Nesting Loops
All the loops support nesting concepts which means you can put one loop inside another similar one or different loops. This nesting can go up to an unlimited number of times based on your requirement.
Here is an example of nesting while loop. The other loops can be nested based on the programming requirement in a similar way −
Nesting while Loops
It is possible to use a while loop as part of the body of another while loop.
Syntax
while command1 ; # this is loop1, the outer loop
do
Statement(s) to be executed if command1 is true
while command2 ; # this is loop2, the inner loop
do
Statement(s) to be executed if command2 is true
done
Statement(s) to be executed if command1 is true
done
Example
Here is a simple example of loop nesting. Let’s add another countdown loop inside the loop that you used to count to nine −
#!/bin/sh
a=0
while [ “$a” -lt 10 ] # this is loop1
do
b=”$a”
while [ “$b” -ge 0 ] # this is loop2
do
echo -n “$b “
b=`expr $b – 1`
done
echo
a=`expr $a + 1`
done
This will produce the following result. It is important to note how echo -n works here. Here -n option lets echo avoid printing a new line character.
0
1 0
2 1 0
3 2 1 0
4 3 2 1 0
5 4 3 2 1 0
6 5 4 3 2 1 0
7 6 5 4 3 2 1 0
8 7 6 5 4 3 2 1 0
9 8 7 6 5 4 3 2 1 0
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