Bash Scripting Language Cheat Sheet Cheat Sheet (DRAFT) by danilobanjac

ECHOING VARIABLES:
	- var=10
	  echo var (This will echo "var")
	  echo $var (This will echo the actual value, this is why we are using dollar sign "$" before variable. So that program knows that we want a value from the variable)
UNSET:
	- var=10
	  unset var 
	  echo $var ("unset" will actually reassign the value of "var" to null)
ASIGN VALUE TO THE VARIABLE THROUGH USER:
	- echo "type in some value"
	  read var2 (This will ask user for input same like "prompt" in javascript and store this value to variable)
	  echo $var2

PROPERTIES OF VARIABLES:
	- var="T r a l a l a        lalalal  l"
	  echo $var (This will output the string but we will miss some spaces, this means that ot all of our spaces will be printed that is why we are using "")
	  var="T r a l a l a        lalalal  l"
	  echo "$var" (When we use "" to wrap our variable with as well the "$" dollar sign then all of our spaces will be outputed. This is recommended way of doing "echo" in bash)
DEFINE NULL VARIABLE:
	- var= (This will define variable with a value of "null")

DECLARING VARIABEL ON SAME LINE:
	- var1=11 var2=22 var3=33
	  echo "$var1 $var2 $var3" (Same like in javascript)

ASSIGNING, REASSIGNING AND UNSETTING THE VALUE:
	- var=
	  echo "$var"
	  var=9
	  echo "$var"
	  var=10
	  echo "$var"
	  unset var
	  echo "$var"

ASSIGN VALUE FROM BASH COMMAND TO VARIABLE:
	- hi=$(ls -la) (This is the way of doing it)
          echo "$hi" (Will print value of "hi")


ADDING TO A VARIABLE OR DOING ARITHMETIC OPERATIONS ON VARIABLES:
	- var=
	  let "var ++"
	  echo "$var" (This will output 1)
	  let "var += 10"
	  echo "$var" (This will output 10)
	
REPLACING VALUES IN NUMBER:
	- num=1100
	  var=${num/10/B}
	  echo "$var" (This will echo "1B0")

ENVIRONMENTAL VARIABLES:
        - #!/bin/bash

          MIN=10 (Minimum passed arguments in order to execute the script)

          if [ -n "$1" ]; then echo "1st one is $1"; fi  ("$1" first argument that is being passed to script and "-n" check if argument exists. Returning either "true" or "false")
          if [ -n "$2" ]; then echo "2st one is $2"; fi
          if [ -n "$3" ]; then echo "3rd one is $3"; fi
          if [ -n "$4" ]; then echo "4th one is $4"; fi
          if [ -n "$5" ]; then echo "5th one is $5"; fi
          if [ -n "$6" ]; then echo "6th one is $6"; fi
          if [ -n "$7" ]; then echo "7th one is $7"; fi
          if [ -n "$8" ]; then echo "8th one is $8"; fi
          if [ -n "$9" ]; then echo "9th one is $9"; fi
          if [ -n "${10}" ]; then echo "10th one is ${10}"; fi

         echo "List of arguments: "$"" ("$" this will take all the arguments that are being passed to script)
         echo  "Name of Script: \""$0"\"" ("$0" is used to grab the name of the file)

         if [ $# -lt "$MIN" ]; then echo "Not enought arguments, need "$MIN" to run!"; fi  (This check "$#" number of all arguments being passed to script and compare it to our defined variable.)
         - sh filename.sh 1 2 3 4 5 6 7 8 9 10 (Passing arguments to our script and printing them)

RETURN VALUES:
         - #!/bin/bash

           NO_OF_ARGS=2
           E_BADARGS=85
           E_UNREADABLE=86

           if [ $# -ne "$NO_OF_ARGS" ]; then echo "Usage: "$0" fileOne fileTwo"; exit $E_BADARGS; fi
           if [ ! -r "$1" ] || [ ! -r "$2" ]; then echo "One or both files does not exist: "$1" or "$2""; exit "$E_UNREADABLE"; fi
           (
           cmp $1 $2
           ) > /dev/null 2>&1
           if [ $? -eq 0 ]; then echo "Files are the same!"; else echo "Files are not the same!"; fi
           exit 0

         - Explanation: This script will accept two files and than compare if they are identical. We are defining our "EXIT CODES" and returning them depending on situation. (exit code 0 == good (true); exit code 1 == bad (false))

