---
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 some different needs while programming.
While loops take this form:
while [condition] do command1 command2 command3 ... doneHere is a first simple example:
i=0; while [$i -lt 4] do echo $i i=$((i + 1)) doneIn 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 while the varibale
i 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 3Sometimes it required to decleare infinite loops for various programming purposes.
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)) doneNo termination codinition 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. 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 WorldIf you want your shell to hang forever doing nothing you can write out the following infinite loop:
while : do : doneIn scripting, while loops, are often used to process files line by line.
while read -r first_name last_name phone; do printf '%s\n' "$last_name" done < "$file"The
read
command is used to read 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.