I have a confession to make. Up until recently I didn’t know how to write a bash script. I started my career on Windows, so PowerShell was the logical choice and for cross platform scripts I always reached for Node.js. But recently I’ve been moving away from writing JavaScript and I also didn’t want to have to install PowerShell on every Linux machine I wanted to run a script on. So I decided to level up a bit and learn how to write bash scripts.

This is the guide I wish I had. It’s not comprehensive, but it is enough to make you feel comfortable writing bash scripts and hit the ground running. It also serves as a reference for the most common things you’ll (read I’ll) need to do.

Variables

In PowerShell, you declare variables and reference them both by prefixing the variable name with a dollar sign:

$myVariable = "Hello, World!"
Write-Host $myVariable

In bash, you declare variables by assigning a value to them without a dollar sign:

my_variable="Hello, World!"
echo $my_variable

In bash convention for variable names is to use all lower, snake case letters. As opposed to PowerShell where the convention is to use PascalCase or camelCase.

Arrays

For arrays in PowerShell you use the @() syntax:

$myArray = @("Hello", "World")
Write-Host $myArray[0]

In bash, you declare an array by using parentheses and you access elements by using square brackets similar to PowerShell. The declare -a afterwards defines it as an array. Unlike PowerShell, you don’t have a comma between elements in the array:

declare -a my_array=("Hello" "World")
echo ${my_array[0]}

Conditionals

When writing conditionals in PowerShell you use the if, elseif, and else keywords:

if ($myVariable -eq "Hello, World!") {
    Write-Host "It's true"
} elseif ($myVariable -eq "Goodbye, World!") {
    Write-Host "It's false"
} else {
    Write-Host "It's neither. The Universe is a lie."
}

In bash, you use the if, elif, and else keywords:

if [ $my_variable = "Hello, World!" ]; then
    echo "It's true"
elif [ $my_variable = "Goodbye, World!" ]; then
    echo "It's false"
else
    echo "It's neither. The Universe is a lie."
fi

The syntax here is admittedly a little weird and takes some getting used to. The weirdest thing to me is not using a double equals in the equality, but hey, PowerShell uses -eq so who am I to judge?

Loops

In PowerShell you have the foreach loop:

foreach ($item in $myArray) {
    Write-Host $item
}

And the while loop:

$i = 0
while ($i -lt $myArray.Length) {
    Write-Host $myArray[$i]
    $i++
}

In bash you have the for loop:

for item in "${my_array[@]}"; do
    echo $item
done

Also available in bash is the while loop:

i=0
while [ $i -lt ${#my_array[@]} ]; do
    echo ${my_array[$i]}
    i=$((i + 1))
done

Again, the syntax is a little weird, but it’s not too hard to understand. The @ symbol is used to reference the entire array in bash, and the # symbol is used to get the length of the array. -lt is used to compare if one number is less than another.

Functions

The syntax for functions in PowerShell is:

function SayHello {
    param (
        [string]$name
    )

    Write-Host "Hello, $name!"
}

SayHello -name "World"

When in bash, you declare and call a function like this:

function say_hello {
    name=$1

    echo "Hello, $name!"
}

say_hello "World"

Command Line Arguments

In PowerShell you can declare your script to take arguments like this:

param (
    [string]$name
)

Write-Host "Hello, $name!"

It looks exactly like declaring a function parameter, but it’s at the top of the script. Similarly, you use the $1, $2, etc., syntax to access the arguments passed to the script itself as well.

In bash, you access the arguments passed to the script using $1, $2, etc.:

name=$1

echo "Hello, $name!"

Pipes

Now here is where things get a very different. In PowerShell | is used to pipe objects from one command to another. In bash, | is used to pipe the output of one command to the input of another. That output in bash is just text, it’s completely unstructured. So you can’t pipe objects from one command to another without using something like jq to convert the text to JSON. They syntax though is the same:

echo "Hello, World!" | grep "World"

Conclusion

There’s so much more to bash than what I’ve covered here. There are things like redirection of outputs, doing more advanced arithmetic, handling errors, getting input from user and more. If you want to know more checkout this excellent bash guide by Mike G., or checkout the man pages for bash by running man bash in your terminal.

Now you have the basics, you know where to look for more information, and you can start writing your own bash scripts. Good luck out there!