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Two other possible looping constructs are the while…do loop and the until…do loop. The structure of each is presented here:

 while condition until condition do do { body } { body } done done

The while statement executes while the condition is true. The until statement executes until the condition is true—in other words, while the condition is false.

Here is an example of a while loop that outputs the number 0123456789:

 N=0 while [ $N -lt 10 ] ; do echo -n $N let N=$N+1 done

Another way to output the number 0123456789 is to use an until loop as follows:

 N=0 until [ $N -eq 10 ] ; do echo -n $N let N=$N+1 done