The while/until structure

while/until are looping structures.

while - while command returns success,
  execute the loops action body.

until - until command returns success,
  execute the loops action body.

Like if, while/until tests the exit status of specified command.

Loop structure


while cmd
do
  action list.
done

until cmd
do
  action list.
done


while or until - keyword.

cmd - a command or command sequence.
  The exit status of command determine if loop body executed.

do - keyword, marks start of the action block (loop body).

action list - a sequence of commands to run.
   This may include other bash flow of control statement.

done - keyword marking the end of the action block.

Special commands

break - used to break out of loop for reasons other than cmd's
  success.  Usually listed as an action of an if statement.

continue - used to skip the rest of the action block
  and return to the top of the loop (re-run the test command).

# prompts user for value 
read -p "Input a number : " num

# check that it is integer of 1 or more digits.
# Until a good number given or q typed, keep asking.
until echo "$num" | grep "^[0-9][0-9]*$" > /dev/null 2>&1
do
  # if q entered, user Signaling quit.
  if [ "$num" = 'q' ]
  then
    break
  fi

  # ask again
  read -p "Bad number, Input a number : " num
done


#!/bin/bash

function getnum ()
{
  # Sets the variable num to an integer number

  # prompt user for a number
  read -p "Input a number : " num

  # check that it is only an integer
  # Until a good number given or q typed, keep asking.
  until echo "$num" | grep "^[0-9][0-9]*$" > /dev/null 2>&1
  do
    # if q entered, user Signaling quit.
    if [ "$num" = 'q' ]
    then
      return 1
      break
    fi

    # ask again
    echo "Bad number"
    read -p "Input a number : " num
  done
}

# Loop is endless and must be exited from by an
#   internal action.
while [ 1 = 1 ]
do

  # get 1st input, if q, then exit program
  getnum
  num1=$num
  if [ "$num1" = 'q' ]
  then
    exit
  fi

  # get 2nd input, if q, then exit program
  getnum
  num2=$num
  if [ "$num2" = 'q' ]
  then
    exit
  fi

  # if either number > 1000, skip calculation and re-prompt
  if [ "$num1" -gt 1000 -o  "$num2" -gt 1000 ]
  then
    echo "User smaller values"
    continue
  fi

  # sum numbers and display
  total=$( expr $num1 + $num2 )
  echo "$num1 + $num2 = $total"

done