Iterating over arrays in Bash Scripting

Introduction

Iterating over arrays lets you do something with each item in a list, like checking or printing them one by one.

You want to print all items in a list of names.

You need to check each file in a list to see if it exists.

You want to add a prefix to every word in a list.

You want to count how many items in a list meet a condition.

Syntax

Bash Scripting

array=(item1 item2 item3)
for element in "${array[@]}"; do
  # commands using $element
  echo "$element"
done

Use double quotes around "${array[@]}" to handle items with spaces correctly.

The variable 'element' holds each item from the array during the loop.

Examples

Prints each fruit in the array one by one.

Bash Scripting

fruits=(apple banana cherry)
for fruit in "${fruits[@]}"; do
  echo "$fruit"
done

Handles an empty array gracefully by not running the loop body.

Bash Scripting

empty_array=()
for item in "${empty_array[@]}"; do
  echo "$item"
done

Works with an array that has only one item.

Bash Scripting

single_item=(onlyone)
for element in "${single_item[@]}"; do
  echo "$element"
done

Handles array items that contain spaces correctly.

Bash Scripting

words=("hello world" "foo bar" "baz")
for word in "${words[@]}"; do
  echo "$word"
done

Sample Program

This script creates an array of colors, prints them all at once, then prints each color on its own line using a loop, and finally prints the whole array again.

Bash Scripting

#!/bin/bash

# Create an array of colors
colors=(red green blue yellow)

# Print all colors before loop
printf "Colors before loop: %s\n" "${colors[*]}"

# Iterate over the array and print each color
for color in "${colors[@]}"; do
  echo "Color: $color"
done

# Print all colors after loop
printf "Colors after loop: %s\n" "${colors[*]}"

Important Notes

Time complexity is O(n) where n is the number of items in the array because each item is visited once.

Space complexity is O(n) for storing the array items.

A common mistake is not quoting "${array[@]}" which can cause items with spaces to split incorrectly.

Use array iteration when you need to process each item individually; use other methods like joining if you want to handle the whole list as one string.

Summary

Iterate over arrays using 'for element in "${array[@]}"; do ... done'.

Always quote "${array[@]}" to handle spaces in items safely.

Loops run zero times if the array is empty, which is safe.

Practice

1. What is the correct way to iterate over all elements in a Bash array named fruits?

easy

A. for fruit in ${fruits[*]}; do echo "$fruit"; done

B. for fruit in ${fruits}; do echo "$fruit"; done

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

D. for fruit in fruits; do echo "$fruit"; done

Solution

1. Step 1: Understand array iteration syntax

   In Bash, to iterate over all elements of an array safely, you use "${array[@]}" which expands each element as a separate word, preserving spaces.

2. Step 2: Check each option's correctness

   for fruit in "${fruits[@]}"; do echo "$fruit"; done uses "${fruits[@]}" correctly. Options B and C omit quotes or use ${fruits} or ${fruits[*]} without quotes, which can break elements with spaces. for fruit in fruits; do echo "$fruit"; done treats fruits as a string, not an array.

3. Final Answer:

   for fruit in "${fruits[@]}"; do echo "$fruit"; done -> Option C

4. Quick Check:

   Use "${array[@]}" to iterate safely [OK]

Hint: Always quote "${array[@]}" to handle spaces safely [OK]

Common Mistakes:

• Not quoting "${array[@]}" causing word splitting
• Using ${array} instead of ${array[@]}
• Iterating over the array name as a string

2. Which of the following is the correct syntax to iterate over an array colors in Bash?

easy

A. for color in colors; do echo $color; done

B. for color in ${colors}; do echo $color; done

C. for color in ${colors[@]}; do echo $color; done

D. for color in "${colors[@]}"; do echo $color; done

Solution

1. Step 1: Identify correct quoting for array iteration

   To iterate over array elements safely, you must quote "${array[@]}" to preserve elements with spaces.

2. Step 2: Evaluate each option

   for color in "${colors[@]}"; do echo $color; done correctly uses quotes around "${colors[@]}". for color in colors; do echo $color; done treats 'colors' as a string, not an array. for color in ${colors[@]}; do echo $color; done misses quotes, risking word splitting. for color in ${colors}; do echo $color; done uses ${colors} which is incorrect for arrays.

