! /bin/bash

ECHO COMMAND
echo Hello World!

VARIABLES
Uppercase by convention
Letters, numbers, underscores
NAME="Bob"
echo "My name is $NAME"
echo "My name is ${NAME}"

USER INPUT
read -p "Enter your name: " NAME
echo "Hello $NAME, nice to meet you!"

SIMPLE IF STATEMENT
if [ "$NAME" == "Bappy" ]
then
  echo "Your name is Bappy"
fi

IF-ELSE
if [ "$NAME" == "Bappy" ]
then
  echo "Your name is Bappy"
else 
  echo "Your name is NOT Bappy"
fi

ELSE-IF (elif)
if [ "$NAME" == "Bappy" ]
then
  echo "Your name is Bappy"
elif [ "$NAME" == "Jack" ]
then  
  echo "Your name is Jack"
else 
  echo "Your name is NOT Bappy or Jack"
fi

COMPARISON
NUM1=31
NUM2=5
if [ "$NUM1" -gt "$NUM2" ]
then
  echo "$NUM1 is greater than $NUM2"
else
  echo "$NUM1 is less than $NUM2"
fi

#######
val1 -eq val2 Returns true if the values are equal
val1 -ne val2 Returns true if the values are not equal
val1 -gt val2 Returns true if val1 is greater than val2
val1 -ge val2 Returns true if val1 is greater than or equal to val2
val1 -lt val2 Returns true if val1 is less than val2
val1 -le val2 Returns true if val1 is less than or equal to val2
#######

FILE CONDITIONS
FILE="test.txt"
if [ -e "$FILE" ]
then
  echo "$FILE exists"
else
  echo "$FILE does NOT exist"
fi

#######
-d file   True if the file is a directory
-e file   True if the file exists (note that this is not particularly portable, thus -f is generally used)
-f file   True if the provided string is a file
-g file   True if the group id is set on a file
-r file   True if the file is readable
-s file   True if the file has a non-zero size
-u    True if the user id is set on a file
-w    True if the file is writable
-x    True if the file is an executable
#######

CASE STATEMENT
read -p "Are you 21 or over? Y/N " ANSWER
case "$ANSWER" in 
  [yY] | [yY][eE][sS])
    echo "You can have a beer :)"
    ;;
  [nN] | [nN][oO])
    echo "Sorry, no drinking"
    ;;
  *)
    echo "Please enter y/yes or n/no"
    ;;
esac

SIMPLE FOR LOOP
NAMES="Bappy Kevin Alice Mark"
for NAME in $NAMES
  do
    echo "Hello $NAME"
done

FOR LOOP TO RENAME FILES
FILES=$(ls *.txt)
NEW="new"
for FILE in $FILES  
  do
    echo "Renaming $FILE to new-$FILE"
    mv $FILE $NEW-$FILE
done

WHILE LOOP - READ THROUGH A FILE LINE BY LINE
LINE=1
while read -r CURRENT_LINE
  do
    echo "$LINE: $CURRENT_LINE"
    ((LINE++))
done < "./new-1.txt"

FUNCTION
function sayHello() {
  echo "Hello World"
}
sayHello

FUNCTION WITH PARAMS
function greet() {
  echo "Hello, I am $1 and I am $2"
}

greet "Bappy" "36"

CREATE FOLDER AND WRITE TO A FILE
mkdir hello
touch "hello/world.txt"
echo "Hello World" >> "hello/world.txt"
echo "Created hello/world.txt"