Shell or Bash Scripts

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

. path/to/script.sh

Executing. Execute in a NEW shell - e.g. cd doesn't modify current shell's path

./path/to/script.sh

Syntax

declare Function

Returning result is done via echo

#!/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

declare Array

Small a!!!

	declare -a map

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

	echo ${map[1]}

declare Map

Big A!!!

	declare -A map

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

	echo ${map[test2]}

iterate array keys

Access via ${!array[@]}

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

dereference an array / map value

${ARRAY[2]}
${map[key1]}

If Else

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

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

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:

$(command)

or like this using backticks:

`command`

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.

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

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

# 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

Tohuwabohu excorcism

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