+
+### Functions
+
+```bash
+fname() {
+	...
+}
+
+function fname {
+	...
+}
+```
+
+A function can store a block of code (compound command), so it can be reused by calling its name:
+
+```bash
+fname
+```
+
+Any variables defined inside the function 
+
+#### Arguments
+
+```bash
+fname() {
+	echo $1 $2
+}
+
+fname "a" "b"
+```
+
+#### Scope
+
+```bash
+var1='A'
+var2='B'
+
+fname () {
+  local var1='C'
+  var2='D'
+  echo "var1: $var1, var2: $var2" # C, D
+}
+
+echo "$var1, var2: $var2"       # A, B
+fname                           # C, D
+echo "var1: $var1, var2: $var2" # A, D
+```
+
+Defining a variable inside the function overwrites the global scope. To prevent this, use the `local` keyword.
+
+#### `return`
+
+```bash
+fname() {
+	return 1;
+}
+
+fname
+echo $? # 1
+```
+
+Use the `return` command to exit the function and return a value.
+
+### Variables (Parameters)
+
+```bash
+name="Trude"
+echo $name # Trude
+
+name+=" EH"
+echo $name # Trude EH
+echo ${name}
+```
+
+Variables can be of any type, and grow to any needed size.
+
+#### Special Variables
+
+- `$*` Expands to every positional parameter: `$1$2$3`.
+- `$@` Expands to every positional parameter, separated by spaces: `"$1" "$2" "$3"`.
+- `$#` Number of positional arguments.
+- `$?` Exit status of last command / pipeline.
+- `$-` Current option flags set by `set`, or by the shell itself.
+- `$$` Process ID of the shell. In a subshell, it expands to the process ID of the parent shell.
+- `$!` Process ID of the latest job placed into the background.
+- `$0` Name of the shell or script.
+
+### Shell Expansions
+
+#### Brace Expansion
+
+```bash
+echo a{d,c,b}e # ade ace abe
+```
+
+#### Tilde Expansion
+
+- `~` = `$HOME`
+- `~+` = `$PWD`
+- `~-` = `$OLDPWD`
+
+#### Shell Parameter Expansion
+
+### Builtins ---------------------
+
+Bash doesn't come with any programs such as `cat`, `grep` and `ls` by default, for example. Those