Kali Linux Penetration Testing Bible. Gus Khawaja

Table 2.3 File/Directory Conditions

File exists?	[[ ‐a filename ]]
Directory exists?	[[ ‐d directoryname ]]
Readable file?	[[ ‐r filename ]]
Writable file?	[[ ‐w filename ]]
Executable file?	[[ ‐x filename ]]
File not empty?	[[ ‐s filename ]]

      Loops

      You can write loops in two different ways: using a while loop or using a for loop. Most of the programming languages use the same pattern for loops. So, if you understand how loops work in Bash, the same concept will apply for Python, for example.

      Let's start with a while loop that takes the following structure:

      while [[ condition ]] do do something done

      The best way to explain a loop is through a counter from 1 to 10. We'll develop a program that displays a progress bar:

      #!/bin/bash #Progress bar with a while loop #Counter COUNTER=1 #Bar BAR='##########' while [[ $COUNTER -lt 11 ]] do #Print the bar progress starting from the zero index echo -ne "\r${BAR:0:COUNTER}" #Sleep for 1 second sleep 1 #Increment counter COUNTER=$(( $COUNTER +1 )) done

Note that the condition ( [[ $COUNTER ‐lt 11]] ) in the while loop follows the same rules as the if condition. Since we want the counter to stop at 10, we will use the following mathematical formula: counter<11 . Each time the counter is incremented, it will display the progress. To make this program more interesting, let it sleep for one second before going into the next number.

      On the other hand, the for loop will take the following pattern:

      for … in [List of items] do something done

      We will take the same example as before but use it with a for loop. You will realize that the for loop is more flexible to implement than the while loop. (Honestly, I rarely use the while loop.) Also, you won't need to increment your index counter; it's done automatically for you:

      #!/bin/bash #Progress bar with a For Loop #Bar BAR='##########' for COUNTER in {1..10} do #Print the bar progress starting from the zero index echo -ne "\r${BAR:0:$COUNTER}" #Sleep for 1 second sleep 1 done

      File Iteration

      Here's what you should do to simply read a text file in Bash using the for loop:

      for line in $(cat filename) do do something done

      In the following example, we will save a list of IP addresses in a file called ips.txt . Then, we will reuse the Nmap ping program (that we created previously) to check whether every IP address is up or down. On top of that, we will check the DNS name of each IP address:

      #!/bin/bash #Ping & get DNS name from a list of IPs saved in a file #Prompt the user to enter a file name and its path. read -p "Enter the IP addresses file name / path:" FILE_PATH_NAME function check_host(){ #if not the IP address value is empty if [[ -n $IP_ADDRESS ]] then ping_cmd=$(nmap -sn $IP_ADDRESS| grep 'Host is up' | cut -d '(' -f 1) echo '------------------------------------------------' if [[ -z $ping_cmd ]] then printf "$IP_ADDRESS is down\n" else printf "$IP_ADDRESS is up\n" dns_name fi fi } function dns_name(){ dns_name=$(host $IP_ADDRESS) printf "$dns_name\n" } #Iterate through the IP addresses inside the file for ip in $(cat $FILE_PATH_NAME) do IP_ADDRESS=$ip check_host done

If you have followed carefully through this chapter, you should be able to understand everything you see in the previous code. The only difference in this program is that I used Tab spacing to make the script look better. The previous example covers most of what we did so far, including the following:

       User input

       Declaring variables

       Using functions

       Using if conditions

       Loop iterations

       Printing to the screen

Summary

I hope you have practiced all the exercises in this chapter, especially if you're new to programming. A lot of the concepts mentioned will apply to many programming languages, so consider the exercises as an opportunity to learn the basics.

      I personally use Bash scripting for small and quick scenarios. If you want to build more complex applications, then you can try doing that in Python instead. Don't worry! You will learn about Python at the end of this book so you can tackle any situation you want in your career as a penetration tester.

      Finally, this chapter covered a lot of information about Bash scripting. However, there is a lot more information than what is in this chapter. In practice, I use internet search engines to quickly find Bash scripting references. In fact, you don't need memorize everything you learned in this chapter. Remember that this book is a reference on which you can always rely to remember the syntaxes used in each case.