How to Loop Through Array Using “for” Loop in Bash

Looping through arrays is essential for processing data efficiently in Bash scripting. Arrays provide a structured way to organize and manage data. The “for” loop offers a simple yet powerful mechanism for iterating through array elements. This article will discuss how to use the “for” loop in Bash scripting to iterate over array elements.

7 Methods of Iterating Through Bash Array Using “for” Loop

Unlocking the potential of the bash array using the for loop opens up a world of possibilities in script automation. From simple iteration of array elements to more advanced techniques like accessing elements by index and traversing multiple arrays simultaneously, this article covers 7 different array manipulation strategies in Bash scripting.

1. Simple Iteration of Array Elements

Iterating over arrays is a fundamental operation for processing array elements in bash scripting. The following script demonstrates a basic iteration process applied to an array named ‘fruits’ using array expansion. Array expansion in Bash scripting refers to the process of accessing or referencing all the elements within an array. Here is the script to do so:

#!/bin/bash

fruits=("apple" "banana" "orange" "grape")

for fruit in "${fruits[@]}"; do
echo "fruit: $fruit"
done

Here a basic for loop echoes all elements of an array.

2. Using a User-Defined Function with Variable Arguments

Using user-defined functions with variable arguments is a versatile technique for handling array elements dynamically. This script showcases the utilization of such a function named Loop to iterate over an array named fruits using a straightforward for loop. As the function iterates over each argument, it echoes the value to the console. Here is the script to demonstrate this functionality:

#!/bin/bash

fruits=("apple" "banana" "orange" "grape")

Loop(){
for fruit in ${*} ;
do
echo ${fruit}
done
}

Loop ${fruits[*]}

Here the user-defined function iterates over the elements of an array and echoes them into the terminal.

3. Accessing Elements by Index Using Positional Numbers

In bash scripting, accessing array elements by their index positions is a common task for array manipulation. The provided script showcases this approach by utilizing a for loop to iterate over the indices of the fruits array. Using the syntax ${!fruits[@]} enables the script to access each index position dynamically and allows direct mapping between the index and the array element. Here’s how:

#!/bin/bash

fruits=("apple" "banana" "orange" "grape")

for i in ${!fruits[@]}; do
echo "element $i is ${fruits[$i]}"
done

Here array elements are echoed with their corresponding index number.

4. Traditional C-Style “for” Loop

In bash scripting, the traditional C-style for loop offers an alternative method for iterating over array elements. This script demonstrates how to leverage this loop style to efficiently iterate through an array named fruits:

#!/bin/bash

fruits=("apple" "banana" "orange" "grape")

for (( i=0; i<${#fruits[@]}; i++ )); do
echo ${fruits[$i]}
done

Here a C-style for loop iterates over all elements of an array and prints them into the terminal using the echo command.

5. Iterating Over Multiple Arrays Simultaneously

Simultaneously iterating over multiple arrays is a useful technique in bash scripting for coordinating related data stored across different arrays. The following script initializes 2 arrays fruits and colors each containing 4 elements. It iterates over the indices of the fruits array and echoes the element at each index, followed by the corresponding element from the colors array. Here is the script to do so:

#!/bin/bash

fruits=("apple" "banana" "orange" "grape")
colors=("red" "yellow" "orange" "green")

for (( i=0; i<${#fruits[@]}; i++ )); do
echo ${fruits[$i]} ${colors[$i]}
done

Here elements of 2 arrays are echoed simultaneously.

6. Looping Through Key-Value Pairs in an Associative Array

An associative array is a collection of key-value pairs where each key is unique and associated with a specific value. Looping through an associative array is a crucial task in bash scripting for processing organized data. The following script starts with defining an associative array and then utilizes a for loop to iterate over the key-value pair of the array. Here is the script to iterate over an associative array using a for loop:

#!/bin/bash

declare -A fruits

fruits["apple"]="red"
fruits["banana"]="yellow"
fruits["orange"]="orange"
fruits["grape"]="green"

for fruit in "${!fruits[@]}"
do
echo "The $fruit is ${fruits[$fruit]}"
done

Here key-value pairs of the associative array “fruits” are echoed in a formatted way.

