While Loops and Until Loops | Spring Into Linux

The commands in a while loop continue to execute as long as the condition is true. The general format of a while loop is:

 while [ condition ] do       commands done

The while loop first tests the condition. If it's true, the commands are executed. When the script reaches done, it returns to the while line and tests the condition again. If the condition's still true, it executes the commands again. If the condition is not true, it proceeds to the command following the while loop.

The simple shell script below executes the while loop four times:

 #!/bin/bash # Script name: numbers - outputs a list of numbers n=0 while [ $n -lt 3 ] do       echo $n       let n=n+1 done echo After loop

The program execution is shown below:

 ./numbers 0 1 2 After loop

The until loop is very similar, except it stops looping when the condition is true. To get the above output, you would use the following until loop, changing -lt to -eq:

 n=0 until [ $n -eq 3 ] do       echo $n       let n=n+1 done echo After loop