ECHO:
	- echo "Some text" (This is same like "print" in python. It will print to command line)
	- echo "Some text" #This is comment ("This is comment" will not be executed)
DEFINING VARIABLES:
	- name=10 (This desfines a variable with integer)
	- name=tea (This defines a variable)
	USING HASH "#":
	- echo "The word $name contains ${#name} chars" (This is how to do a string formatting in bash)
	- $(#name) (This will return a lenght of the string in bash)
	- $name will replace this part of the string with the current variable.
USING SEMICOLON:
	- echo "hi there"; echo "you there?" (; semicolon sign will tell bash to run this as a next line of the code)
IF/THEN/ELSE STATEMENT:
	- var=10
	- if [ "$var" -gt 5 ]; then echo "YES"; else echo "NO"
	  fi (Please note the spaces inside the brackets they need to be there for statement to run, and every semicolon will be seen as a next line and at the end we have "fi" this closes the if statement)
FOR LOOP:
	- colors="red black white" (This defines a variable as a string)
	- for col in $colors
	  do (This will run the desired action)
	  	echo "$col" (This will echo "col")
	  done (This will finish the for loop)
	- if we run this script now the result below will be printed to console:
	  red
	  black
	  white
	- as you can see for loop threated variable colors as a list. Because "for col in $colors" dollar sign and colors will make a list from the string. If we put $colors inside double quotes like this "$colors" this will be then threated as a string and the ouput to the console will be:
	  red black white
	- if we put single quotes '$colors' this would take the actual word $colors and print it.
USING LET:
	- let "y=((x=20, 10/2))" (let in bash will let us to perform arithemic operations on variables)
	- echo $y (This will return 5 because we separated the operation with comma)
CHANGING THE STRING TO UPPER OR LOWER:
	- var = DSLConnection
	- echo ${var,} (This will change the first character of the string to lower)
	- echo ${var,,} (This will change the whole string to lower)
USING "\" ESCAPE CHARACTER:
	- echo ""Linux is awesome""
	  will output this Linux is awesome to the console.
	- echo "\"Linux is awesome"\" (This will take quotes literaly)
	  will output this "Linux is awesome" to the console
REASIGN THE VALUE:
	- let val=500/2
	  val2=
echo $val
 (This will allow us to reasign the value from the first variabel to the second variabel)
	  echo "$val2"
	  will give same output "250"
	  IMPORTANT
	  In order to get the value from any oder script example Python. Bash script will only recognise the value if value is printed, that means that function can return number but at the end when we call the function in python script it should look like this:
	  print return_value()
	USING ":" SIGN:
	- var=20
	  if [ "$var" -gt 15  ]; then :;else echo "$var";fi (This ":" sign after then will actually tell our code to do nothing this will come very useful, same like "pass" in python)
IF STATEMENT USING "?" MARK:
	- var=10
	  echo $(( var2=var1<20?1:0  )) (This will return the first value "1" if statement is true and second value if statement is false. In this case we don't need the "$" sign before "var1" to tell the program to use value, this is special case)
CREATE ARRAY:
	- Colors=(red blue green white) (This will create an array in bash)
WRAP STRINGS INTO SOME CHARACTERS:
	- echo \+{test1,test2,test3}\+ (You can replace the "+" sign with any other sing for example "$")
	  will output +test1+ +test2+ +test3+ to console
CREATE RANGE:
	- echo {0..9} (This will print all numbers between 0 and 9 (including 9) same like range() in python)
SEPARATE THE BLOCK OF CODE:
	- var1=1
	  var2=2
	  {
	  var1=10
	  var2=12
	  }
	  echo "$var1 $var2" (The output to the console will be 10 and 12 because "{}" will separate this part of code)
SAVE EXIT CODE FROM THE LAST COMMAND:
	- python myPythonScript.py
	  ret=$?
	  if [ $ret -ne 0 ]; then
	       #Handle failure
	       #exit if required
	  fi
USE EXIT CODE TO MANIPULATE SCRIPT:
	- #!/bin/bash

	  touch /root/test 2> /dev/null

	  if [ $? -eq 0 ]
	  then
	    echo "Successfully created file"
	    exit 0
	  else
	    echo "Could not create file" >&2
	    exit 1
	  fi
HIDE WHOLE OUTPUT FROM THE SCRIPT:
	- (
    	  	./manage.py create_test_database
	  ) > /dev/null 2>&1

CONNECTING IF STATEMENTS:
	- var=1
	  if [ "$var" -gt 0  ] && [ "$var" -eq 10  ]; then echo "THEN PART"; else echo "HELLOOO"; fi (Example with logical "and" statment)
	  var=1
	  if [ "$var" -gt 0  ] || [ "$var" -eq 10  ]; then echo "THEN PART"; else echo "HELLOOO"; fi (Example with logical "or" statment)
	MODULO:
 	- let var=5%4
	  echo "$var" (Result will be one)
STRING UPPER CASE:
	- some_word=tEsT
	  echo "${some_word^}" (This will grab first letter and make it upper case)
	  echo "${some_word^^}" (This will grab whole word and make it upper case)