Question
Answer and Explanation
A "do while" loop in Bash is a control flow statement that executes a block of code repeatedly as long as a specified condition remains true. Unlike a "while" loop, which checks the condition before executing the code block, a "do while" loop executes the code block at least once before checking the condition.
Here's the basic structure of a Bash "do while" loop:
do
command1
command2
...
done while [ condition ]
Let's break down the components:
- do: This keyword marks the beginning of the code block to be executed repeatedly.
- command1, command2, ...: These are the commands or statements that will be executed in each iteration of the loop. They can include any valid Bash commands or scripts.
- done: This keyword marks the end of the code block.
- while [ condition ]: After the 'done' keyword, this part specifies the condition that will be checked after the execution of the code block. The loop will continue iterating as long as the condition evaluates to true. The condition is often a test expression, such as comparisons or file checks.
Key Characteristics and Use Cases:
1. Guaranteed First Execution: The key feature of a "do while" loop is that it always runs the code block at least once, even if the initial condition is false. This is useful when you need to perform some actions before testing a condition.
2. Iteration Control: The loop continues to iterate as long as the condition remains true. Once the condition becomes false, the loop terminates.
3. Common Uses: "Do while" loops are often used for tasks where you need to prompt the user for input and continue prompting until a valid input is given. Another use case is when you need to execute a process at least once, then decide whether to continue, based on some output or change in the system.
Example:
Let's create a script that asks the user for a number, and keeps prompting until they input a number greater than 10.
#!/bin/bash
number=0
do
read -p "Enter a number greater than 10: " number
done while [ "$number" -le 10 ]
echo "You entered: $number"
In this example, the `read` command prompts the user to enter a number. The loop continues until the entered number is greater than 10. The loop executes at least once.
In conclusion, the Bash "do while" loop is a useful construct when you need to guarantee at least one execution of a code block before a condition is checked. Its primary difference from a standard "while" loop lies in the placement of the condition check, which is after each iteration.