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

echo "${USER_INPUT} is a file"

3 elif [[ -d "${USER_INPUT}" ]]; then

echo "${USER_INPUT} is a directory"

else

4 echo "${USER_INPUT} is not a file or a directory"

fi

Listing 2-7

Using if and elif statements

We begin with an if statement that checks whether the variable

USER_INPUT is null 1. This allows us to exit the script early using exit 1 if we

receive no command line arguments from the user. We then begin a second if

condition that uses the file test operator to check whether the input is a file 2.

Below this condition, we use elif to test whether the argument is a directory 3.

This condition won’t be tested unless the file test fails. If neither of these

conditions is true, the script responds that the argument is neither a file nor a

directory 4.

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

Bash/blob/master/ch02/if_elif.sh.

Functions

Functions help us reuse blocks of code so we can avoid repeating them. They

allow us to run multiple commands and other bash code together by simply

entering the function’s name. To define a new function, enter a name for it,

followed by parentheses (). Then place the code you would like the function to

run within curly brackets {}:

#!/bin/bash

say_name(){

echo "Black Hat Bash"

}

Here, we define a function called say_name that executes a single echo

command. To call a function, simply enter its name:

say_name

If the function is not called, the commands within it won’t run.

Returning Values

Like commands and their exit statuses, functions can return values using the

return keyword. If there is no return statement, the function will return the

code of the last command it ran. For example, the function in Listing 2-8 returns a

different value based on whether the current user is root or not:

#!/bin/bash

1 check_if_root(){