9.2.2.1. Simple example using while

Here is an example for the impatient:

#!/bin/bash

# This script opens 4 terminal windows.

i="0"

while [ $i -lt 4 ]
do
xterm &
i=$[$i+1]
done

9.2.2.2. Nested while loops

The example below was written to copy pictures that are made with a webcam to a web directory. Every five minutes a picture is taken. Every hour, a new directory is created, holding the images for that hour. Every day, a new directory is created containing 24 subdirectories. The script runs in the background.

Note the use of the true statement. This means: continue execution until we are forcibly interrupted (with kill or Ctrl+C).

This small script can be used for simulation testing; it generates files:

#!/bin/bash

# This generates a file every 5 minutes

while true; do
touch pic-`date +%s`.jpg
sleep 300
done

Note the use of the date command to generate all kinds of file and directory names.

9.2.2.3. Using keyboard input to control the while loop

This script can be interrupted by the user when a Ctrl+C sequence is entered:

#!/bin/bash

# This script provides wisdom

FORTUNE=/usr/games/fortune

while true; do
echo "On which topic do you want advice?"
cat << topics
politics
startrek
kernelnewbies
sports
bofh-excuses
magic
love
literature
drugs
education
topics

echo
echo -n "Make your choice: "
read topic
echo
echo "Free advice on the topic of $topic: "
echo
$FORTUNE $topic
echo

done

A here document is used to present the user with possible choices. And again, the true test repeats the commands from the CONSEQUENT-COMMANDS list over and over again.

9.2.2.4. Calculating an average

This script calculates the average of user input, which is tested before it is processed: if input is not within range, a message is printed. If q is pressed, the loop exits:

#!/bin/bash

# Calculate the average of a series of numbers.

SCORE="0"
AVERAGE="0"
SUM="0"
NUM="0"

while true; do

  echo -n "Enter your score [0-100%] ('q' for quit): "; read SCORE;

  if (("$SCORE" < "0"))  || (("$SCORE" > "100")); then
    echo "Be serious.  Common, try again: "
  elif [ "$SCORE" == "q" ]; then
    echo "Average rating: $AVERAGE%."
    break
  else
    SUM=$[$SUM + $SCORE]
    NUM=$[$NUM + 1]
    AVERAGE=$[$SUM / $NUM]
  fi

done

echo "Exiting."

Note how the variables in the last lines are left unquoted in order to do arithmetic.