Bash For Loop Examples

bash for loop

Imagine you have a list of things to do, like a shopping list, and you check each item off one by one. 

A for loop in Bash scripting is similar. It goes through a list of things and does a specified set of tasks for each item. For practical purposes, a loop is like a helper that repeats tasks for you, making sure each item in your list gets equal attention.

The idea of for loops is really handy in Bash, which is the language we use to tell computers running Linux what to do. 

Consider the common scenario where you have a bunch of numbers or a list of file names. You can use a for loop to work through them one by one, doing the same thing for each item. 

In this article, we’ll explore how to use for loops in Bash. It’s a fun and easy way to make your computer do repetitive tasks without having to do each step yourself!

Table Of Contents

  1. The Basics of Bash for Loop
  2. How To Use Bash for Loop in Scripts?
    1. The Prerequisites
    2. Example #1: Handling Each Item Separately
    3. Example #2: Looping Through a Number Range
    4. Example #3: Looping Through a Range With Increment
    5. Example #4: Generate Sequences
    6. Example #5: Use C-Style Syntax for Bash for loops
    7. Example #6: The Problem of Endless Loops
    8. Example #7: Exit a Loop Early
    9. Example #8: Skip Loop Iterations
    10. Example #9: Looping Through Array Elements
    11. Example #10: Access Array Indices
    12. Example #11: Nesting Loops Within Loops
    13. Example # 12: Iterating Strings
    14. Example #13: Processing Files in a Directory
    15. Example #14: Command Substitution
    16. Example #15: Command Line Arguments
  3. Conclusion
  4. FAQ

The Basics of Bash for Loop

A for loop in Bash allows us to repeat actions. A typical syntax of the loop is as follows:

for item in [list]

do

  [commands]

done

In this syntax, the item variable takes each value from the list, and the commands are executed for each item.

How To Use Bash for Loop in Scripts?

Now that you know the fundamentals of the Bash for loop, let’s see how you can use it to carry out practical everyday server management and administration tasks. Let’s start with the prerequisites. 

The Prerequisites

Before diving into the intricacies of for loops, ensure you have:

  • A Linux distribution that supports the Bash shell.
  • A basic grasp of using the command line.

Example #1: Handling Each Item Separately

#!/bin/bash

# For loop with colors

for color in red green blue

do

  echo $color

done

This script uses the for loop to print each color name.

1 Handling Each Item Separately

Example #2: Looping Through a Number Range

#!/bin/bash 

# For loop with a number range

for number in {1..5}

do

   echo $number

done

This script loops through the range and prints numbers from 1 to 5.

2 Looping Through a Number Range

Example #3: Looping Through a Range With Increment

#!/bin/bash 

# For loop with number range with increment

for number in {0..10..2}

do

 echo $number

done

This script loops through a range and prints even numbers up to 10. During the process, the loop will skip odd numbers to print only the even numbers. 

3 Looping Through a Range With Increment

Example #4: Generate Sequences

#!/bin/bash 

# For loop with seq command

for number in $(seq 1 5)

do

  echo $number

done

The seq statement uses a starting (1) and ending point (5) to generate a sequence from 1 to 5.

4 Generate Sequences

Example #5: Use C-Style Syntax for Bash for loops

#!/bin/bash 

# For loop in c-style syntax

for ((i = 0; i < 5; i++))

do

echo $i

done

You can use the C language’s syntax for Bash for loops because many sysadmins are familiar with C/C++. 

Here’s a quick breakdown of the script:

  1. #!/bin/bash: This line instructs the terminal to use Bash to run the script.
  2. for ((i = 0; i < 5; i++)): This is the main syntax of the for part. It’s a loop that starts with i equal to 0. After each round of the loop, it adds 1 to i (i++). The loop keeps going as long as i is less than 5.
  3. do … done: These stop words show where the loop starts and ends.
  4. Inside the loop: echo $i is the command that runs in each round of the loop. It prints the current value of i to the screen.

So, when you run this script, it prints:

Each number is shown one after the other, from 0 up to 4. This script is a good example of how to do something repeatedly, like counting numbers or using a loop in Bash.

5 Use C-Style Syntax for Bash for loops

Example #6: The Problem of Endless Loops

#!/bin/bash 

# Creating endless loops

for (( ; ; ))

do

echo "Infinite loop, press CTRL +C to exit"

done

It is easy to create endless for loops that run infinitely until you stop the process or the computer runs out of resources. That is why you should be very careful about creating endless loops because your script will continue execution in the terminal. 

The easiest way to exit an infinite loop is CTRL+C.

Here’s how an infinite Bash for loop looks like: 

6 The Problem of Endless Loops

Example #7: Exit a Loop Early

#!/bin/bash 

# Break the loop early

for number in {1..10}

do

if [ $number -eq 5 ]

then

break

fi

echo $number

done

The usual behavior of a Bash for loop is to exit when the loop execution hits the exit condition. However, you can exit earlier by using the break statement. 

In the above script, the for loop is written to exit when the loop counter hits 10. However, the break statement ensures the loop exits earlier (when the counter hits 5). 

7 Exit a Loop Early

Example #8: Skip Loop Iterations

#!/bin/bash 

#Skipping loop iterations

for number in {1..5}

do

if [ $number -eq 3 ]

then

continue

fi

echo $number

done

By design, a Bash for loop runs through every iteration. However, there are scenarios when you wish to skip the processing of the task for a particular iteration and then continue the loop execution.

