Looping through an explicit list of items

#!/bin/bash

for i in a B c 1 2 3
do
  echo $i
done

Looping through a range of numbers.

#!/bin/bash

for i in {1..5}
do
  echo $i
done

Looping through a range of letters.

#!/bin/bash

for i in {a..z}
do
  echo $i
done

Looping through the output of the ls command.

#!/bin/bash

for i in $(ls)
do
  echo $i
done