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

if [[ "${VARIABLE_ONE}" -gt "${VARIABLE_TWO}" ]]; then

echo "${VARIABLE_ONE} is greater than ${VARIABLE_TWO}"

else

echo "${VARIABLE_ONE} is less than ${VARIABLE_TWO}"

fi

We create two variables, VARIABLE_ONE and VARIABLE_TWO, and assign

them values of 10 and 20, respectively. We then use the -gt operator to compare

the two values and print the one that is greater. This script is available at

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

Bash/blob/master/ch02/integer_comparison.sh.

Linking Conditions

So far, we’ve used if to check whether a single condition is met. But like

most programming languages, we can also use the or (||) and and (&&) operators

to check for multiple conditions at once.

For example, what if we wanted to check that a file exists and also that its size

greater than zero? Listing 2-4 does so.

#!/bin/bash

echo "Hello World!" > file.txt

if [[ -f "file.txt" ]] && [[ -s "file.txt" ]]; then

echo "The file exists and its size is greater than zero".

fi

Listing 2-4

Chaining two file-test conditions using an and condition

This code writes some content to a file, then checks whether that file exists

and whether its size is greater than zero. Both conditions have to be met in order

for the echo command to be executed. If either returns false, nothing will happen.

To demonstrate an or condition, Listing 2-5 checks whether a file is either of

file or directory type:

#!/bin/bash

DIR_NAME="dir_test"

mkdir "${DIR_NAME}"

if [[ -f "${DIR_NAME}" ]] || [[ -d "${DIR_NAME}" ]]; then

echo "${DIR_NAME} is either a file or a directory."

fi

Listing 2-5

Chaining two file test conditions using or

This code first creates a directory, then uses an if condition with the or (||)

operator to check whether the variable is a file (-f) or directory (-d). The second

condition should evaluate to true, and the echo command should execute.