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Until Loops

The until loop is exactly like the while loop but the test is negated. This can improve readability and in certain circumstances makes a string of conditionals easier to write. Here, the code in until.sh is a clear description of how long the loop should run for: until $a is greater than 12, or $b is less than 100. On each iteration of the loop, $a is incremented by 1, and 10 is taken away from $b.

$ cat until.sh
#!/bin/bash
read -p "Enter a starting value for a: " a
read -p "Enter a starting value for b: " b
until [ $a -gt 12 ] || [ $b -lt 100 ]
do
  echo "a is ${a}; b is ${b}."
  let a=$a+1
  let b=$b-10
done
$ ./until.sh
Enter a starting value for a: 5
Enter a starting value for b: 200
a is 5; b is 200.
a is 6; b is 190.
a is 7; b is 180.
a is 8; b is 170.
a is 9; b is 160.
a is 10; b is 150.
a is 11; b is 140.
a is 12; b is 130.
$ ./until.sh
Enter a starting value for a: 10
Enter a starting value for b: 500
a is 10; b is 500.
a is 11; b is 490.
a is 12; b is 480.
$ ./until.sh
Enter a starting value for a: 1
Enter a starting value for b: 120
a is 1; b is 120.
a is 2; b is 110.
a is 3; b is 100.
$

until.sh

To write this in a while loop, everything has to be negated: -gt becomes -le, -lt becomes -ge, and || becomes &&. The description of the code changes too; without-until.sh continues incrementing a and reducing b by 10 for as long as a is less than (or equal to) ...