Mini completion of the bash tutorial

CCNA/readme.md

@@ -293,6 +293,143 @@ exit
 waldek@helloworld:~$ 
 ```
 
+### Using variables to store the output of command
+
+Bash only really knows *characters*, both for sending and receiving.
+We can temporarily store the output	of a command using variables.
+The syntax is a bit tricky at first but quickly becomes quite natural.
+We can try this out on the command line.
+Next we'll write a small script to leverage the power of variables and pipes.
+
+```
+waldek@metal:~$ grep $USER /etc/passwd
+waldek:x:1000:1000:waldek,,,:/home/local/waldek:/bin/zsh
+waldek@metal:~$ my_name=$(grep $USER /etc/passwd)
+waldek@metal:~$ echo $my_name 
+waldek:x:1000:1000:waldek,,,:/home/local/waldek:/bin/zsh
+waldek@metal:~$ 
+```
+
+A little but more complicated.
+
+```
+waldek@metal:~$ count=$(grep "/home" /etc/passwd | wc -l)
+waldek@metal:~$ msg="there are $count users on this machine"
+waldek@metal:~$ echo $msg
+there are 4 users on this machine
+waldek@metal:~$ 
+```
+
+Now a small script.
+
+```bash
+#!/bin/bash
+
+FULLNAME="wouter gordts"
+CITY="Brussels"
+
+echo "this script was written by $FULLNAME in $CITY"
+
+IP=$(ip a | grep -v "127.0.0.1" | grep -o -E "[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\/[[:digit:]]{1,3}")
+TIME=$(date +%X)
+DAY=$(date +%A)
+YEAR=$(date +%Y)
+
+echo "this computer has $IP as IP address"
+echo "it is $TIME and we are a $DAY in $YEAR"
+```
+
+Which if we run it gives us the following output.
+
+```
+
+waldek@metal:~$ bash test.sh 
+this script was written by wouter gordts in Brussels
+this computer has 10.1.12.53/24 as IP address
+it is 02:17:02 PM and we are a Tuesday in 2022
+waldek@metal:~$ 
+```
+
+### Getting input into the script
+
+#### With `read`
+
+Observe the output of the following *program*.
+It's not really complicated but it will demonstrate we can do arithmetic in bash scripts as well!
+
+```
+waldek@metal:~$ bash test.sh 
+In which year where you born?
+1986
+your are probably around 36...
+waldek@metal:~$ 
+```
+
+The output above was generated with the following code.
+The **two** things to notice are the `read year` line and the `$(( $this_year - $year ))`.
+The former offers the possibility to **prompt** the user for input, the latter performs a mathematical calculation with two numbers.
+
+```bash
+#!/bin/bash
+
+echo "In which year where you born?"
+read year
+this_year=$(date +%Y)
+echo "your are probably around $(( $this_year - $year ))..."
+waldek@metal:~$ 
+```
+
+#### With command line arguments
+
+We can create a similar behaviour but with command line arguments.
+By doing so we don't have to answer any questions the script poses at runtime.
+If we create a script that will run for a long time, it doesn't require any interaction mid way!
+All necessary information is supplied by at execution time.
+
+```
+waldek@metal:~$ bash test.sh 1986
+your are probably around 36...
+waldek@metal:~$
+```
+
+```bash
+#!/bin/bash
+
+year=$1
+this_year=$(date +%Y)
+echo "your are probably around $(( $this_year - $year ))..."
+```
+
+The magic behind is the `$1` variable.
+This variable represents the *first* argument on the command line.
+Knowing this, what would `$4` mean?
+Indeed, the *fourth* argument...
+
+# Coding Challenge - output the exact output below
+
+```
+waldek@metal:~$ bash test.sh hello world 1986 35 foo bar linux rulez...
+hello waldek, my name is test.sh
+you supplied 8 arguments on the command line...
+here are all of them on one line: hello world 1986 35 foo bar linux rulez...
+waldek@metal:~$ 
+```
+
+<details>
+<summary>Spoiler warning!</summary>
+
+```bash
+#!/bin/bash
+
+echo "hello $USER, my name is $0"
+echo "you supplied $# arguments on the command line..."
+echo "here are all of them on one line: $@"
+```
+
+</details>
+
+
+
 ### What is a Bash Script - What are they, how do they work and how to run them.
 
 [Ryan's tutorials](https://ryanstutorials.net/bash-scripting-tutorial/bash-script.php)