Black Hat Bash - Bash Scripting for Hackers and Pentesters (Early Access) (Dolev Farhi, Nick Aleks)

IP_ADDRESS="${1}"

case ${IP_ADDRESS} in

192.168.*)

echo "Network is 192.168.x.x"

;;

10.0.*)

echo "Network is 10.0.x.x"

;;

*)

echo "Could not identify the network."

;;

esac

Listing 2-18

A case statement to check an IP address and determine its network

We define a variable that expects one command line argument to be passed

(${1}) and saves it to the IP_ADDRESS variable. We then use a pattern to check

whether the IP_ADDRESS variable starts with 192.168. and a second pattern

to checks whether it starts with 10.0..

We also define a default wildcard pattern using *, which returns a default

message to the user if nothing else has matched.

Save this file as case_ip_address_check.sh and run it:

$ chmod u+x case_ip_address_check.sh

$./case_ip_address_check.sh 192.168.12.55

Network is 192.168.x.x

$./case_ip_address_check.sh 212.199.2.2

Could not identify the network.

This script is available at https://github.com/dolevf/Black-Hat-

Bash/blob/master/ch02/case_ip_address_check.sh.

Text Processing and Parsing

One of the most common things you’ll find yourself doing in bash is

processing text. You can parse text on the command line by running one-off

commands, or use a script to store parsed data in a variable that you can act on in

some way. In both cases, the skill is important to many scenarios.

To test the commands in this section on your own, download the sample log

file from https://github.com/dolevf/Black-Hat-Bash/blob/master/ch02/log.txt. This

file is space-separated, and each segment represents a specific data type, such as

the client’s source IP address, timestamp, HTTP method, HTTP path, HTTP User

Agent field, HTTP status code, and more.