For instance, the Bash for loop starts with the execution and prints the numbers 1 and 2. At the third iteration, the loop execution “skips” printing the number 3 and then continues printing the numbers.

8 Skip Loop Iterations

Example #9: Looping Through Array Elements

#!/bin/bash 

# Looping through array elements

colors=('red' 'green' 'blue')

for color in "${colors[@]}"

do

echo $color

done

Arrays are very popular data structures that store multiple values in consecutive memory locations. This results in fast data retrieval and simplification of data management in scripts.

The above script contains an array named colors. On every iteration, the for loop accesses the array element and prints the value (the name of colors). 

9 Looping Through Array Elements

Example #10: Access Array Indices

#!/bin/bash 

# Access array indices

colors=('red' 'green' 'blue')

for index in "${!colors[@]}"

do

echo $index

done

In some cases, you wish to access the indices of the array elements. In the example script, the for loop prints out the indices of the array instead of the array elements. 

10 Access Array Indices

Example #11: Nesting Loops Within Loops

#!/bin/bash 

# Combining loops within loops

for outer in 1 2

do

for inner in A B

do

echo "$outer$inner"

done

done

You can include for loops within other for loops. This allows you to execute multiple operations without writing multiple loops. Programmers often use nested loops to work with multi-dimensional arrays. 

In the sample script, the first for loop (called the outer loop) prints numbers 1 and 2. The second loop (called the inner loop) prints alphabets A and B. 

11 Nesting Loops Within Loops

Example # 12: Iterating Strings

#!/bin/bash 

# Iterating over strings

for word in "this is a sentence"

do

echo $word

done

Working with strings is a common activity in Bash scripting. The for loop is a great tool for working with strings. In the sample script, the for loop iterates over the string and prints the characters. 

12 Iterating Strings

Example #13: Processing Files in a Directory

#!/bin/bash 

# Processing files in a directory

for file in *.sh

do

echo $file

done

In this sample script, the for loop goes through the entire directory and prints the name of the files. This script presents a basic idea of using the for loop to access all files in a directory that you can expand for more complex tasks.

13 Processing Files in a Directory

Example #14: Command Substitution

#!/bin/bash 

# Using command output as list

for user in $(cat users.txt)

do

echo $user

done

Instead of outputting to the standard output, you can use the for loop to run commands. This iterative application of commands allows you to process data, fetch multiple resources, and perform similar sysadmin tasks. 

In the sample script, we used the for loop to apply the cat command on each line in the file.

14 Command Substitution

Example #15: Command Line Arguments

#!/bin/bash 

# Handling script arguments

for arg in "$@"

do

echo $arg

done

The for loop in this sample script accesses the standard Bash argument array (represented by $@) to get the parameters passed to the script execution command. 

While the sample script simply prints the arguments passed to it, you can extend this idea into scripts that carry out complex tasks.

15 Command Line Arguments

Conclusion

The for loop in Bash scripting offers a versatile way to automate tasks. They save time and reduce human error, allowing us to focus on more complex problems. Remember, practice is key to mastering these concepts. 

If you’re looking for a reliable hosting provider to test and run your Bash scripts, RedSwitches Bare Metal Hosting offers robust and scalable solutions that cater to all your hosting needs.

RedSwitches offers the best dedicated server pricing and delivers instant dedicated servers, usually on the same day the order gets approved. Whether you need a dedicated server, a traffic-friendly 10Gbps dedicated server, or a powerful bare metal server, we are your trusted hosting partner.

FAQ

Q. What is the syntax for a for loop in Bash?

In Bash, the syntax for a for loop is:

for item in list_of_items;

do

[commands];

done.

Q. How can I use a for loop to iterate through a list of items in Bash?

You can use a for loop in Bash to iterate through a list of items by specifying the list of items after the “in” keyword in the for loop syntax.

Q. Can you provide some useful examples of using a for loop in Bash?

Sure! Some useful examples of using a for loop in Bash include iterating through files in a directory, processing command-line arguments, and iterating through an array of elements.

Q. What is the syntax for an infinite for loop in Bash?

The syntax for an infinite for loop in Bash is: f

or (( ; ; ));

do

[commands];

done.

Q. How do I use a for loop with a command in Bash?

You can use a for loop with a command in Bash by placing the command inside the backticks or using command substitution within the for loop.

Q. What is the break statement used for in a Bash for loop?

The break statement is used in a Bash for loop to exit the loop prematurely based on a certain condition.

Q. Can I use the for loop with conditional statements in Bash?

Yes, you can use the for loop with conditional statements such as if, elif, and else to perform different actions based on specific conditions.

Q. How does the continue statement work in a Bash for loop?

The continue statement in a Bash for loop is used to skip the remaining commands in the loop and move to the next iteration of the loop.

Q. What are some practical examples of using a while loop in Bash?

Some practical examples of using a while loop in Bash include reading input from a file, processing data until a certain condition is met, and continuously executing a script based on certain criteria.

Q. How can I use the Bash for loop in a command-line environment?

You can use the Bash for loop in a command-line environment to perform repetitive tasks, iterate through a list of items, and automate certain processes using Bash commands.