Until loop

Just like while loop, until loop is also based on a condition.

Syntax

The until loop continues running commands as long as the item in list continues to evaluate true. Once an item evaluates false, the loop is exited. The syntax is:

until [ condition ]
do
   command1
   command2
   ...
   ....
   commandN
done

The while loop vs the until loop

1. The until loop executes until a nonzero status is returned.
2. The while command executes until a zero status is returned.
3. The until loop always executes at least once.

Example

Create a shell script called until.sh:

#!/bin/bash
i=1
until [ $i -gt 6 ]
do
	echo "Welcome $i times."
	i=$(( i+1 ))
done

Save and close the file. Run it as follows:

chmod +x until.sh
./until.sh

Sample outputs:

Welcome 1 times.
Welcome 2 times.
Welcome 3 times.
Welcome 4 times.
Welcome 5 times.
Welcome 6 times.

The loop in the above example initializes the variable i to 1, and then increments and displays out the message until it equals 6.