Bash Script For Loop

Mastering bash scripting can unlock powerful capabilities in managing repetitive tasks efficiently. In this guide, we'll explore the fundamentals of the for loop in bash scripting, a versatile tool that can save both time and effort. Whether you're a seasoned programmer or new to scripting, understanding for loops will enhance your scripting skills significantly.

What is a Bash Script For Loop?

A for loop in bash scripting is a control flow statement that allows code to be executed repeatedly based on a set of conditions. Imagine needing to execute a command multiple times — instead of rewriting the code repeatedly, a for loop does the work for you, iterating over a sequence of items or numbers. It's like a chef preparing multiple dishes using the same recipe — they just need to change the ingredients.

Basic Syntax of a For Loop in Bash

The structure of a for loop in bash follows a simple syntax:

for item in [LIST]
do
   [COMMANDS]
done

• for item in [LIST]: The loop starts with the keyword for followed by a variable that represents each item in the list.
• do: Indicates the beginning of the command block.
• [COMMANDS]: Commands that you want to execute for each item.
• done: Ends the loop.

Example: Iterating Over a List of Words

To better understand how a for loop works, let's start by iterating over a list of words.

for word in Hello World Bash Script
do
   echo $word
done

Explanation:

• for word in Hello World Bash Script: The loop initializes the variable word and assigns it each of the listed words sequentially.
• do: Begins the block of commands to execute.
• echo $word: Prints the current value of word.
• done: Completes the loop execution once all words are printed.

When executed, this script prints:

Hello
World
Bash
Script

Using Numbers in a For Loop

You might encounter situations where you need to iterate over a series of numbers. Here's how you can do that using a for loop:

for number in {1..5}
do
   echo "Number: $number"
done

Explanation:

• for number in {1..5}: Sets up a sequence from 1 to 5.
• do: Initiates the action block.
• echo "Number: $number": Outputs the current number.
• done: Closes the loop.

This script will output:

Number: 1
Number: 2
Number: 3
Number: 4
Number: 5

Practical Application: Iterating Over Files

A for loop also becomes handy when working with files. Imagine you need to perform operations on multiple files, such as renaming or moving them.

Example:

for file in *.txt
do
   mv "$file" "${file%.txt}.backup"
done

Explanation:

• for file in *.txt: Loops through all files ending with .txt.
• do: Begins operations on each file.
• mv "$file" "${file%.txt}.backup": Renames each .txt file to .backup.
• done: Ends the loop.

This renames every .txt file by changing its extension to .backup.

Nested For Loops

Sometimes your tasks require iterating through multiple sequences, that's where nested for loops come in.

Example:

for i in {1..3}
do
   for j in {1..3}
   do
      echo "i: $i, j: $j"
   done
done

Explanation:

• First for loop: Iterates i through values 1 to 3.
• Second for loop: For each i, loops j through values 1 to 3.
• echo "i: $i, j: $j": Prints combinations of i and j.

This script outputs all pair combinations:

i: 1, j: 1
i: 1, j: 2
i: 1, j: 3
...
i: 3, j: 3

Conclusion

Understanding for loops in bash scripting empowers you to automate repetitive tasks efficiently. By practicing with different lists and sequences, you can apply these concepts to real-world scripting challenges, saving time and reducing errors.

For further reading on scripting and loops in various languages, you might find Mastering the C For Loop - The Code Journal or For Loops in R: A Beginner's Guide - The Code Journal beneficial. Enhance your scripting skills and make your work more efficient with these powerful tools!