Until Loop

The Bash until loop is a control flow construct that repeatedly executes a block of commands as long as a specified condition evaluates to false (returns a non-zero exit status). It functions as the logical inverse of the while loop, terminating execution the moment the condition evaluates to true (returns an exit status of 0).

Syntax

The standard multi-line syntax for the until loop is:

until [ condition ]
do
    # Commands to execute
done

For inline or single-line execution, semicolons are required to separate the structural keywords from the commands:

until [ condition ]; do commands; done

Execution Mechanics

1. Condition Evaluation: Bash executes the command or test provided in the condition slot.
2. Exit Status Check: Bash inspects the exit status ($?) of that condition.
   • If the exit status is non-zero (logical false), the commands inside the do...done block are executed.
   • If the exit status is zero (logical true), the loop immediately terminates, and control flow passes to the next statement following the done keyword.
3. Iteration: After executing the do...done block, control returns to step 1.

Condition Types

The condition is not limited to boolean expressions; it can be any valid Bash command. The loop’s continuation depends strictly on the command’s exit code.

Using the test construct ([[ ]] or [ ]): This is the most common approach, evaluating arithmetic or string comparisons.

counter=0

until [[ $counter -eq 3 ]]; do
    ((counter++))
done

Execution flow: The loop runs while $counter -eq 3 returns 1 (false). Once $counter reaches 3, the test returns 0 (true), and the loop exits.

Using standard commands: The loop can evaluate the success or failure of standard executables.

until command_that_might_fail; do
    sleep 1
done

Execution flow: The loop body executes repeatedly as long as command_that_might_fail returns an exit code greater than 0.

Loop Control Statements

The until loop respects standard Bash loop control statements:

• break: Immediately terminates the loop entirely, regardless of the condition’s current state.
• continue: Halts the current iteration, skipping any remaining commands in the do...done block, and immediately re-evaluates the condition for the next iteration.

Infinite Loops

Because the until loop requires a 0 exit status to terminate, providing a command that permanently returns a non-zero exit status creates an infinite loop. The false command (which always exits with 1) is the standard idiom for an infinite until loop:

until false; do
    # This block will execute indefinitely
done

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