shell scripting notes

shell script:
=============

-> to automate our compleate administration process

Introduction :
==============

shebang : #!

ex: #!/bin/bash, #!/bin/ksh

Comment: comments are not executable. comments are added using '#'

ex: # comment added

ex:

helloworld.sh
-------------
#!/bin/bash

# printing hello world
echo "Hello world!"

execution:

1. permissions : chmod +x helloworld.sh

2. ./helloworld.sh

Handling script arguments :
---------------------------
wish.sh
-------
#!/bin/bash

echo "Hello $1 $2 $3"


Ex: ./wish.sh Ramu Swapna Aishwarya

$0  --> wish.sh
$1  --> Ramu
$2  --> Swapna
$3  --> Aishwarya


1. $1,$2.... are used to refere 1st arg, 2nd arg ....etc
2. $0 refers name of the current script
3. args which are stored in $1 etc., are read only. we can't change from our script.
4. $@ --> all args provided (where all args can  be treated one by one)
5. $# --> to count the amount of args provided
6. $* --> if you need a single string that contains all args (use in specific cases only)

Example :

#!/bin/bash

echo "Hello $1 $2"

Example arges 2 :

#!/bin/bash

echo "\$* gives $*"
echo "\$@ gives $@"
echo "\$# gives $#"
echo "\$0 gives $0"
for i in "$*"
do 
   echo $i
done
for j in "$@"
do 
   echo $j
done
exit 0

Pattern matching: (substitution operator or string operator )
=============================================================

Ex:

DATE=
echo ${DATE:-today}
echo ${DATE:=today}
echo ${DATE:?message}

DATE=$(date +%d-%m-%y)
echo date is ${DATE:0:2}

Reggular Expressions :c
======================
-> grep, cut, awk, sed etc are used for search patterns


Variables :
===========

we can declare variables using lowercase letters with underscore between words. EX: mission_name= lunar-mission

to access mission_name variable, echo $mission_name

Environment Variables:
======================

which are predefined variables in the shell. which uses all capital letters. EX: $PATH, $USER

Command line arguments :
========================

-> design our script to be reusable
-> script should not required to be edited before running
-> use command line arguments to pass inputs

Read Inputs : 
==============
 read -p "enter the mission name: " mission_name

Arithmatic operations in shell scripting :
==========================================

-> using expr command : expr 6 + 3, expr 6 \* 3
-> using double parenthsis : echo $((A+B)), echo $(( ++A ))
                             echo $((A/B)) | bc -l  ---> for to get exact result in float point representation

Flow control :
==============

conditional logic :
--------------------

simple-if:
-----------
if [ somecondition ]
then
    statement
fi

if-else:
--------
if [ somecondition ]
then 
     statement 1
elif [ some condition ]
     statement 2
else
    statement 3
fi

==> condition should be enclosed in square brackets '[]'
==> for strings only we should use '=' or '!=' operators
==> for other types, we can use '-eq, -ne, -lt, -gt'
==> example : if [ "abc" = "abc" ]
              if [ 5 -gt 3 ] 
==> in bash scripting., we can also use '[[ ... ]]' for to write conditions
==> examples in pattern management : [[ "abcd" = *bc* ]]  --> abcd contains bc
                                     [[ "abc" = ab[cd] ]] --> third character may be c or d 
==> conditional operators : [ cond1 ] && [ cond2 ] or [[ cond1 && cond2 ]]
                            [ cond1 ] || [ cond2 ] or [[ cond1 || cond2 ]]
==> file level operators : [ -e FILE ] --> if file exists
                           [ -d FILE ] --> if file exists and it is dir
                           [ -s FILE ] --> if file exists and size greter than 0
                           [ -x FILE ] --> if file exists and it is executable
                           [ -w FILE ] --> if file exists and it is writable
						   
*** example to check month number with month name : 
================================================

#!/bin/bash

month_number=$1

if [ -z $month_number ]
then
  echo "No month number given. Please enter a month number as a command line argument."
  echo "eg: ./print-month-number 5"
  exit
fi

if [[ $month_number -lt 1 ]] || [[ $month_number -gt 12 ]]
then
  echo "Invalid month number given. Please enter a valid number - 1 to 12."
  exit
fi

if [ $month_number -eq 1 ]
then
  echo "January"
elif [ $month_number -eq 2 ]
then
  echo "February"
elif [ $month_number -eq 3 ]
then
  echo "March"
elif [ $month_number -eq 4 ]
then
  echo "April"
elif [ $month_number -eq 5 ]
then
  echo "May"
elif [ $month_number -eq 6 ]
then
  echo "June"
elif [ $month_number -eq 7 ]
then
  echo "July"
elif [ $month_number -eq 8 ]
then
  echo "August"
elif [ $month_number -eq 9 ]
then
  echo "September"
elif [ $month_number -eq 10 ]
then
  echo "October"
elif [ $month_number -eq 11 ]
then
  echo "November"
elif [ $month_number -eq 12 ]
then
  echo December
fi


LOOPS :
=======

Array, List, Set  ---> Group of elements
 example: class=student1, student2, student3...
 
 for student in student1 student2 .. student40; do
   
   total_marks=
   
DRY(don't repeat yourself) code and WET(write everything twice) code

for:
----
syntax:

for mission in <--list of missions-->
do
   command $mission
done

or

for mission in `cat mission-names.txt` --> $(cat mission-names.txt)   
do
   # command $mission
   echo $mission
done

or

for (( mission = 0 ; mision <= 100 ; mission++ ))
do
   echo $mission
done

==> execute a set commands multiple times
==> Iterate through files
==> Iterate through lines within a file
==> Iterate through the output of a command

