Using for loops

The for loop is often used to iterate over a collection or a list of items and carry out an action.

Four ingredients are required to execute a for loop in Bash, for, in, do, and done.

Consider the array of rainbow colors we mentioned earlier while learning about arrays

#!/bin/bash

rainbows=(red orange yellow green blue indigo violet)

echo "${rainbows[@]}"

We can use a for loop to iterate over the array and print each item like this:

#!/bin/bash

rainbows=(red orange yellow green blue indigo violet)

for color in ${rainbows[@]}
    do
    echo "$color"
done

The combination of these four keywords literally means: for (every item) in (this container/variable/list), do (print the item), and (after the last item, you're) done.

So rewording this to suit the above example, we can say for every color in the rainbows array variable, do print the color, and after printing the last color, you're done.

Make the script executable and run it.

You should see the following output:

red
orange
yellow
green
blue
indigo
violet

The key takeaway here is that, while the echo command simply outputs the value of a variable or a string passed to it, a for loop executes nested commands for each item in the array, typically on a new line, and terminates after executing the last item.

In advanced lessons, you will learn how you can nest a for loop within an echo command and within another for loop.