Do Unless Loop Bash

Shell Programming

As a bash scripting enthusiast, I often find myself exploring different ways to control the flow of my scripts. One construct that I’ve come to appreciate is the “do unless” loop in bash. This unique loop provides a concise and elegant way to execute a block of code unless a certain condition is met. In this article, I’ll delve into the intricacies of the “do unless” loop in bash and share some personal insights on its practical applications.

Understanding the “do unless” Loop

The “do unless” loop in bash is a lesser-known alternative to the traditional “if-else” or “while” loops. It allows you to execute a block of code unless a specified condition is true. The syntax of the “do unless” loop follows a simple structure:

unless condition; do
    # Code block to be executed

In this context, the unless keyword serves as a conditional check, and the subsequent do indicates the beginning of the code block to be executed. Once the condition evaluates to false, the statements within the block are executed.

An Example of the “do unless” Loop in Action

Let’s consider a practical scenario where the “do unless” loop can be particularly useful. Suppose we want to iterate through a list of files and perform a specific operation on each file unless it is a backup file. Here’s how we can achieve this using the “do unless” loop:

unless \[ "\$file" == "*.bak" \]; do
    # Execute operation on the file

In this example, the loop iterates through each file, and the operation within the block is executed for every file that is not a backup file.

Personal Insights and Practical Applications

From my experience, the “do unless” loop offers a more expressive and succinct way of handling scenarios where the default action should be performed unless a specific condition is met. Its readability and clarity make it a valuable addition to my bash scripting toolkit.

I’ve found the “do unless” loop to be particularly handy in cases where I need to process a list of items, excluding those that meet certain criteria. Whether it’s filtering out specific file types, skipping certain entries in a data file, or performing default actions in a controlled manner, the “do unless” loop provides a clean and intuitive solution.

Exploring Further Possibilities

While the “do unless” loop may not be as widely recognized as other looping constructs, its effectiveness and elegance make it worthy of consideration in bash scripting endeavors. As I continue to explore its capabilities, I discover new ways to leverage its power in diverse scripting scenarios.


In conclusion, the “do unless” loop in bash presents a compelling approach to conditionally executing code in a clear and concise manner. Its unique structure and practical applications make it a valuable tool for enhancing the readability and logic of bash scripts. Whether you’re a seasoned bash scripter or just starting out, incorporating the “do unless” loop into your scripting repertoire can bring a fresh perspective to your coding endeavors.