====== Shell or Bash Scripts ======

|Shebang|Header, which tells where the interpreter is located <code>#!/bin/sh</code> | http://stackoverflow.com/questions/10376206/what-is-the-preferred-bash-shebang |
|Sourcing| <WRAP>launch script in a new shell using point with an empty space between sript path Souring: 
<code>. path/to/script.sh</code> 
vs Execute in current shell:
<code>./path/to/script.sh</code> </WRAP>| http://superuser.com/questions/176783/what-is-the-difference-between-executing-a-bash-script-and-sourcing-a-bash-scrip |

===== Guides===== 

|https://www.shellcheck.net/|Check the syntax of the shell live|
|http://www.artificialworlds.net/blog/2012/10/17/bash-associative-array-examples/| |
|https://linuxconfig.org/bash-scripting-tutorial| |
|http://www.tldp.org/LDP/Bash-Beginners-Guide/html/Bash-Beginners-Guide.html#chap_11| |
|http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html| |

===== Executing or Sourcing the script=====

Sourcing. 
Execute in the **CURRENT** shell
<code>. path/to/script.sh</code> 

Executing.
Execute in a **NEW** shell - e.g. cd doesn't modify current shell's path
<code>./path/to/script.sh</code>


===== Syntax =====

==== declare Function====
Returning result is done via **echo**

<code>
#!/bin/bash

#Define bash global variable
#This variable is global and can be used anywhere in this bash script
VAR="global variable"

function bash {
  #Define bash local variable
  #This variable is local to bash function only
  local VAR="local variable"
  echo $VAR
}
echo $VAR
bash

# Note the bash global variable did not change
# "local" is bash reserved word
echo $VAR
</code>

== declare Array==
Small a!!!
<code>
	declare -a map

	map[1]="myvalue"
	map[1]="myvalue2"

	echo ${map[1]}
</code>

==== declare Map ====
Big A!!!
<code>
	declare -A map

	map[test]="myvalue"
	map[test2]="myvalue2"

	echo ${map[test2]}
</code>

==== iterate array keys ====
Access via **${!array[@]}**

<code>
for i in "${!array[@]}"
do
  echo "key  : $i"
  echo "value: ${array[$i]}"
done
</code>

==== dereference an array / map value ====
<code>
${ARRAY[2]}
${map[key1]}
</code>

== If Else ==
  * Empty spaces after the parenthesis are critical: **[ $a==$b ]**
  * 

<code>
# check if it is the root device
if [ $device_name==$root_device_name ]
then
	echo "$device_name is root"
else
	echo "NOT root"
fi
</code>

==== Command Substitution ====

Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed like this:

<code>
$(command)
</code>

or like this using backticks:

<code>
`command`
</code>


==== Input from command line ====
If a command expects **a file** as input - one can pass input from STDOUT instead,
without storing the output to a file first by using following syntax

**oc create -f** expects a file here.

<code>
oc process -f build/my-build-template.yaml  GITSERVER=$GITSERVER | oc create -f - 
oc process -f build/my-build-template.yaml  GITSERVER=$GITSERVER | oc create -f /dev/stdin 
</code>


==== Double-Dash -- in Shell commands ====
A double-dash in a shell command signals the end of options and disables further option processing.
https://www.baeldung.com/linux/double-dash-in-shell-commands


<sxh shell>
# If we use the same approach, we'll receive an error:
# That's because grep treats "--hello" as a multi-character command option.
grep "--hello" data.txt

#works
grep -- --hello data.txt
</sxh>