> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # Bash While Loop A `while` loop in Bash is a control flow statement that repeatedly executes a block of commands as long as a specified test command (or list of commands) evaluates to true (returns an exit status of `0`). The loop terminates immediately when the evaluated command returns a non-zero exit status. ## Syntax The standard multi-line syntax requires the `while`, `do`, and `done` keywords. Placeholders are denoted by angle brackets (`< >`): ```bash theme={"dark"} while do done ``` For inline execution, statements must be separated by semicolons: ```bash theme={"dark"} while ; do ; done ``` ## Condition Evaluation Mechanics Unlike many high-level languages that evaluate boolean expressions, a Bash `while` loop evaluates the **exit status** of a command or a list of commands. Crucially, `` is not limited to a single command. It can be a pipeline or a list of multiple commands separated by `;`, `&&`, or `||`. When a list of commands is provided, the `while` loop evaluates the exit status of the **last** command executed in that list. * **Exit Status `0`:** Interpreted as success (true). The loop executes the `do` block. * **Exit Status `>0`:** Interpreted as failure (false). The loop terminates, and execution continues after the `done` keyword. The `` typically utilize one of the following evaluation constructs: 1. **Command Lists:** Multiple commands where the final command dictates the loop continuation. ```bash theme={"dark"} while echo "Checking..."; ping -c 1 10.0.0.1 > /dev/null; do ``` 2. **POSIX Test Command (`[ ]` or `test`):** Used for standard file, string, and integer comparisons. ```bash theme={"dark"} while [ "$var" -lt 10 ]; do ``` 3. **Bash Extended Test (`[[ ]]`):** Provides advanced string manipulation, pattern matching, and logical operators without word splitting issues. ```bash theme={"dark"} while [[ "$string" != *"pattern"* ]]; do ``` 4. **Arithmetic Evaluation (`(( ))`):** Used strictly for C-style integer arithmetic and mathematical comparisons. ```bash theme={"dark"} while (( var < 10 )); do ``` 5. **Standard Commands:** Any executable command or pipeline. The loop continues as long as the command executes successfully. ```bash theme={"dark"} while grep -q "active" status.log; do ``` ## Loop Control Statements Bash provides two built-in commands to alter the standard execution flow within the `do` block: * `break`: Immediately terminates the loop entirely, bypassing the condition check, and transfers control to the command following `done`. It accepts an optional integer argument (e.g., `break 2`) to break out of nested loops. * `continue`: Halts the current iteration, skips any remaining commands in the `do` block, and returns control to the top of the loop to re-evaluate the `while` condition. ## Infinite Loops An infinite loop is created by providing a condition command that always returns an exit status of `0`. This is typically achieved using the `true` command or the `:` (null utility) built-in, which does nothing and always succeeds. ```bash theme={"dark"} while : do done ``` ## Input Redirection The `while` loop can process data streams by redirecting Standard Input (`stdin`) directly into the loop structure. This is most commonly paired with the `read` built-in command, which returns `0` as long as it successfully reads a line. ```bash theme={"dark"} while IFS= read -r line do done < "filename.txt" ``` In this construct, the redirection `< "filename.txt"` is applied to the entire `while` loop block, making the file's contents available to the `read` command on each iteration until the End-of-File (EOF) is reached, at which point `read` returns a non-zero exit status and the loop terminates.

Tired of Poor Bash Skills? Fix That With Deep Grasping! Learn More