Note: These notes are for my personal reference!

𝐁𝐚𝐬𝐡 𝐒𝐜𝐫𝐢𝐩𝐭𝐢𝐧𝐠 for Hackers & Bug Bounty Hunters

𝐁𝐚𝐬𝐢𝐜𝐬

• Telling interpreter that the file is bash file

!#/bin/bash

• Make the file executable

$ chmod +x script.sh

𝐂𝐨𝐦𝐦𝐞𝐧𝐭𝐬

# this is a single line comment 

# Multi line comment

# - Method I
<<COMMENTS
  This 
  is a 
  multiline
  comment
COMMENTS

# - Method II
:'
 This 
  is a 
  multiline
  comment
'

𝐕𝐚𝐫𝐢𝐚𝐛𝐥𝐞𝐬

• Syntax: variable_name=variable_value
• Note: There should not be any white spaces on either side of the =
• Single quotes (') helps to treat every character as it is
• Double quotes (") helps to do the substitution

---

System Defined Variables	Meaning
BASH	represents the Shell Name.
BASH_VERSION	specifies the shell version which the Bash holds.
COLUMNS	specify the no. of columns for our screen
HOME	specifies the home directory for the user
LOGNAME	specifies the logging user name.
OSTYPE	tells the type of OS.
PWD	represents the current working directory.
USERNAME	specifies the name of currently logged in user.

                                                                                      
#!/bin/bash

echo $BASH
echo $BASH_VERSION
echo $COLUMNS
echo $HOME
echo $LOGNAME
echo $OSTYPE
echo $PWD
echo $USERNAME
                                                                                       

---

𝐑𝐞𝐚𝐝𝐢𝐧𝐠 𝐈𝐧𝐩𝐮𝐭

• Syntax: read [flag] varname
  • -s : for silent mode
  • -p : for prompt mode
  • -a : for arrays

┌──(shreyas㉿kali)-[~/practise/bash_scripting]
└─$ cat read.sh  
#!/bin/bash

echo "firstname: "
read firstname
echo "lastname: "
read lastname

echo "Hello Mr. $firstname $lastname"

┌──(shreyas㉿kali)-[~/practise/bash_scripting]
└─$ ./read.sh  
firstname: 
shreyas
lastname: 
chavhan
Hello Mr. shreyas chavhan

┌──(shreyas㉿kali)-[~/practise/bash_scripting]
└─$ vim read_prompt.sh
                                                                                       
┌──(shreyas㉿kali)-[~/practise/bash_scripting]
└─$ chmod +x read_prompt.sh 
                                                                                       
┌──(shreyas㉿kali)-[~/practise/bash_scripting]
└─$ cat read_prompt.sh     
#!/bin/bash

read -p "Your Name: " name

echo "Hello Mr. $name"
                                                                                       
┌──(shreyas㉿kali)-[~/practise/bash_scripting]
└─$ ./read_prompt.sh 
Your Name: Shreyas
Hello Mr. Shreyas
                          

┌──(shreyas㉿kali)-[~/practise/bash_scripting]
└─$ vim read_silent.sh
                                                                                       
┌──(shreyas㉿kali)-[~/practise/bash_scripting]
└─$ ./read_silent.sh  
Your Password 
Your Password is mypassword, it's secret - don't tell anyone!
                                                                                       
┌──(shreyas㉿kali)-[~/practise/bash_scripting]
└─$ cat read_silent.sh 
#!/bin/bash


read -sp "Your Password " password

echo ""
echo "Your Password is $password, it's secret - don't tell anyone!"

• Multi-line input

while read -r line; do
   printf '%s\n' "$line"
done

---

𝐁𝐚𝐬𝐡 𝐢𝐟-𝐞𝐥𝐢𝐟-𝐞𝐥𝐬𝐞

• Syntax

if [condition]; then
  <blah blah>
elif [condition]; then
  <blah blah>
else
  <blah blah>
fi

• || : OR
• && : AND

if [] || []; then
  <blah blah>
elif [] && []; then
  <blah blah>
else
  <blah blah>
fi

Operators	Description
! EXPRESSION	To check if EXPRESSION is false.
-n STRING	To check if the length of STRING is greater than zero.
-z STRING	To check if the length of STRING is zero (i.e., it is empty)
STRING1 == STRING2	To check if STRING1 is equal to STRING2.
STRING1 != STRING2	To check if STRING1 is not equal to STRING2.
INTEGER1 -eq INTEGER2	To check if INTEGER1 is numerically equal to INTEGER2.
INTEGER1 -gt INTEGER2	To check if INTEGER1 is numerically greater than INTEGER2.
INTEGER1 -lt INTEGER2	To check if INTEGER1 is numerically less than INTEGER2.
-d FILE	To check if FILE exists and it is a directory.
-e FILE	To check if FILE exists.
-r FILE	To check if FILE exists and the read permission is granted.
-s FILE	To check if FILE exists and its size is greater than zero (which means that it is not empty).
-w FILE	To check if FILE exists and the write permission is granted.
x FILE	To check if FILE exists and the execute permission is granted.

