`

Elements of a Bash Script

In this section, you’ll learn the building blocks of a bash script,

including how to use comments to document what a script does, how

to tell Linux to use a specific interpreter to execute the script, and

how to style your scripts for better readability.

Bash doesn’t have an official style guide, but we recommend

adhering to Google’s shell style guide

(https://google.github.io/styleguide/shellguide.html), which outlines

best practices to follow when developing bash code. If you work on

a team of penetration testers and have an exploit code repository,

using good code styling practices will help your team maintain it.

The Shebang Line

Every script should begin with the shebang line, a character

sequence that starts with the hash and exclamation mark symbols

(#!), followed by the full path to the script interpreter. Listing 1-3

shows an example of a shebang line for a typical bash script.

#!/bin/bash

--snip--

Listing 1-3

The shebang line

The bash interpreter is typically located at /bin/bash. If you

instead wrote scripts in Python or Ruby, your shebang line would

include the full path to the Python or Ruby interpreter.

You’ll sometimes encounter bash scripts that make use of a

shebang line like the following.

#!/usr/bin/env bash

--snip—

Listing 1-4

A portable shebang line

You may want to use this shebang line because it is more

portable than the one in Listing 1-3. Some Linux distributions place

the bash interpreter in different system locations, and this shebang

line will attempt to find that location. This approach could be

particularly useful in penetration tests, where you might not know

the location of the bash interpreter on the target machine. For

simplicity, however, we’ll use the shebang version from Listing 1-3

throughout this book.

Black Hat Bash (Early Access) © 2023 by Dolev Farhi and Nick Aleks