Everything You Need to Know About the Until Loop in Shell Script

In shell scripting, loops are used to execute a set of commands repeatedly until a certain condition is met.

There are various types of loops available in shell scripting such as for loop, while loop, until loop, etc.

In this article, we will discuss the until loop in shell script and provide examples to demonstrate its usage.

What is an until loop?

The until loop is a type of loop in shell scripting that is used to execute a set of commands repeatedly until a particular condition is true.

The syntax for the until loop is similar to the while loop, but the until loop executes the commands until the condition becomes true, whereas the while loop executes the commands while the condition is true.

Syntax:

The basic syntax of the until loop is as follows:

until [ condition ]
do
    # Statements to be executed
done

The condition is a Boolean expression that is evaluated
before executing the statements inside the loop.

If the condition is
true, the statements inside the loop will not be executed, and the loop
will be terminated.

If the condition is false, the statements inside the
loop will be executed repeatedly until the condition becomes true.

Example

#!/bin/bash

# Initialize the counter
counter=1

# Execute the loop until the counter becomes greater than 5
until [ $counter -gt 5 ]
do
    echo "Counter value: $counter"
    counter=$((counter+1))
done

echo "Loop executed successfully"

In this example, we have initialized the counter variable to 1 and used the until loop to execute a set of commands until the value of counter becomes greater than 5.

The echo statement inside the loop is used to print the value of the counter variable, which is incremented by 1 in each iteration.

Once the value of the counter variable becomes greater than 5, the loop terminates, and the message "Loop executed successfully" is printed.

Output

Counter value: 1
Counter value: 2
Counter value: 3
Counter value: 4
Counter value: 5
Loop executed successfully

Conclusion:

The until loop is a useful construct in shell scripting that allows you to execute a set of commands repeatedly until a certain condition is met. In this article, we have discussed the syntax of the until loop and provided examples to demonstrate its usage. By mastering the until loop, you can write more efficient and effective shell scripts.