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

Listing 2-16

The use of continue in a for loop

We start a for loop using the example_file* glob, which will expand to

match the names of all files starting with example_file in the directory where the

script runs 1. As a result, the loop should iterate over all three files we created

earlier. Within the loop, we use a file test operator to check whether the filename is

equal to example_file1 because we want to skip this file and not make any changes

to it. If the condition is met, we use the continue statement 2 to proceed to the

next iteration, leaving the file unmodified. Later in the loop, we use the echo

command with the environment variable ${RANDOM} to generate a random

number and write it into the file.

Save this script as for_loop_continue.sh and execute it in the same directory

as the three files.

$ chmod u+x for_loop_continue.sh

$./for_loop_continue.sh

Skipping the first file.

If you examine the files, you should see that the first file is empty while the

other two contain a random number as a result of the script echoing the value of

the ${RANDOM} environment variable into them.

Case Statements

In bash, case statements allow you to test multiple conditions in a cleaner

way by using more readable syntax. Often, they help you avoid many if

conditions, which can become harder to read as they grow in size.

Listing 2-17 shows the case statement syntax.

case EXPRESSION in

PATTERN1)

# do something if the first condition is met

;;

PATTERN2)

# do something if the second condition is met

;;

esac

Listing 2-17

The case statement syntax

Case statements start with the keyword case followed by some expression,

such as a variable you want to match a pattern against. PATTERN1 and

PATTERN2 in this example represent some pattern case (such as a regular

expression, a string, or an integer) that you want to compare to the expression. To

close a case statement, you use the keyword esac (case inverted).

Let’s take a look at an example case statement that checks whether an IP

address is present in a specific private network (Listing 2-17).

#!/bin/bash