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

Next, we executed ls -l, which simply listed the directories.

The result of the ls command succeeded without any specific

errors, so it was sent to the standard output stream.

You’ll practice working with the standard input stream when we

introduce redirection in “Redirection Operators” on page XX.

Control Operators

Control operators in bash are tokens that perform a control

function. Table 1-5 gives an overview of control operators.

Table 1-5

Bash Control Operators

Operator

Description

Sends a command to the background.

Used as a logical AND. The second command in the expression will be

evaluated only if the first command evaluated to true.

( and )

Used for command grouping

;

Used as a list terminator. A command following the terminator will run af-

ter the preceding command has finished, regardless of whether it evalu-

ates to true or not.

;;

Ends a case statement.

Redirects the output of a command as input to another command.

Used as a logical OR. The second command will run if the first one evalu-

ates to false.

Let’s see some of these control operators in action. The &

operator sends any command to the background. If you have a list of

commands in a shell script, sending the first command to the

background will allow bash to continue to the next line even if the

previous command hasn’t finished its work. Commands that are

long-running are often sent to the background to prevent scripts from

hanging.

#!/bin/bash

# This script will send the sleep command to the background

echo "Sleeping for 10 seconds..."

sleep 10 &

# Creates a file

echo "Creating the file test123"

touch test123

# Delete a file

echo "Deleting the file test123"

rm test123