Penetration Test

Bash Scripting Basics

COMMENTS

• Help you remember what you were thinking

  • All comments start with the '#' character

  • Anything after '#' is ignored by the interpreter

  • Ex:

    # This is a comment
    

VARIABLES

• varName=value

  • Ex:

    name=Eric
    

• echo $name

• Common to read data into variables, as opposed to hard coding too much

• Bash variables are untyped

SUBSTITUTIONS

• "$" prefix refers to the contents of an identifier(ex. echo $name)

• Can refer to

  • Variables

    $name
    

  • Input parameters

    $1
    

  • Environment variables

    $PATH
    

  • Values from utilities

    $(whoami)
    

  • And, bash will set defaults when no other value is provided

    JAVAPATH=${JAVAPAHOME:=/usr/lib/java}
    

    OUTPUTDIR=${1:/tmp} #IMPORTANT DIFFERENCE
    

COMMON OPERATIONS

• String operations

  • Concatenate

    var="Hello";var="$var World"
    

  • Length

    ${#string} OR expr length $string ex.${#name}
    

  • Extract a substring

    echo ${string:position} ex.${name:3}
    

  • Replacing substring

    ${string/substring/replacement} ex.${name/ch/xx}
    

• Compound operations

  • AND: -a
  • OR: -o
  • if["(varA" -eq ")varB"]
  • Equal: -eq OR ==
  • Not equal: -ne OR !=
  • Greater than, greater than or equal to: -gt or >, -ge OR >=
  • Less than, less than or equal to: -lt OR <, -le OR <=
  • Not null (empty string): -n
  • Null (empty string): -z

LOGIC

• Looping - for

  for var in list
  do
    Statement(s)
  done
  

  • Examples

    for in in 1 2 3 4 5
    

    for i in $(seq 1 5)
    

FLOW CONTROL

if condition
then
  commands
endif condition
then
  commands
else
  commands
fi

if name=Michael
  then <run some command>

If name doesn't equal Michael...

elif name=Eric
  then <run some command>

If name doesn't equal Michael OR Mary ...

else
  then <run some command>
fi

BASH if CONDITIONS

Expression	Description
-d file	True if file is a directory
-e file	True if file exists
-f file	True if file exists and is a regular file
-z string	True if string is a null (empty) string
-n string	True if string is not a null(empty string)
stringA = stringB	True if strings are equal
stringA != stringB	True if strings are not equal

BASH SCRIPTING

• test / []

  if test -eq $name "Eric"
  if [$name = "Eric"]
  

• break

  • Exits the current loop iteration

• exit

  • Exits a script and returns a value (exit code)

BASH PORT SCANNER

#!/bin/bash

target=$1
minPort=$2
maxPort=$3

function scanports
{
for ((counter=$minPort; counter<=$maxPort; counter++))
do
	(echo >/dev/tcp/$target/$counter) > /dev/null 2 >&1 && echo "$counter open"
done
}

scanports

QUICK REVIEW

• Bash is the default shell in Linux
• Bash makes it easy to combine multiple commands that can react to input
• Learn basic loops and conditional logic
• A few lines of a bash script can automatically execute many commands, such as scans