completes some more bash scripting

.gitignore

@@ -1 +1,2 @@
 *.swp
+tmp.*

advanced/learning_bash_scripting.md

@@ -351,7 +351,6 @@ waldek@metal:~$
 
 # Coding challenge - Output system stats
 
-
 Write a program that prints information about your computer such as:
 
 * the hostname
@@ -410,6 +409,9 @@ hmmm, I don't know how to compare helloworld to supersecret
 waldek@metal:~$ 
 ```
 
+We haven't learned how to *evaluate* values but if you're eager and quick you can try to figure it out yourself.
+If not, no worries, we'll get to *conditional logic* soon enough.
+
 <details>
 <summary>Spoiler warning!</summary>
 
@@ -425,11 +427,32 @@ echo "hmmm, I don't know how to compare $pass to $my_pass"
 
 </details>
 
-
-
 ### `read` multiple variables
 
-TODO
+`read` can be used to unpack multiple values right on the spot!
+By *unpacking* I mean that each value is separated by **white space**.
+A demonstration can be seen below, plus the actual code right after.
+
+```
+waldek@debian:~$ bash test.sh 
+first and last name please: wouter gordts
+hello mr gordts...
+or may I call you wouter?
+waldek@debian:~$ 
+```
+
+```
+#!/bin/bash
+
+read -p "first and last name please: " first last
+echo "hello mr $last..."
+echo "or may I call you $first?"
+```
+
+#### Exercise - unpack values
+
+It is worth discovering what happens when you supply too many or to little values.
+Please try this out!
 
 ## With command line arguments
 
@@ -457,14 +480,6 @@ This variable represents the *first* argument on the command line.
 Knowing this, what would `$4` mean?
 Indeed, the *fourth* argument...
 
-## From a file
-
-TODO
-
-## From a pipe
-
-TODO
-
 # Coding Challenge - output the exact output below
 
 ```
@@ -900,14 +915,208 @@ fi
 
 </details>
 
+# Coding challenge - pipe or argument?
+
+Can you code me a script that depending on how it is called, with argument or via pipe, prints a different message?
+Along these lines...
+
+```
+waldek@debian:~$ bash test.sh 
+No input was found on stdin, skipping!
+No input given!
+waldek@debian:~$ bash test.sh shopping.list 
+No input was found on stdin, skipping!
+Filename specified: shopping.list
+Doing things now..
+waldek@debian:~$ cat shopping.list | bash test.sh 
+Data was piped to this script!
+waldek@debian:~$ 
+```
+
+<details>
+<summary>Spoiler warning!</summary>
+
+**It might take you some time but we've seen all the necessary building blocks!**
+
+<details>
+<summary>Spoiler warning!</summary>
+
+```bash
+#!/bin/bash
+
+if [ -p /dev/stdin ]; then
+        echo "Data was piped to this script!"
+else
+        echo "No input was found on stdin, skipping!"
+        if [ -f "$1" ]; then
+                echo "Filename specified: ${1}"
+                echo "Doing things now.."
+        else
+                echo "No input given!"
+        fi
+fi
+```
+
+</details>
+
+</details>
 
 # Loops - A variety of ways to perform repetitive tasks.
 
-[Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-loops.php)
+Up until now all our scripts run from top to bottom, executing each line as they are read.
+Most, if not all, programming languages offer some form of looping to **repeat** lines of code either based on a **condition** or for a predefined number of *items*.
 
