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

2 if [[ "${EUID}" -eq "0" ]]; then

return 0

else

return 1

fi

}

3 is_root=$(check_if_root)

4 if [[ "${is_root}" -eq "0" ]]; then

echo "user is root!"

else

echo "user is not root!"

fi

Listing 2-8

An if condition to test whether a function returned true or false

We define the check_if_root function 1. Within this function, we use an

if condition with an integer comparison test 2, accessing the environment

variable EUID to get the effective running user’s ID and checking whether it

equals 0. If so, the user is root, and the function returns 0; if not, it returns 1. Next,

we call the function and assign the result returned to the variable is_root 3.

Then we use another integer comparison test to determine this value 4. This code

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

Bash/blob/master/ch02/check_root_function.sh.

Bash scripts that perform privileged actions often check whether the user is

root before attempting to install software, create users, delete groups, and so on.

Attempting to perform privileged actions on Linux without the necessary

privileges will result in errors, so this check helps handle these cases.

Accepting Arguments

In the previous chapter, we covered the passing of arguments to commands on

the command line. Functions can also take arguments using the same syntax. For

example, the function in Listing 2-9 prints the first three arguments it receives:

#!/bin/bash

print_args(){

echo "first: ${1}, second: ${2}, third: ${3}"

}

1 print_args No Starch Press

Listing 2-9

A function with arguments

To call a function with arguments, simply enter its name and the arguments

separated by spaces 1. Save this script as function_with_args.sh and run it:

$ chmod u+x function_with_args.sh

$ ./function_with_args.sh

first: No, second: Starch, third: Press