010-bash-conditionals.md

Bash Conditionals

In the last section, we covered some of the most popular conditional expressions. We can now use them with standard conditional statements like if, if-else and switch case statements.

If statement

The format of an if statement in Bash is as follows:

if [[ some_test ]]
then
    <commands>
fi

Here is a quick example which would ask you to enter your name in case that you've left it empty:

#!/bin/bash

# Bash if statement example

read -p "What is your name? " name

if [[ -z ${name} ]]
then
    echo "Please enter your name!"
fi

If Else statement

With an if-else statement, you can specify an action in case that the condition in the if statement does not match. We can combine this with the conditional expressions from the previous section as follows:

#!/bin/bash

# Bash if statement example

read -p "What is your name? " name

if [[ -z ${name} ]]
then
    echo "Please enter your name!"
else
    echo "Hi there ${name}"
fi

You can use the above if statement with all of the conditional expressions from the previous chapters:

#!/bin/bash

admin="devdojo"

read -p "Enter your username? " username

# Check if the username provided is the admin

if [[ "${username}" == "${admin}" ]] ; then
    echo "You are the admin user!"
else
    echo "You are NOT the admin user!"
fi

Here is another example of an if statement which would check your current User ID and would not allow you to run the script as the root user:

#!/bin/bash

if (( $EUID == 0 )); then
    echo "Please do not run as root"
    exit
fi

If you put this on top of your script it would exit in case that the EUID is 0 and would not execute the rest of the script. This was discussed on the DigitalOcean community forum.

You can also test multiple conditions with an if statement. In this example we want to make sure that the user is neither the admin user or the root user to ensure the script is incapable of causing too much damage. We'll use the or operator in this example, noted by ||. This means that either of the conditions needs to be true. If we used the and operator of && then both conditions would need to be true.

#!/bin/bash

admin="devdojo"

read -p "Enter your username? " username

# Check if the username provided is the admin

if [[ "${username}" != "${admin}" ]] || [[ $EUID != 0 ]] ; then
    echo "You are not the admin or root user, but please be safe!"
else
    echo "You are the admin user! This could be very destructive!"
fi

If you have multiple conditions and scenerios, then can use elif statement with if and else statements.

#!/bin/bash

read -p "Enter a number: " num

if [[ $num -gt 0 ]] ; then
    echo "The number is positive"
elif [[ $num -lt 0 ]] ; then
    echo "The number is negative"
else
    echo "The number is 0"
fi

Switch case statements

As in other programming languages, you can use a case statement to simplify complex conditionals when there are multiple different choices. So rather than using a few if, and if-else statements, you could use a single case statement.

The Bash case statement syntax looks like this:

case $some_variable in

  pattern_1)
    commands
    ;;

  pattern_2| pattern_3)
    commands
    ;;

  *)
    default commands
    ;;
esac

A quick rundown of the structure:

• All case statements start with the case keyword.
• On the same line as the case keyword, you need to specify a variable or an expression followed by the in keyword.
• After that, you have your case patterns, where you need to use ) to identify the end of the pattern.
• You can specify multiple patterns divided by a pipe: |.
• After the pattern, you specify the commands that you would like to be executed in case that the pattern matches the variable or the expression that you've specified.
• All clauses have to be terminated by adding ;; at the end.
• You can have a default statement by adding a * as the pattern.
• To close the case statement, use the esac (case typed backwards) keyword.

Here is an example of a Bash case statement:

#!/bin/bash

read -p "Enter the name of your car brand: " car

case $car in

  Tesla)
    echo -n "${car}'s car factory is in the USA."
    ;;

  BMW | Mercedes | Audi | Porsche)
    echo -n "${car}'s car factory is in Germany."
    ;;

  Toyota | Mazda | Mitsubishi | Subaru)
    echo -n "${car}'s car factory is in Japan."
    ;;

  *)
    echo -n "${car} is an unknown car brand"
    ;;

esac

With this script, we are asking the user to input a name of a car brand like Telsa, BMW, Mercedes and etc.

Then with a case statement, we check the brand name and if it matches any of our patterns, and if so, we print out the factory's location.

If the brand name does not match any of our case statements, we print out a default message: an unknown car brand.

Conclusion

I would advise you to try and modify the script and play with it a bit so that you could practice what you've just learned in the last two chapters!

For more examples of Bash case statements, make sure to check chapter 16, where we would create an interactive menu in Bash using a cases statement to process the user input.