Shell Scripting: Expert Recipes for Linux, Bash, and More

Nested Loops

It is possible to put one loop inside another, even to put different kinds of loops within one another. Although there is no real limit to the number of loops that can be nested, the indentation becomes complicated, and it soon gets difficult to keep track of where all of the loops end. Nested loops are useful because you can use the best attributes of any type of loop that you need. Here, the while loop is best suited for running continuously until the user types the word “quit” to exit the loop. Inside the while loop, the for loop is best suited to iterating over a fixed set of items (fruits in the case of the code that follows). Although $myfruit is listed at the end of the loop, on the first iteration it is blank (myfruit="") so the first run lists only three fruits. Subsequent runs include the user’s favorite fruit at the end of the list.

$ cat nest.sh
#!/bin/sh
myfruit=""
while [ "$myfruit" != "quit" ]
do
  for fruit in apples bananas pears $myfruit
  do
    echo "I like $fruit"
  done # end of the for loop
  read -p "What is your favorite fruit? " myfruit
done # end of the while loop
echo "Okay, bye!"
$ ./nest.sh
I like apples
I like bananas
I like pears
What is your favorite fruit? grapes
I like apples
I like bananas
I like pears
I like grapes
What is your favorite fruit? plums I like apples I like bananas I like pears I like plums What is your favorite fruit?  ...