bashscript

command in a variable:
d=$(pwd)
echo $d

numerical expression in a variable:
d=$((2/3))
For floating calculation use bc -l instead:
d=$(echo 2/3 | bc -l)

String comparison, double square brackets
d=[[ $a > $b ]]
note the spaces
d=[[ $a == $b ]]

For number comparisons, use keywords:
d=[[ 1 -lt 2 ]]
echo $?
d=[[ 2 -le 2 ]]
echo $?

To check null (-z) or not (-n):
e=""
[[ -z $e ]]

&& || ! works

String operations:
a="hello"
b="world"
concatanate with: c=$a$b

echo $(#a) gives  size of a.

substring: echo $(a:4) gets from 5th character of a..i.e o
echo $(c:2:3) gets 3 number of characters from 2. i.e. lowo
use negative number and count from end of the string
eg echo $(a: -2) gets lo
echo $(a: -2:1) gets l
notice the space when using minus

replace string:
fruit="apple banana banana cherry"
echo ${fruit/searchstring/replacestring}
echo ${fruit/banana/durian}
replaces only one occurance, to replace all occurance, change / before banana to double slash //

if you use # in front of the term, it will only replace the term if its on the very first.
similarly if you use ! in front, it will only replace it if it is the last term
eg echo ${fruit/#banana/durian} does not replace anything.
o

array
a=()
empty array
b=("apple" "banana" "cherry") creates an array
echo ${b[2]} prints the 2nd element fo the array
b[5]="kiwi" adds to the fifth element of the array`
b+=("mango") adds to the array
echo ${b[@]}  prints the whole array
echo ${b[@]: -1}  gets the last array

associative array
declare -A myarray
myarray[color]=blue
myarray["office building"]="HQ West"

echo ${myarray["office building"]} is ${myarray[color]}

working with text files
using while loop
i=1
while read f; do
	echo "Line $i: $f"
	((i++))
done < file.txt


heredoc

cat <<- EndOfText
This is a
multiline
text string
EndOfText

using <<- removes leading tabs from the text.
otherwise tabs will be present.

ftp -n <<- DoneWithTheUpdate
	open mirrors.xmission.com
	user anonymous nothinghere
	ascii
	cd gutenberg
	get GUTINDEX.01
	bye
DoneWithTheUpdate


'if' statement
if expression; then
or if expression
then 
	eho "true"
elif expression2; then
	echo "ex is false"
fi

a=5
if [ $a -gt 4 ]; then
	echo $a is greater than 4!
else
	echo $a is not greater than 4!
fi

checking to see if a string contains numbers using regex
a="This is my string"
if [[ $a =~ [0-9]+ ]]; then
	echo "There are numbers in the string: $a"
else
	echo "There are no numbers in the string: $a"
fi

While loop
i=0
while [ $i -le 10 ]; do
	echo i:$i
	((i+=1))
done

j=0
until [ $j -ge 10 ]; do
	echo j:$j
	((j+=1))
done

For loop
for i in 1 2 3
do
	echo $i
done

for i in {1..100..2}

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

arr=("apple" "banana" "cherry")
for i in ${arr[@]}
do
	echo $i
done

for assc. array
declare -A arr
arr["name"]="Scott"
arr["id"]="1234"
for i in "${!arr[@]}"
do
	echo "$i: ${arr[$i]}"
done

using for loop with command substitution
for i in $(ls)
do
	echo "$i"
done

FUNCTION
function greet {
	echo "Hi $1!"
}
echo "And now, a greeting!"
greet Scott


function numberthings {
	i=1
	for f in $@; do
		echo $i: $f
		((i+=1))
	done
}

CASE
a="dog"
case $a in 
	cat) echo "Feline";;
	dog|puppy) echo "Canine";;
	*) echo "No match!";;
esac

WORKING WITH USER INPUTS

flags
while getopts u:p: option; do
	case $option in
		u) user=$OPTARG;;
		p) pass=$OPTARG;;
	esac
done
echo "User: $user / Pass: $pass"

OPTIONS
echo "What is your name?"
read name

echo "What is your password?"
read -s pass    (won't show the password s stands for silent)

read -p "What's your favorite animal? " animal  (writes out a prompt before the input area)

echo name: $name, pass: $pass, animal: $animal

select animal in "cat" "dog" "bird" "fish" "quit"
do
	case $option in 
	cat) echo "Cats like to sleep.";;
	dog) echo "Dogs like to play catch.";;
	quit) break;;
	*) echo "I'm not sure what that is.";;
	esac
done

ERROR TOLERANCE
if [ $# -lt 3]; then
	cat <<- EOM
	This command requires three arguments:
	username, userid, and favorite number.
	EOM
else	
	# the program goes here
	echo "Username: $1"
	echo "UserID: $2"
	echo "Favorite number: $3"
fi

OR

read -p "Favorite animal?" a
while [[ -z "$a" ]]; do
	read -p "I need an answer! " a
done
echo "$a was selected."

Have a default answer

read -p "Favorite animal? [cat]" a
while [[ -z "$a" ]]; do
	a="cat"
done
echo "$a was selected."


validation - rejecting input that does not match a regex

read -p "What year? [nnnn] " a
while [[ ! $a =~ [0-9]{4} ]]; do
	read -p "A year, please! [nnnn] " a
done
echo "Selected year: $a"