The while loop is perfect for a situation where you need to execute a set of commands while some condition is true. Sometimes you need to execute a set of commands until a condition is true.
until command do Statement(s) to be executed until command is true done
Here the Shell command is evaluated. If the resulting value is false, given statement(s) are executed. If the command is true then no statement will be executed and the program jumps to the next line after the done statement.
Here is a simple example that uses the until loop to display the numbers zero to nine −
#!/bin/sh a=0 until [ ! $a -lt 10 ] do echo $a a=`expr $a + 1` done
Upon execution, you will receive the following result −
0 1 2 3 4 5 6 7 8 9