In this tutorial, we’ll explore the concept of loops in the Linux Bash scripting. Loops are control structures that allow you to repeatedly execute a block of code until a certain condition is met.

Usage of Loops

Loops are commonly used in shell scripting for various purposes, such as:

  • Iterating over files and directories and moving certain files
  • Processing lines from a file or input stream. For example, reading each line and appending some text.
  • Executing commands until a certain condition is met.
  • Performing repetitive tasks with different inputs.

There are three types of loops commonly used in Bash.

1. For Loop

The for loop allows you to iterate over a sequence of values and execute a block of code for each value.

1for variable in value1 value2 ... valuen
2do
3    # Code statements
4done

For example, let’s use a for loop to print the numbers from 1 to 5:

1#!/bin/bash
2for i in 1 2 3 4 5
3do
4    echo $i
5done

If we have to iterate over many times, writing those many numbers as sequence is cumbersome. In such conditions, we can uses braces operator like this.

1#!/bin/bash
2for i in {1..20}
3do
4    echo "Number: $i"
5done

Output:

bash script.sh
Number: 1
Number: 2
... ...
Number: 20

Looping over Array

Similar to this we can iterate over array elements using ${array_name[*]} syntax.

1names=(John Jenny Zoe)
2for name in ${names[*]}
3do
4    echo "Name: $name"
5done

Output:

Name: John
Name: Jenny
Name: Zoe

Looping through lines of a file

Similarly, for reading file content, we can use for loop along with cat command. We will see more examples of this in upcoming lesson.

1for line in `cat /path/to/file`
2do
3    echo $line
4done

2. While Loop

The while loop executes a block of code as long as a given condition is true.

1while [ condition ]
2do
3    # Statements
4done

Here’s an example that uses a while loop to count down from 5 until 0.

Example:

1#!/bin/bash
2counter=5
3
4while [ $counter -gt 0 ]
5do
6    echo $counter
7    ((counter--))
8done

3. Until Loop

Similar to the while loop, the until loop also executes a block of code repeatedly until a given condition becomes true.

1until [ condition ]
2do 
3   # Statements
4done

Below is an example where we use an until loop to print numbers from 1 to 3:

1#!/bin/bash
2number=1 
3
4until [ $number -gt 3 ] 
5do 
6   echo $number 
7   ((number++)) 
8done

Output:

1
2
3

Loops are an essential constructs in Linux Bash scripting that allow you to automate repetitive tasks and iterate over sequences of values.