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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).

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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

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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.

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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.

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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.

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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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