---

𝐁𝐚𝐬𝐡 𝐂𝐚𝐬𝐞𝐬

• Syntax:

#!/bin/bash  
  
echo "Which Operating System are you using?"  
echo "Windows, Android, Chrome, Linux, Others?"  
read -p "Type your OS Name:" OS  
  
case $OS in  
    Windows|windows)  
        echo "That's common. You should try something new."  
        echo  
        ;;  
    Android|android)  
        echo "This is my favorite. It has lots of applications."  
        echo  
        ;;  
    Chrome|chrome)  
        echo "Cool!!! It's for pro users. Amazing Choice."  
        echo  
        ;;  
    Linux|linux)  
        echo "You might be serious about security!!"  
        echo  
        ;;  
    *)  
        echo "Sounds interesting. I will try that."  
        echo  
        ;;  
esac  

---

𝐁𝐚𝐬𝐡 𝐟𝐨𝐫 𝐋𝐨𝐨𝐩

• C++ like for loop

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

• To read a range

for num in {1..10}  
  do  
  echo $num  
done  

• a range with increment

for num in {1..10..1}  
  do  
  echo $num  
done  

• a range with decrement

for num in {10..0..1}  
  do  
  echo $num  
done  

• Array variables

array=(  "element1" "element 2" .  .  "elementN" )  
  
for i in "${arr[@]}"  
  do  
  echo $i  
done  

• white spaces in String as word separators

#!/bin/bash  
  
for word in $str;  
do  
  <Statements>  
done  

• Each line in string as a word

#!/bin/bash  
  
for word in "$str";  
  do  
  <Statements>  
done  

• Infinite loop

i=1;  
for (( ; ; ))  
  do  
  sleep 1s  
  echo "Current Number: $((i++))"  
done  

---

𝐁𝐚𝐬𝐡 𝐰𝐡𝐢𝐥𝐞 𝐥𝐨𝐨𝐩

• C++ Style while loop

i=1  
while((i <= 10))  
  do  
  echo $i  
  let i++  
done  

𝐂𝐮𝐭 𝐂𝐡𝐞𝐚𝐭𝐬𝐡𝐞𝐞𝐭

Command	Explanation
cut -c 3	display 3rd character from each line of text
cut -c 2,7	display the 2nd and 7th character from each line of text
cut -c 2-7	display a range of characters starting at the 2nd position of a string and ending at the 7th position(both positions included)
cut -c -4	display the first four characters from each line of text
cut -d $'\t' -f -3	display first three fields of a tab delimited file
cut -c 13-	display the characters from 13th position to the end
cut -d ' ' -f 4	display 4th word with space ' ' as a delimiter
cut -d ' ' -f -3	display first three words with space ' ' used as a delimiter
cut -d $'\t' -f 2-	given a tab delimited file, display the fields from second fields to last field

𝐎𝐭𝐡𝐞𝐫 𝐍𝐨𝐭𝐞𝐬

• Chop off the arithmetic operations to decimal points: bc <<< "scale=3; $expression"

• Round of the arithmetic operation result: printf %.3f $(echo $expression | bc -l)

• Performing Arithmatic Operations (add, subtract, multiply divide) on two variables: $((EXPR))

read -s X;
read -s Y;

echo $(($X + $Y));
echo $(($X - $Y));
echo $(($X * $Y));
echo $(($X / $Y));

• head : output the first part of files

head -n <number of lines>       # display first n lines from a text file
head -c <number of characters>  # display first n characters from a text file

• tail : output the last part of files

tail -n <number of lines>      # display last n lines from a text file
tail -c <number of characters> # display last n characters from a text file

• Reading a file line by line

while read -r line; do <command> "$line" done < filename

Multi-threading or Parallel Execution in bash

• We make use of parallel command to execute multiple threads in parallel
• Installing parallel

sudo apt install parallel

• Reading Man Page

man parallel

• Usage

parallel [options] [command [arguments]] < list_of_arguments

parallel [options] [command [arguments]] ( ::: arguments | :::+ arguments | :::: argfile(s) | ::::+ argfile(s) ) ...

parallel --semaphore [options] command

#!/usr/bin/parallel --shebang [options] [command [arguments]]

#!/usr/bin/parallel --shebang-wrap [options] [command [arguments]]

---

Done