Arrays and String Manipulation

# CHAPTER 10

Arrays and String Manipulation #

1. Introduction #

*(Important POSIX Note: Traditional Bourne Shell sh does NOT support true arrays. Arrays are a feature of bash, zsh, and ksh. Because arrays are mandatory for modern DevOps, this chapter will explicitly leverage Bash-compatible array syntax).*

2. Learning Objectives #

• Declare and initialize an indexed array.

• Retrieve specific elements from an array using zero-based index mapping.

• Dynamically calculate the total length (number of items) of an array.

• Iterate over an entire array systematically using a for loop.

• Perform variable string manipulation (substring extraction and replacement).

3. Declaring and Accessing Arrays #

1. Declaring an Array: You define an array by wrapping a space-separated list of words in parentheses.

#!/bin/bash
# Define an array of 4 distinct IP addresses
CLUSTER_IPS=("10.0.0.1" "10.0.0.2" "10.0.0.50" "10.0.0.99")

2. Accessing Elements: To pull a specific piece of data out of the array, you must explicitly use curly braces ${} and specify the slot number in square brackets [].

# Print the FIRST IP (Index 0)
echo "Primary DB IP is: ${CLUSTER_IPS[0]}"

# Print the THIRD IP (Index 2)
echo "Backup DB IP is: ${CLUSTER_IPS[2]}"

4. Advanced Array Operations #

1. Print All Elements (@): If you need to view every single item in the array simultaneously, use the @ (At) symbol as the index.

echo "All nodes in the cluster: ${CLUSTER_IPS[@]}"

2. Get the Length of the Array (#): How many IPs are currently stored in the list? Prepend the Hash (#) symbol to the array name.

echo "Total number of nodes active: ${#CLUSTER_IPS[@]}"
# Output: 4

3. Appending to an Array: To add a brand-new IP address to the end of the existing list without overwriting the old data, use the += operator.

CLUSTER_IPS+=("10.0.0.100")

5. Iterating Over Arrays #

#!/bin/bash
SERVERS=("Web01" "Web02" "DB01")

# The [@] expands the array into a list of words the loop can read
for TARGET in "${SERVERS[@]}"
do
    echo "Initiating deployment sequence on: $TARGET"
    # Execute actual deployment SSH commands here...
done

*Crucial Syntax:* Always wrap "${SERVERS[@]}" in double quotes. If one of the items in your array happens to contain a space, the double quotes prevent the for loop from accidentally splitting it into separate words.

6. String Manipulation #

1. String Length: Just like array length, use the Hash (#) to count the characters in a string.

PASSWORD="SuperSecret123"
echo "Password length is: ${#PASSWORD} characters."

2. Substring Extraction: Extract a chunk of text. Syntax: ${VARIABLE:START_INDEX:LENGTH}

STRING="abcdefg"
# Start at index 2 (c), and grab 3 characters
echo "${STRING:2:3}" 
# Output: cde

3. String Replacement: Find and replace text directly inside the variable.

FILENAME="report_2026.txt"
# Replace .txt with .csv
NEW_FILE="${FILENAME/.txt/.csv}"
echo "Renamed to: $NEW_FILE"

7. Diagrams/Visual Suggestions #

8. Best Practices #

• Use Arrays for Complex Command Flags: If your script needs to execute a complex command that requires 10 different flags, do not write them as one massive, unreadable string. Build an array of flags (FLAGS=("-v" "--force" "--secure")), and then pass the expanded array into the command (my_command "${FLAGS[@]}"). This makes your code infinitely easier to read, maintain, and modify.

9. Common Mistakes #

• Forgetting the Curly Braces: When beginners attempt to print the first item of an array, they often type echo $SERVERS[0]. The shell evaluates $SERVERS (which defaults to the first item), prints it, and then literally prints the text "[0]" immediately after it. You MUST isolate the entire array call within curly braces: echo ${SERVERS[0]}.

10. Mini Project: Build a Simple Contact Manager #

1. 1. nano contacts.sh

1. 2. Write the code:

#!/bin/bash

# Define an array of employees
EMPLOYEES=("Alice Smith" "Bob Johnson" "Charlie Davis")

echo "Generating Corporate Email Directory..."
echo "-------------------------------------"

# Loop through the array
for PERSON in "${EMPLOYEES[@]}"
do
    # 1. Convert the entire name to lowercase (Bash 4.0+ feature)
    LOWER_NAME="${PERSON,,}"
    
    # 2. Replace the space with a dot to format the email
    EMAIL="${LOWER_NAME/ /.}@corporate.com"
    
    echo "Employee: $PERSON | Email: $EMAIL"
done

1. 3. Run it. You have dynamically processed and formatted bulk text data utilizing arrays and built-in string replacement.

11. Practice Exercises #

1. 1. Given the array FILES=("config.ini" "data.csv" "script.sh"), write the exact shell syntax required to mathematically determine the total number of items stored inside the array.

1. 2. Provide the syntax to extract exactly the first 4 characters from a variable named PRODUCTCODE.

13. Interview Questions #

• Q: A junior engineer writes a script containing the variable call echo $SERVERS[1]. The terminal outputs Web01[1] instead of the actual second server in the array. Explain the mechanical syntax error the engineer made regarding variable expansion evaluation.

• Q: Explain the precise structural difference between utilizing "${ARRAY[*]}" and "${ARRAY[@]}" within a script, particularly focusing on how double quotes interact with the expansion regarding word splitting.

• Q: You are tasked with writing a script that receives a massive, comma-separated string (DATA="ip,hostname,mac,status"). While you could use external tools, describe how you might utilize built-in variable string manipulation to replace those commas with spaces directly in the shell.

14. FAQs #

15. Summary #

16. Next Chapter Recommendation #