theshell.ch/site/pages/scripts/while-loop.html

---
layout: page
category-page: scripts
category-title: Scripting
tags: loop while do script scripting read
author: Matteo Omenetti
title: While Loop
---
<p>

<!-- Introduction -->
Loops are an important concept in programming and therefore also in scripting. Thanks to loops you are able to repeat an instruction
automatically several times, until a certain condition turns false. <br>

Two are the main types of loops: while and for. They both generate a repeating piece of code, but with some key differences
that make them suitable for different needs while programming. <br>

While loops take this form:
<pre>
  while [condition]
  do
    command1
    command2
    command3
    ...
  done
  </pre>
<!-- End of Introduction -->

<!-- First Example -->
  Here is a first simple example:
<pre>
  i=0;

  while [$i -lt 4]
  do
  echo $i
  i=$((i + 1))
  done
  </pre>

  In this first example, you simply create a variable called i and evaluate it to 0.
  Then you acces the while loop: the condition <code> [$i -lt 4] </code> means that this while
  loop will run until the <code> i </code> varibale is less than 4.
  Every cycle of this loop, you print out the value of variable i with <code> echo $i </code>
  and finally you increase its value by 1 with <code> i=$((i + 1)) </code>.
  Therefore in 4 cycles the value of i will be 4. This will make the condition of the while loop false.
  The output of this piece of code is:
  <pre>
    0
    1
    2
    3
  </pre>
  <!-- End of First Example -->


  <!-- Infinite Loop First Example -->
  Sometimes it is required to decleare infinite loops for various programming purposes. <br>
  Here is an example:
  <pre>
  i=1

  while :
  do
    printf "i=$i\i: Hello World"
    if [ $i == 3 ]
    then
            echo "I love programming"
    elif [ $i == 5]
    then
            echo "I love Bash"
    elif [ $i == 7 ]
    then
            echo "I love this website"
    elif [ $i == 9 ]
    then
            exit 0
    i=$((i + 1))
    done
    </pre>
    No termination condition is set for this loop in this example. This type of loop is called infinite loop.
    The <code> exit </code> statement is used to quit the loop. This loop will iterate for 9 times, then
    as soon as <code> i </code> becomes equal to 0, the condtition of the last if statement will evaluate to true and the
    loop will be terminated. <br>
    The output of this piece of code is:
     <pre>
       1: Hello World
       2: Hello World
       3: Hello World
       I love programming
       4: Hello World
       5: Hello World
       I love Bash
       6: Hello World
       7: Hello World
       I love this website
       8: Hello World
       9: Hello World
      </pre>

<!-- End of Infinite Loop First Example  -->

<!-- Infinite Loop Second Example  -->
  If you want your shell to hang forever doing nothing you can write out the following infinite loop:
  <pre>
    while :
    do
    :
    done
  </pre>
  <!-- End of Infinite Loop Second Example  -->

<!-- Read  -->
  In scripting, while loops are often used to process files line by line. <br>
  Here is an examaple:
  <pre>
  while read -r first_name last_name phone;
  do
  printf '%s\n' "$last_name"
  done <!-- Find out how to write "<"  --> "$file"
  </pre>

  The <code> read </code> command is used to read a file line by line. The flag <code> -r </code> is used to tell the
  command read to intepret backslashes (/) literally, instead as escape characters. This command, expect for some few
  rare cases, should always be used with this flag.
  In this example, <code> <!-- Find out how to write "<" --> "$file" </code> redirects the loop's input from a file whose name is stored in a variable.
  This file has 3 colums, <code> first_name last_name phone </code>, separated by blank space (or a tab).
  This piece of code only prints out the second column.
  <!-- End of Read -->

  </p>