3. Final Answer:

   for color in "${colors[@]}"; do echo $color; done -> Option D

4. Quick Check:

   Quote "${array[@]}" for safe iteration [OK]

Hint: Quote "${array[@]}" to avoid splitting elements [OK]

Common Mistakes:

• Forgetting quotes around "${array[@]}"
• Using array name without $ or quotes
• Using ${array} instead of ${array[@]}

3. What will be the output of the following Bash script?

names=("Alice" "Bob Smith" "Charlie")
for name in "${names[@]}"; do
  echo "$name"
done

medium

A. Alice
Bob Smith
Charlie

B. Alice
Bob
Smith
Charlie

C. Alice Bob Smith Charlie

D. Syntax error

Solution

1. Step 1: Understand array elements and quoting

   The array has three elements: "Alice", "Bob Smith", and "Charlie". Quoting "${names[@]}" preserves each element as one word, even if it contains spaces.

2. Step 2: Trace the loop output

   The loop prints each element on its own line: first "Alice", then "Bob Smith", then "Charlie".

3. Final Answer:

   Alice Bob Smith Charlie -> Option A

4. Quick Check:

   Quoted "${array[@]}" keeps elements intact [OK]

Hint: Quoted "${array[@]}" keeps multi-word elements together [OK]

Common Mistakes:

• Assuming elements split on spaces inside quotes
• Using unquoted ${array[@]} causing word splitting
• Expecting all elements on one line

4. Identify the error in this Bash script that tries to print all elements of array items:

items=("pen" "notebook" "eraser")
for i in ${items[@]}
do
  echo $i
done

medium

A. Array declaration syntax is wrong

B. Missing quotes around ${items[@]} causes word splitting

C. for loop syntax is incorrect

D. echo command is missing quotes

Solution

1. Step 1: Check array declaration

   The array declaration is correct with parentheses and quoted elements.

2. Step 2: Analyze the for loop iteration

   The loop uses unquoted ${items[@]}, which causes word splitting. If any element had spaces, it would split incorrectly. Quoting "${items[@]}" fixes this.

3. Final Answer:

   Missing quotes around ${items[@]} causes word splitting -> Option B

4. Quick Check:

   Always quote "${array[@]}" to avoid splitting [OK]

Hint: Quote "${array[@]}" to prevent splitting elements [OK]

Common Mistakes:

• Not quoting array expansion in loops
• Assuming echo needs quotes for correctness here
• Misreading array declaration syntax

5. You have a Bash array files containing filenames, some with spaces. You want to print only those filenames that contain the word "report". Which script correctly iterates and filters the array?

hard

A. for file in "${files[@]}"; do if [[ $file == *report* ]]; then echo "$file"; fi; done

B. for file in ${files[@]}; do if [[ $file == *report* ]]; then echo "$file"; fi; done

C. for file in "${files[*]}"; do if [[ $file == *report* ]]; then echo "$file"; fi; done

D. for file in files; do if [[ $file == *report* ]]; then echo "$file"; fi; done

Solution

1. Step 1: Choose correct array iteration with quoting

   To handle filenames with spaces correctly, iterate using "${files[@]}" quoted. This preserves each filename as one item.

2. Step 2: Apply filtering condition inside loop

   The condition [[ $file == *report* ]] correctly checks if the filename contains "report". for file in "${files[@]}"; do if [[ $file == *report* ]]; then echo "$file"; fi; done uses this with proper quoting and iteration.

3. Final Answer:

   for file in "${files[@]}"; do if [[ $file == *report* ]]; then echo "$file"; fi; done -> Option A

4. Quick Check:

   Quote "${array[@]}" and filter inside loop [OK]

Hint: Quote "${array[@]}" and filter inside loop with [[ ]] [OK]

Common Mistakes:

• Using unquoted ${files[@]} splitting filenames
• Using ${files[*]} which joins all elements into one string
• Iterating over literal 'files' string