A for loop can be used to loop through a list of items in a variable or list.

The following will loop through the values in a variable. By default, a for loop will use whitespace as the delimiter.

#!/bin/bash

items="apple banana orange grape"

for item in $items; do
  echo $item
done

Running the script should produce the following.

apple
banana
orange
grape

If the following is returned, the Internal Field Separator, commonly just known as IFS, may be set to split at new lines, and not whitespaces. 

apple banana orange grape

By the way, declare can be used to determine if the element is a variable or an list. Two dashes (--) means it is a variable.

~]# declare -p items
declare -- items="apple banana orange grape"

Lets say the file named fruit.txt contains "apple banana orange grape". Here is how you can loop through a file.

~]# for line in $(cat /path/to/fruit.txt); do echo $line; done;
apple banana orange grape

And here is how you can loop through the elements in a list.

#!/bin/bash

items=(apple banana orange grape)

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

Running the script should produce the following output.

apple
banana
orange
grape

By the way, declare can be used to determine if the element is a variable or an list. -a means it is a list.

~]# declare -p items
declare -a items='([0]="apple" [1]="banana" [2]="orange" [3]="grape")'

Internal Field Separator (IFS) whitespace delimiter

Let's say you have a variable where each line is separated by a new line.

#!/bin/bash

items="
apples are red
bananas are yellow"

for item in $items; do
  echo $item
done

In this example, the for loop will produce the following result because the Internal Field Separator, commonly known as IFS, uses whitespace as the delimiter.

apples
are
red
bananas
are
yellow

Here is now you can specify a delimiter (new lines in this example).

#!/bin/bash

items="
apples are red
bananas are yellow"

IFS=$'\n'
for item in $items; do
  echo $item
done
unset IFS

Once IFS is set to use new lines as the delimiter, the for loop will split at new lines.

apples are red
bananas are yellow

Loop through directories

Let's say the /opt/servers directory contains sub directories, such as server01, server02, and server03. You can loop through each server.

#!/bin/bash

for server in $(ls /opt/servers); do
  echo $server
done

Running the script will produce the following output. The benefit to this type of approach as that as directories are added or removed from /opt/servers, there is no need to update your script, making the script much more stable and scalable.

server01
server02
server03

Until

In this example, $i is zero, and is incremented by 1, until $i is 7.

for (( i=0; i < 7; i++ )); do
  echo $i
done

Running this script will produce the following.

0
1
2
3
4
5
6