7. Reading Elements from a File into an Array

Reading elements from a file into an array is a common task in bash scripting for processing data from an external source. The following demonstrates how to read elements of a text file in an array and iterate over them using the mapfile command. The mapfile command in Bash is used to read lines from standard input or a file into an array variable.

The content of the fruits.txt text file is as follows:

apple red
banana yellow
orange orange
grape green

Execute the following script to read the elements of a text file and iterate through them:

#!/bin/bash

mapfile -t fruits < fruits.txt
count=1

for line in "${fruits[@]}"
do
echo "line $count $line"
((count++))
done

Here all lines of the “fruits.txt” file are echoed in a new line in the terminal.

Practice Tasks on Bash “for” Loop Array.

If you aim to be better at using the “for” loop to iterate over an array, then you can try solving the following problems using the concept discussed in this article:

1. Create a script that declares an array of numbers and calculates the sum of all elements using a for loop.
2. Write a script that initializes an array of numbers and uses a for loop to print only the even numbers from the array.
3. Develop a script that takes an array of strings as input and prints the elements in reverse order using a for loop.
4. Create a script that prompts the user to enter a number and checks if it exists in an array of numbers using a for loop.
5. Write a script that sorts an array of integers in ascending order using a simple sorting algorithm implemented with a for loop.

Conclusion

Understanding the intricacies of looping through arrays using the “for” loop in Bash is paramount for effective script development. Whether you’re a novice or an experienced scripter I hope these array-looping techniques discussed in this article will enhance your bash scripting journey.

People Also Ask

What is an Array in Bash Scripting?

An array is a collection of elements stored under a single variable. To declare an array use syntax; array_name=(element1 element2 element3 ...). In bash scripting, the arrays are not typed, which means they can hold elements of different types such as strings, integers, or even other arrays.

Arrays in bash are indexed and the first element is started from zero. All individual elements of an array can be accessed using their index. Bash also supports associative arrays, where elements can be accessed using keys instead of numerical indices.

What is for loop in Bash?

The for loop in Bash scripting is a control flow statement that iterates over a sequence of elements. It allows you to execute a set of commands repeatedly until a specified condition is met. In Bash scripting, the for loop is often used to iterate over arrays, files, or ranges of numbers. With its simple syntax and versatility, the for loop is a fundamental tool for automating tasks and processing data in Bash scripts. It enables users to perform repetitive actions efficiently, making it a cornerstone of Bash scripting for automation and data manipulation.

Can for loop iterate over arrays?

Yes, for loop in Bash can iterate over arrays. By using the ${array[@]} syntax within the for loop, each element of the array can be accessed and processed sequentially. This feature enables efficient handling of array elements in Bash scripts, allowing tasks to be performed on each element individually. Therefore, for loops serve as a fundamental tool for array manipulation and iteration in Bash scripting.

How do I create an array of strings in Bash?

To create an array of strings in Bash, you can use the following syntax: array_name=(element1 element2 element3 ...). Each element is separated by a space within the parentheses. This declaration initializes the array with the specified elements. For example, fruits=(“apple” “banana” “orange” “grape”) create an array named “fruits” containing four string elements. This method allows you to store multiple strings within a single variable, facilitating efficient data management in Bash scripting.

How to use declare in Bash?

In Bash scripting, the declare command is used to explicitly declare variables, including arrays. The basic syntax of the declare command is declare options variable_name. It allows you to define variables with specific attributes, such as declaring an array with the -a option. For instance, declare -a fruits declares an array variable named “fruits”. Using declare helps enforce variable types and enables additional functionality like setting attributes for variables.

How to iterate over a list of strings in the Bash shell script?

To iterate over a list of strings in a shell script, you can use a for loop. The basic syntax for a for loop in shell scripting is for variable in list; do commands; done. To iterate over a list of strings, you replace “list” with the array or list variable containing the strings you want to iterate over. Within the loop, you can perform actions or execute commands using the current string stored in the loop variable.

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