What is a bash UNTIL loop?
Here you will find out:
- what bash UNTIL loop is
- what the difference between a WHILE loop and an UNTIL loop is
- how to use an UNTIL loop in a shell script
- when DiskInternals can help you
Are you ready? Let's read!
About bash UNTIL loop
A bash UNTIL loop is similar to a bash WHILE loop. However, the UNTIL loop is used to run a series of commands based on Boolean-like outcomes; that is, an expression has to return “True” or “False” before your loop commands will execute. But in the case of a bash UNTIL loop, the commands will only be executed if the expression returns “True”. If an expression returns “False”, a bash UNTIL loop will repeat the same set of commands until it returns "True".
The syntax of a bash UNTIL loop
Here’s the syntax of an UNTIL loop. It looks similar to a bash WHILE loop, but the functionality differs:
Single expression syntax
Multiple expressions syntax
Notice the dual brackets in the multiple expression syntax. You should use double brackets for multiple expressions and a single bracket for single expressions.
Things to note:
- A bash UNTIL loop will check for expressions/conditions before running commands.
- Commands will run only if the expression(s) returns “False”.
- The loop will be terminated if the expression returns “True”.
- After termination, program control is passed to the command written below “Done”.
If you want to understand the actual difference between a WHILE and an UNTIL loop, read below.
The difference between a WHILE loop and an UNTIL loop
An UNTIL loop will only run your commands if the defined expression returns a non-zero code. In contrast, a WHILE loop will run your commands when the expression returns a zero status.
Using an UNTIL loop in a shell script
In order to explain the bash UNTIL loop for easy understanding, here are some basic, practical examples.
Single expression:
Multiple expressions:
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