Introduction to Bash For Loops: A Beginner’s Guide

Table of Contents

Why Are ‘For’ Loops Necessary? Basic Syntax of Bash ‘For’ Loop Using Bash ‘For’ Loop Looping through Files and Directories Iterating over a Range of Numbers Using ‘for’ with Command Substitution Nested ‘for’ Loops

1. Why Are ‘For’ Loops Necessary?

Without loops, performing an action on 100 different files would require you to type 100 separate commands. Loops are necessary because they:

Eliminate human error : Manually typing filenames often leads to typos.

Save Time : You write the logic once, and the computer handles the volume.

Enable Scalability: A loop that works for 5 files will work exactly the same way for 5,000 files.

2. Basic Syntax of Bash ‘For’ Loop

The structure of a Bash loop follows a strict "Header, Do, and Done" format.

for variable in list
do
do
    command1
    command2
done

variable: A temporary name (like item or f) that represents the current object being processed.

list: The collection of items (strings, numbers, or files) the loop will cycle through.

do / done: These keywords act as brackets, containing all the code that should be repeated.

3. Using Bash ‘For’ Loop

In its simplest form, you can provide a manual list of strings. The loop will assign each string to the variable one by one.

for color in red green blue
do
    echo "The current color is $color"
done

4. Looping through Files and Directories

This is the most common use case for system administrators. By using wildcards (like *), you can tell Bash to find every file that matches a pattern.

Bash

#Example: Rename all .txt files to .bak
for file in *.txt
do
    mv "$file" "${file%.txt}.bak"
done

Note: Always use double quotes "$file" to handle filenames that contain spaces.

5. Iterating over a Range of Numbers

If you need to perform an action a specific number of times, you can use curly brace expansion {start..end}.

Bash

#Create folders named folder1 through folder5
for i in {1..5}
do
    mkdir "folder$i"
done

6. Using ‘for’ with Command Substitution

You can use the output of another command as the list for your loop. This is done using the $(command) syntax.

Bash

#Loop through all users listed in the system
for user in $(cut -d: -f1 /etc/passwd)
do
    echo "Found system user: $user"
done

7. Nested ‘for’ Loops

A nested loop is a loop inside another loop. The "inner" loop will complete its entire cycle for every single step of the "outer" loop. This is useful for creating grids or processing multi-layered data.

Bash

for letter in A B
do
    for number in 1 2
    do
   echo "Combination: $letter$number"
    done
done

Output:

Combination: A1

Combination: A2

Combination: B1

Combination: B2