Ref- http://www.ramil.pro/2013/01/bash.html

1) If condition

There are 3 options:

1) if..then..else..fi

2) [ condition ] && ( expression-if-true ) || ( expression-if-false )

3) test condition && expression-if-true || expression-if-false

2)  Loops

1) Standard for..in

for arg in list;
do
 commands
done

2) C-like for (attention! double brackets!)

for (( i=1; i <= 10; i++ ));
do
 commands
done

3) While loop

while [ condition ];
do
 commands
done

or 

while (( C-like condition ));
do
 commands
done

4) Until loop

until [ condition ];
do
 commands
done

break [N] - exit the loop
continue [N] - start over

Example:

for (( j=0; j<3 -gt="" arr="" code="" do="" done="" fi="" for="" i="" if="" j="" then="" tmp="">

3) Functions

function function_name {
 commands
}

or

function_name() {
 commands
}

exit and return just work as usually

4) Arrays

declare -a array - declare the array first

define value:
a[13] = "foo"

retrieve value:
${a[13]} - only this way!

5) Strings

String length:
str="string"
len=${#str}

Substring:
${str:pos}
${str:pos:len}

Remove part of string:
${str#substring} - remove the shortest part from the beginning
${str##substring} - remove the longest part from the beginning

${str%substring} - remove the shortest part from the end
${str%%substring} - remove the shortest part from the end

Replacement:
${str#substring/replacement} - from the beginning
${str%substring/replacement} - from the end

6) case..in..esac 

case $variable in
condition 1)
 actions
;;
condition 2)
 actions
;;
esac

7) I/O

Read the input:
read -p "Input string: " str
echo $str

8) Features

$@ - arguments array
$1, $2, $n - arguments in order
$0 - program (self) name
$# - arguments number
$* - all arguments as one sting
$_ - last argument of the last command
$? - previous command return code
$$ - own PID
$! - PID of the last background process
!! - previous command as a string
(( a += 1 )) - double brackets let us code like C

$RANDOM - returns a pseudorandom value within 0..32767

declare -r constant - declare as read-only
declare -a - declare as array
declare -i - declare as integer