Team Scripting: while loop page added

git-svn-id: svn+ssh://atelier.inf.usi.ch/home/bevilj/group-1@165 a672b425-5310-4d7a-af5c-997e18724b81

site/pages/scripts/while-loop.html

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+---
+layout: page
+category-page: scripts
+category:title Scripting
+tags: loop while do script scripting read
+author: Matteo Omenetti
+title: While Loop
+---
+<!-- 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 some 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 while the varibale
+  i 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 -->
+
+  <!-- Second Example -->
+  Sometimes it 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 codinition 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 i 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 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>
+
+
+  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' "$last_name"
+  done < "$file"
+  </pre>
+
+  The <code> read </code> command is used to read 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> < "$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.