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

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.
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.
While loops take this form: {% highlight bash %} while [condition] do command1 command2 command3 ... done {% endhighlight %} Here is a first simple example: {% highlight bash %} i=0; while [$i -lt 4] do echo $i i=$((i + 1)) done {% endhighlight %} In this first example, you simply create a variable called i and evaluate it to 0. Then you acces the while loop: the condition [$i -lt 4] means that this while loop will run until the i varibale is less than 4. Every cycle of this loop, you print out the value of variable i with echo $i and finally you increase its value by 1 with i=$((i + 1)) . 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:

    0
    1
    2
    3
 
Sometimes it is required to decleare infinite loops for various programming purposes.
Here is an example: {% highlight bash %} i=1; while : do printf "i=$i\i: Hello World" if [ $i == 3 ]; then echo "I love DrRacket" 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 {% endhighlight %} No termination condition is set for this loop in this example. This type of loop is called infinite loop.
The exit 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.
The output of this piece of code is:
    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
If you want your shell to hang forever doing nothing you can write out the following infinite loop: {% highlight bash %} while : do : done {% endhighlight %} In scripting, while loops are often used to process files line by line.
Here is an examaple: {% highlight bash %} while read -r first_name last_name phone; do printf '%s\n' "$last_name" done < "$file" {% endhighlight %} The read command is used to read a file line by line. The flag -r 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, < "$file" redirects the loop's input from a file whose name is stored in a variable. This file has 3 colums, first_name last_name phone , separated by blank space (or a tab). This piece of code only prints out the second column.