-Write a script that sets all you cpu's to a desired governor.
+## `while` loop
 
-Rename all files in a folder with an prefix or postfix.
+```bash
+#!/bin/bash
+
+counter=0
+
+while [[ counter -lt 10 ]]; do
+        echo $counter
+        (( counter++ ))
+done
+```
+
+### Read from a file with a `while` loop
+
+Below you can see I have two files, one is my *shopping list*, the other my script.
+The script loops over each item I have to buy and prints me a verbose message.
+
+```
+waldek@debian:~$ cat shopping.list 
+6 eggs
+1 bread
+2 milk
+4 pasta
+waldek@debian:~$ bash test.sh shopping.list 
+you need to buy: 6 eggs
+you need to buy: 1 bread
+you need to buy: 2 milk
+you need to buy: 4 pasta
+waldek@debian:~$
+```
+
+One way to do this is with a `while` loop.
+The syntax is a follows.
+
+```bash
+#!/bin/bash
+
+while read line; do
+        echo "you need to buy: $line"
+done < $1
+```
+
+The syntax is not what I would call *pretty* but it is quite effective.
+The **file** `$1` is redirected **into** the `while` loop which performs a `read`.
+**When** there are no more lines in the file, the condition **fails** and the loop exits.
+
+### Read from a pipe with a `while` loop
+
+We can use a *very* similar construct to read data from a pipe.
+The utilisation is as follows.
+
+```
+waldek@debian:~$ cat shopping.list | ./test.sh 
+you need to buy: 6 eggs
+you need to buy: 1 bread
+you need to buy: 2 milk
+you need to buy: 4 pasta
+waldek@debian:~$
+```
+
+And the code that does this as such.
+
+```bash
+#!/bin/bash
+
+while IFS= read line; do
+        echo "you need to buy: $line"
+done
+```
+
+What on earth is this `IFS=`?
+I'm glad you [asked](https://unix.stackexchange.com/questions/184863/what-is-the-meaning-of-ifs-n-in-bash-scripting)!
+
+## `for` loop
+
+Ah, my favorite loop!
+While the `while` loop runs as long as a condition is `true`, the `for` loop **iterates** over *items*.
+The simplest way to visualize it would be as follows.
+For each *friend* in my list of friends I'll say hello.
+
+```
+waldek@debian:~$ ./test.sh 
+these are my friends: adam camille alice bob steve
+hello adam!
+hello camille!
+hello alice!
+hello bob!
+hello steve!
+waldek@debian:~$ 
+```
+
+The syntax to achieve this goes as follows.
+
+```bash
+#!/bin/bash
+
+friends="adam camille alice bob steve"
+echo "these are my friends: $friends"
+for f in $friends; do
+        echo "hello $f!"
+done
+```
+
+### Counter loops with `for`
+
+We can do a counter loop as well via a `for` loop.
+An easy way to do this is by using the `seq` program.
+Remember to read the `man seq` for more information.
+
+```bash
+#!/bin/bash
+
+for number in $(seq 0 10); do
+        echo "number is $number"
+done
+```
+
+### Finicky behaviour
+
+`bash` is not the prettiest language and the difference between single and double quotes can be a bit confusing.
+
+```bash
+#!/bin/bash
+
+for animal in dog cat horse 'racing horse' whale; do
+        echo "$animal says: I'm an animal..."
+done
+```
+
+Which gives the following output.
+
+```
+waldek@debian:~$ ./test.sh 
+dog says: I'm an animal...
+cat says: I'm an animal...
+horse says: I'm an animal...
+racing horse says: I'm an animal...
+whale says: I'm an animal...
+waldek@debian:~$ 
+```
+
+# Coding challenge - pipe or argument plus action!
+
+Can you expand the previous coding challenge, where you perform a conditional logic on the input of the script and actually *use* the incoming data?
+
+# Coding challenge - Rename files
+
+Rename all files from [this](../assets/simple_sort_01.tar.gz) file with an prefix or postfix.
+Can you give the files that start with an **uppercase** letter a different pre or postfix?
+Can you move the files with lowercase into a different folder?
 
 # Functions - Reuse code to make life easier.
 
@@ -923,6 +1132,7 @@ Rename all files in a folder with an prefix or postfix.
 
 # User Interface - Make your scripts user friendly.
 
+* `case` for command line arguments
 * [dialog tutorial](https://www.linuxjournal.com/article/2807)
 * [better dialog tutorial](https://linuxcommand.org/lc3_adv_dialog.php)
 * [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-user-interfaces.php)