Loops in Shell Scripting: Iterating with Variables

In shell scripting, loops are a crucial construct for executing repetitive tasks. One common requirement is to iterate over a range of numbers, where the upper limit may be variable. In this tutorial, we’ll explore how to achieve this using different approaches.

Understanding the Problem

When working with loops in shell scripts, you might encounter situations where you need to iterate from a starting number to an ending number that is stored in a variable. The challenge arises when trying to use the variable as part of the loop syntax.

Using seq Command

One solution involves using the seq command, which generates a sequence of numbers. Here’s how you can use it:

max=10
for i in $(seq 2 $max)
do
    echo "$i"
done

In this example, $(seq 2 $max) generates numbers from 2 to the value stored in $max, and the loop iterates over these numbers.

Arithmetic for Loop

Another approach is to use the arithmetic-expression version of the for loop. This method allows you to directly specify the range using variables:

max=10
for (( i=2; i <= $max; ++i ))
do
    echo "$i"
done

This syntax is more flexible and doesn’t rely on external commands like seq. It’s a good choice when working with bash or compatible shells.

Manual Looping

If you prefer not to use the for loop, you can achieve similar results with a while loop:

i=2
max=10
while [ $i -le $max ]
do
    echo "$i"
    (( i++ ))
done

This method requires manual increment of the loop variable ($i) but offers an alternative way to iterate over a range.

Best Practices

When working with loops and variables in shell scripts, keep the following tips in mind:

• Use double quotes around variable expansions to prevent word splitting.
• Prefer the arithmetic for loop when possible, as it’s more flexible and efficient.
• Be aware of the differences between various shells (e.g., bash, sh) and their compatibility with certain syntax.

By mastering these techniques, you’ll be able to write more effective and dynamic shell scripts that can handle variable ranges with ease.