Example1 :

for file in $(ls)
do
   echo "Line count of $file is $(cat $file | wc -l )
done

Example2:

for server in $(cat servers.txt)
do
   ssh $server "uptime"
done

Example3:

for package in $(install-packages.txt)
do 
   sudo apt-get -y install $package
done

Example4: Shell script to rename the images

for file in $(ls images)
do
        if [[ $file = *.jpeg ]]
                then
                new_name=$(echo $file| sed 's/jpeg/jpg/g')
                mv images/$file images/$new_name
        fi
done

while loop :
============

==> Execute a command or a set of commands multiple times but we are not sure how many times.
==> Execute a command or a set of commands until a specific condition occurs
==> Create infine loops
==> Menu driven programs

Fibonacci series :
==================

0112358 13 21 ... --> fibonacci numbers

#!/bin/bash
x=0
y=1
i=2
echo "fibonacci series up to 1000 terms"
echo  "$x"
echo  "$y"

#  -lt stands for equal to
while [ $i -lt 1000 ];
do
  i=$(expr $i + 1)
  z=$(expr $x + $y)
  echo "$z"
  x=$y
  y=$z
done

Real time example 1:
--------------

while true
do
  echo "1. Shutdown"
  echo "2. Restart"
  echo "3. Exit Menu"
  read -p "Enter your choice: " choice
  
  if [ $choice -eq 1 ]
  then
      shutdown now
  elif [ $choice -eq 2 ]
  then
      shutdown -r now
  elif [ $choice -eq 3 ]
  then
    break
  else
    continue
  fi 
done

Example 2:
----------

Break --> It stops iteration operation and come out from loop
Continue --> It stops current iteration and continues next iteration.

while true
do
  echo "1. Add"
  echo "2. Subtract"
  echo "3. Multiply"
  echo "4. Divide"
  echo "5. Quit"

  read -p "Enter your choice: " choice

  if [ $choice -eq 1 ]
  then
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( $number1 + $number2 ))
  elif [ $choice -eq 2 ]
  then
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( $number1 - $number2 ))
  elif [ $choice -eq 3 ]
  then
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( $number1 * $number2 ))
  elif [ $choice -eq 4 ]
  then
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( $number1 / $number2 ))
  elif [ $choice -eq 5 ]
  then
    break
  fi

done

case statement :
================

case syntax :

case VARIABLE in
	1) statement1
	;;
	2) statement2
	;;
	.
	.
	.
esac

Example:
--------

while true
do
  echo "1. Shutdown"
  echo "2. Restart"
  echo "3. Exit Menu"
  read -p "Enter your choice: " choice

  case $choice in
    1) shutdown now
       ;;
    2) shutdown -r now
       ;;
    3) break
       ;;
    4) continue
       ;;
  esac
done

example 2:
----------
while true
do
  echo "1. Add"
  echo "2. Subtract"
  echo "3. Multiply"
  echo "4. Divide"
  echo "5. Quit"

  read -p "Enter your choice: " choice

  case $choice in
    1)
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( $number1 + $number2 ))
        ;;
    2)
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( $number1 - $number2 ))
        ;;

    3)
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( $number1 * $number2 ))
        ;;
    4)
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( $number1 / $number2 ))
        ;;
    5)
        break
        ;;
  esac

done

Shebang :
=========

For the best practice, It is good to use shebang ( #!/bin/bash ) before our script

Exit codes :
============

Exit code = 0 --> Sucess
Exit code > 1 (non-zero --> Failure

1 --> file not found
127 --> command not found

==> to see exit code : echo $?
==> It is best practice to use exit codes in our scripts.

Functions :  To avoid duplication of code or in order to write DRY code. We use functions.
===========

Function defination :

function function_name() {
.
.
.
}

Function calling :

function_name <--args if any-->

==> We use return statement in function instead of exit statement

use cases:
----------

-> Break up large script that performs many tasks
-> Installing packages
-> Adding Users
-> Configuring firewalls
-> Perform mathematical calculations

Example 1 :
---------

function addition(){
   echo $(( $1 + $2 ))
}

sum=$(addition 3 6)

Example 2 :
-----------

function addition(){
   return $(( $1 + $2 ))
}

addition 3 6
sum=$?

Example 3 :
-----------

#!/bin/bash
function read_numbers(){
  read -p "Enter Number1: " number1
  read -p "Enter Number2: " number2
}

while true
do
  echo "1. Add"
  echo "2. Subtract"
  echo "3. Multiply"
  echo "4. Divide"
  echo "5. Quit"

  read -p "Enter your choice: " choice

  case $choice in
    1)  read_numbers
        echo $(( $number1 + $number2 )) ;;
    2)
        read_numbers
        echo $(( $number1 - $number2 )) ;;

    3)
        read_numbers
        echo $(( $number1 * $number2 )) ;;

    4)
        read_numbers
        echo $(( $number1 / $number2 )) ;;

    5)  break
  esac

done

Tips and tricks :
=================

1. install shellcheck utility through yum or apt-get (yum install -y shellcheck)
2. IDE's : IntelliJIdea, VSCode, Pycharm, Atom, Shell style guide