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if Conditions

In bash, we can use an if condition to execute some code only when a

condition is met. Its syntax is as follows:

if [[ condition ]]; then

# do something if the condition is met

else

# do something if the condition is not met

fi

We start with the if keyword, followed by a test conditions between double

brackets ([[ ]]). We then use the ; character to separate the if keyword from

the then keyword, which allows us to introduce a block of code that runs only if

the condition is true.

Next, we use the else keyword to introduce a fallback code block that runs if

the condition is not met. Note that else is optional, and you may not always need

it. Finally, we close the if condition with the fi keyword (which is if inversed).

N O T E

In some operating systems, such as those often used in containers, the default shell

might not necessarily be bash. To account for these cases, you may want to use single

brackets ([…]) rather than double brackets to enclose your condition. This use of

single brackets meets the POSIX standard and should work on almost any Unix

derivative, including Linux.

Let’s see an if condition in practice. Listing 2-1 uses an if condition to test

whether a file exists, and if not, creates it.

#!/bin/bash

FILENAME="flow_control_with_if.txt"

if [[ -f "${FILENAME}" ]]; then

echo "${FILENAME} already exists."

exit 1

else

touch "${FILENAME}"

fi

Listing 2-1

An if condition to test for the existence of a file

We first create a variable named FILENAME containing the name of the file

we need. This saves us from having to repeat the filename in the code. We then

introduce the if statement, which uses a condition that tests for the existence of

files with the -f file test operator. If this condition is true, we use echo to print to

the screen a message explaining that the file already exists and then exit the

program using the status code 1 (failure). Using the else block, which will

execute only if the file does not exist, we create the file using the touch

command.

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