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

You can download this script from https://github.com/dolevf/Black-Hat-

Bash/blob/master/ch02/test_if_file_exists.sh. Save the file and execute it. You

should see the flow_control_with_if.txt file in your current directory when you run

ls.

Listing 2-2 shows a different way of achieving the same outcome; it uses the

not operator (!) to check whether a directory doesn’t exist, and if so, creates it.

This example has fewer lines of code and eliminates the need for an else block

altogether.

#!/bin/bash

FILENAME="flow_control_with_if.txt"

if [[ ! -f "${FILENAME}" ]]; then

touch "${FILENAME}"

fi

Listing 2-2

An example of a file test using a negative check

Save and run this script. It should create a directory named downloads if this

directory wasn’t already present.

Let’s explore if conditions that use some of the other kinds of test operators

we’ve covered. Listing 2-3 shows an example of a string comparison test. It tests

whether two variables are equal by performing string comparison with the equal-to

operator (==).

#!/bin/bash

VARIABLE_ONE="nostarch"

VARIABLE_TWO="nostarch"

if [[ "${VARIABLE_ONE}" == "${VARIABLE_TWO}" ]]; then

echo "They are equal!"

else

echo "They are not equal!"

fi

Listing 2-3

A string comparison test comparing two string variables

The script will compare the two variables, both of which have the value

nostarch, and print They are equal! by using the echo command. It is

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

Bash/blob/master/ch02/string_comparison.sh.

Next is an example of an integer comparison test, which takes two integers

and checks which one is the larger number.

#!/bin/bash

VARIABLE_ONE="10"

VARIABLE_TWO="20"