Difference between "while" and "until" in Bash

Question

What is the real difference between while and until ?

I'd like to know if it's possible to do an increasingly loop with "until" and a descending loop with while.

Because I saw that if I do this

COUNTER=0      while [ $COUNTER -lt 10 ]; do          echo The counter is $COUNTER          let COUNTER+=1       done 

and this

COUNTER=20      until [ $COUNTER -lt 10 ]; do          echo COUNTER $COUNTER          let COUNTER-=1      done 

they work well.

But if I do the opposite, for example:

COUNTER=20      while [ $COUNTER -lt 10 ]; do          echo COUNTER $COUNTER          let COUNTER-=1      done 

the script doesn't end.

Does this means that we cannot do a reverse loop with a while in bash?

Answer 1

while runs the loop while the condition is true. until runs the loop until the condition is true (i.e. while the condition is false).

See http://www.gnu.org/software/bash/manual/bashref.html#Looping-Constructs.

Answer 2

While Loop executes the block of code (enclosed in do...done) when the condition is true and keeps executing that till the condition becomes false. Once the condition becomes false, the while loop is terminated.

Until Loop executes the block of code (enclosed in do...done) when the condition is false and keep executing that till the condition becomes true. Once the condition becomes true, the until loop is terminated.

COUNTER=20 while [ $COUNTER -lt 10 ]; do     echo COUNTER $COUNTER     let COUNTER-=1 done 

On the very first occasion, the condition mentioned in while, i.e. [ $COUNTER -lt 10 ], holds false, so the block of code inside the while loop will not run at all.